HIPPARCHOS The Hellenic Astronomical Society Newsletter Volume 3, Issue 4

ISSN: 1790-9252 Contents HIPPARCHOSHIPPARCHOS Volume 3, Issue 4 • July 2021

ISSN: 1790-9252 Hipparchos is the official newsletter of the Hellenic Astronomical Society. Message from the President ...... 3 It publishes review papers, news and comments on topics of interest to as- tronomers, including matters concern- REVIEWS ing members of the Hellenic Astro- nomical Society. Sensing the pulsations of empty space by Theocharis A. Apostolatos ...... 4 Editorial board • Kostas Gourgouliatos A revolution in transient astrophysics (Physics Dept., University of Patras) by Giorgos Leloudas ...... 8 • Spiros Patsourakos Predicting Solar Energetic Particle (SEP) events: (Physics Dept., University current status and future perspectives of Ioannina) by A. Anastasiadis and A. Papaioannou ...... 14 • Kostas Tassis (Physics Dept., University of Crete) Characterising exoplanetary atmospheres: • Nektarios Vlahakis the legacy of HST/WFC3 (Physics Dept., University by Angelos Tsiaras ...... 21 of Athens)

Contact person Nektarios Vlahakis Physics Department, National and The 15th Hellenic Astronomical Conference Kapodistrian University of Athens, University Campus, The Hellenic Astronomical Conference, organized by the Hellenic Astronomical 157 84 Zografou, Society (Hel.A.S.), is the major scientific event of the greek astronomical com- Athens, Greece munity. The Conference, which takes place every two years in a different part of Tel: +30-210-7276903 Greece, typically brings together over 100 scientists with research interests in As- E-mail: [email protected] trophysics, Planetary Science and Space Physics. The 15th Conference of Hel.A.S. was originally planned to take place in Patras, Editorial Advisors from 5 to 8 July, 2021. However, the persistence of the COVID pandemic forced • Vassilis Charmandaris us to host the conference virtualy. (Physics Dept., University of Crete) We would have much preferred to seeing you in Patras, but we hope to least see you online instead! • Antonis Georgakakis Sponcored by: (National Observatory of Athens) • Costis Gontikakis (RCAAM, Academy of Athens)

Printed by ZITI Publications • www.ziti.gr

Cover Image: The Galactic Center: A detailed panorama explores regions just above and below the galactic plane. Different bands of X-rays from the Chandra Observatory (orange - hot, green - hotter, purple - hottest) have been combined with radio data (gray) from the MeerKAT telesccope. These data reveal threads of superheated gas and magnetic fields near the center of the Milky Way. Image Credit: X-ray: NASA/CXC/UMass/Q.D. Wang; Radio: NRF/SARAO/MeerKAT. Message from the President

ver this past year, we have all been in person in Patras during the 15th major astronomy events that take place Oexperiencing the abrupt changes Conference of our Society, as it was in Greece, as well as an Instagram and in our daily lives imposed by the pan- originally planned. Our conference, Twitter account. Moreover, Hel.A.S. demic and several of us have had fam- like many others, will be held “virtu- supported the public outreach activi- ily and friends who have become sick. ally” during the second week of July ties of a dynamic group of Junior Mem- Moreover, the way we work, teach, and 2021. This appears to have facilitated bers, who established the “2 minute do science has been modified and in participation since over 200 astrono- Science” page, both on the web and on the process we became experts in using mers from 24 countries, both mem- Facebook, in order to share with the ZOOM, Webex and eClass along with a bers and non-members of Hel.A.S., general public in Greece their excite- variety of other teleconference tools. have registered to attend it, mak- ment about astrophysics and space ing it the largest ever conference or- physics. I do hope that they keep their Despite these challenges, our training ganized by our Society. The Scientif- enthusiasm and with the help of the in the new “tools of the trade” also en- ic Organizing Committee has select- next generation of astronomy students abled new opportunities. We have at- ed four exceptional scientists as ple- and postdocs will continue to provide, tended lectures and seminars from in- nary speakers, Dr. Francisco Colom- in Greek, interesting and high quality stitutes across the globe and participat- er (JIVE, The Netherlands), Dr. Athe- information about astronomy to our ed in conferences remotely. Sharing in- na Coustenis (Obs. de Paris, France), fellow citizens. formation in a reliable and user-friend- Prof. Avishay Gal-Yam (Weizmann In- ly manner has become much easier for stitute, Israel) and Prof. Kostas Kok- The Vice Chair of the Society, Prof. all. Our Society, following the popu- kotas (Univ. of Tuebingen, Germany), Nektarios Vlahakis, was also responsi- lar trend, commenced a monthly vir- and worked hard to prepare a dense ble in selecting and editing the four in- vited articles on pertinent areas of as- tual colloquium. A committee of three and diverse scientific program. In ad- trophysics, which appear in the pres- members of the Governing Council dition, the Governing Council decided ent issue of Hipparchos. Prof. Theo- – Nektarios Vlahakis, Spiros Patsou­ to name one of the plenary lectures as haris Apostolatos is presenting the lat- rakos and Kostas Tassis – selected a “John H. Seiradakis Plenary Lecture”, est results in the quest for gravitation group of distinguished colleagues who in memory of Prof. John Seiradakis waves, Dr. George Leloudas is shar- presented 9 very interesting colloquia who passed away last year. As we all ing with us some of the wonders of the this past academic year. These were know Prof. Seiradakis played a critical transient sky, Dr. Anastasios Anasta- typically attended by over 70 of our role in founding the Hellenic Astro- siadis with Dr. Athanasios Papaioannou members and are now available as re- nomical Society and also was the Sec- provide the latest information on trac- corded videos in the YouTube channel retary of the Society from 1994 un- ing energetic particles emanating from of our Society. I would like to personal- til 1998 and President from 1998 un- the Sun, and Dr. Angelos Tsiaras is de- ly thank everyone involved, and in par- til 2002. scribing the ongoing efforts in charac- ticular the speakers who invested their terizing the atmospheres of exoplanets, time to share with us their results, and Over the past year, the Society, thanks on the eve of the upcoming launch of I do look forward for these colloquia to the investment of time and energy the James Webb Space Telescope from to continue for many year into the fu- of several members, also obtained a NASA and the Ariel mission from ESA. ture. more prominent presence in the so- I am certain that we will all enjoy and cial media. We now maintain an active learn a lot reading them. However, due to the restrictions of the Facebook page, where we share sci- pandemic, it was not possible to meet ence news and information about all

Vassilis Charmandaris President of Hel.A.S.

HIPPARCHOS | Volume 3, Issue 4 3 Sensing the pulsations of empty space by Theocharis A. Apostolatos Department of Physics, National and Kapodistrian University of Athens, Greece

1. Introduction orem, substantial developments in dif- The lack of physical experiments, ferential geometry, the tensor calculus where one could measure the differ- Struggling for almost a decade, after by Levi­-Civita etc. ences between the new theory and the the annus mirabilis when he formulat- The idea of waves permitted by this celebrated for three centuries newto- ed Special Relativity, Albert Einstein new field (the field of the metric of the nian theory of gravity, as long as the ad- moved on to formulate General The- space­-time itself) concerned Einstein vent of quantum mechanics lead Gen- ory of Relativity, which was completed himself, from the very beginning. In anal- eral Relativity to remain confined in a in 1915. Although the idea of a curved ogy to Maxwell equations which per- few small groups of mainly mathemati- space-­time sounds rather simple, and mitted the existence of electromagnet- cians, scattered around the globe. Grav- well known, today, it was a radical idea ic waves, Einstein felt that his new set of itational waves were not an exception. at the dawn of the 20th century and full equations should allow for similar type In the famous book “Classical Theory of surprises. Famous physicists, like Lev of waves. However, the fact that the co- of Fields” of Landau and Lifshitz [1] that Landau, considered the idea of Einstein ordinates one could use to map space-­ was first published during the World as a unique idea in the history of Phys- time, apart of being continuous and dif- War II, and was translated in English in ics and have jealously described Gener- ferentiable, were completely free, led the 50’s, it is noted that the amount of al Relativity as “probably the most beau- him to a very difficult situation. One energy radiated by gravitational waves tiful of all existing physical theories”. Al- should somehow discern the true waves in he case of a binary system would be though the basic idea was conceived by ­if that had any meaning­ from the ficti- of the order of 10–12 of the total energy Einstein rather early, it took him years tious waves showing up just as an arti- of the system itself, and thus the time to overcome the mathematical obsta- fact of choosing crazy coordinates. Ein- scale expected to see its effects on the cles before he presented it in its right, stein faced that problem and was soon motion of the binary would be of cos- final form. Actually, it was David Hil- led to wave solutions moving at any pos- mic order. No one was crazy enough bert, a famous mathematician inspired sible speed! As Arthur Eddington, one to suspect that something so tiny would by long discussions with Einstein, who of the first believers of Einstein’s theory, ever be observed. managed to formulate General Theory commented “gravitational waves should of Relativity a few days before Einstein, propagate at the speed of thought”. Ein- following the much easier path of least stein was so puzzled that for almost 2. The golden era action principle. two decades, he was opposed to the of General Relativity and a At first the Einstein equations, as idea that there was any kind of propa- bold experimental device they came to be known, were most- gating gravitational waves correspond- ly neglected by the physical communi- ing to some real physical context as en- In the 50’s a new generation of physi- ty, since the mathematical notions in- ergy carriers. After the repeated deni- cists of high caliber, such as Bondi, Pira- volved in the theory were not includ- al of the existence of gravitational waves ni, Wheeler, Feynman, DeWitt, Hawk- ed in the basic education of physicists by Einstein, he finally conceded they are ing, Penrose, and others, showed spe- at the time. Apart of a handful of weak-­ real, convinced by his own colleagues cial interest to relativity. Gravity, along gravity relativistic results expressed as (Infeld, Rosen) and other physicists and with the dreams of space exploration, tiny corrections of newtonian gravity, mathematicians. was again an exotic field that allured that eventually turned Einstein into an established figure in sciences, physicists could not work out but a few, though oversimplifying symmetrical physical systems based on the new theory. The mathematics involved were too compli- 1 cated. Soon General Relativity was con- Figure : Joseph Weber sidered a field of mathematics with no working on the much connection to the physical world, electronics of apart of its innovative interpretation of his bar detector. gravitational force. On the other hand, great mathematical ideas with applica- tions in various physical theories sprung from the new field, like Noether’s the-

HIPPARCHOS | Volume 3, Issue 4 4 many brilliant minds. While theoretical issues, like what is the energy content of gravitational waves, were still puz­ zling physicists, a young collaborator of Wheeler, Joseph Weber wrote with Wheeler a paper describing a thought experiment that could be used as a de­ tecting device of gravitational waves. Later on, in the 1960’s, Weber built a resonant detector in Maryland, con­ sisting of pure aluminum cylinders that were constructed to resonate at 1661 Hz, when a gravitational wave signal of that frequency hit them. The cylinders were seismically isolated, and piezo­ electric crystals around the “bar de­ tectors” were adjusted to measure ti­ ny oscillations of their length. Weber, who was a master in electronics, build the whole device by himself, and made it capable to detect gravitational waves of strain 10–16 [2]. As another Colum­ bus he embarked on this journey with­ out knowing either what are the possi­ Figure 2: The evolution of the orbital period of PSR 1913+16 for almost 4 decades. The solid line ble sources of such waves, neither their is the theoretical prediction of General Relativity, while the dots represent actual observations. The physical characteristics (their frequen­ error bars are so small that are not discernible. The graph is adapted from the paper of Weisberg and Huang [4] in 2016. cy and their amplitude). He picked up a frequency (and built bars having that specific normal mode eigenfrequency) in the range where he thought various ery, Weber continued to believe that ed to gravitational waves showed up. astronomical sources, like a star with he was observing gravitational­-wave Hulse and Taylor discovered the first a bump undergoing gravitational col­ signals. It is remarkable that part of pulsar in a binary system, now known lapse, could emit substantial gravita­ the doubt about Weber’s claims was as PSR 1913+16. The 59 ms pulsar in­ tional waves. At that time gravitational driven by theory: If Weber’s detectors volved, was found to be modulated radiation was still an open issue, full of were observing such signals from so with a period of 7.75 h, indicating the controversies, and with no sufficiently distant sources the corresponding en­ presence of a close compact compan­ rigid scientific results. Weber was bold ergy emitted by them would be huge ion around which the pulsar is orbiting. enough to believe that mankind was and our galaxy should disintegrate in a According to General Relativity such ready to detect such miniscule devia­ much shorter time than its age. Despite a close binary is expected to radiate tions from flat metric and he had the this negative environment, some theo­ gravitational waves leading to an adia­ ambition to be the pioneer of this ef­ rists, holding Weber’s work in high re­ batically shrinkage of orbital radius, and fort. In 1969 Weber declared that he gards, thought of imaginative, but rath­ consequently to a slow increase of its had detected for the first time gravi­ er exotic, type of sources that could orbital frequency. Weisberg and Tay­ tational waves from cosmic sources somehow reconcile Weber’s claims and lor followed the evolution of the orbit­ and subsequently, a long series of such theoretical objections. I remember Kip al modulation of PSR 1913+16 for a few “detections” followed during the next Thorne in a Pacific coast gravity con­ years and found that the theoretical ex­ few years. Weber built a second bar­ ference in 1994 trying to reconciliate pectation of General Relativity was ac­ detector in Chicago, in order to make Weber’s opponents with Weber’s ex­ curate to the level of 1.8σ, correspond­ sure that he counted only the coinci­ aggerations regarding his findings, in a ing to a tiny 76.5µs/yr decrease in or­ dent signals in both detectors, thus dis­ hot discussion following Weber’s pre­ bital period. regarding any spurious electronic noise sentation. The paper published by Weisberg in each device. Unfortunately his find­ Nowadays, the detections of We­ and Taylor [3], with all these detailed ings were not confirmed by other ex­ ber are unequivocally rejected by the observations, was quite convincing that perimentalists who were inspired by community. However all recognize him gravitational radiation was a real phe­ Weber’s effort and built their own bar as the father of gravitational­-wave re­ nomenon with measurable impact on detectors. None was able to repro­ search (as Misner put it). If he was not some astrophysical systems, more spe­ duce his results, even though one of bold enough to start this enterprise, cifically for binaries consisting of com­ these new detectors were built at ex­ the actual detection of gravitational pact objects like neutron stars. For the actly the same size as Weber’s detec­ waves would probably be delayed by at first time the scientific community had tor. Although the scientific communi­ least a decade. in hand relativistic evidence regarding ty eventually rejected Weber’s discov­ In 1974 another new finding relat­ strong gravitational fields. All the previ­

HIPPARCHOS | Volume 3, Issue 4 5 ous tests of General Relativity, like the The tiny amounts of energy, at first 4. The LIGO­-VIRGO era ones regarding our Solar system, were glance, turn out to be tremendous for In the late 80’s a few visionary physicists weak field approximations of Einstein’s suitable systems. believed that time had come for build- theory. The moral of this simple, back­-of-­ ing devices capable of detecting grav- the-­envelope calculation shows that itational waves. Rainer Weiss, Vladi- as long as the system consists of very mir Braginsky, Kip Thorne, and Ronald 3. Order compact objects with R/R = O(1), S Drever embarked on a wonderful jour- of magnitude estimations like neuron stars or black holes, that ney to construct huge interferometers are moving so close to each other In order to have a feeling of the num- that could work as broad band (in con- that the velocities involved are close to bers involved in the radiation reaction trast to those of Webers) gravitation- the speed of light, gravitational radia- of a gravitational system, we give here al detectors. The idea was simple: One tion emission could make these sys- the famous quadrupole formula intro- should hang 3 test masses-­reflectors in tems the most explosive machines in duced by Einstein in a paper of 1918 the form of pendulums (forming a Γ). the Universe. On the other hand the [5]. Along the two long arms between the dE ...... high powers of both undimensional GW G corner mass and the two other mass- = 5 Qij Qij . (1) terms in eq. 2, make it rather implau- dt 5c es, monochromatic light would travel sible that one would ever be able to up and down. Whenever a gravitational This is the formula that was disputed observe significant gravitational radia- wave hits the two arms it will make the for many years, as was mentioned ear- tion signals from macroscopic terres- arms oscillate in opposite phase (due to lier, and was later derived in the book tial, or near Earth sources despite, the the tidal nature of gravity waves). The of Landau and Lishitz for binaries, fol- much shorter distances involved (the interference pattern formed by the two lowing a rather dubious methodology. normal objects are far from com- beams should follow the time evolution It gives the rate of energy in the form of pact, and are moving with highly non­- of the signal itself. gravitational waves emitted by a physi- relativistic velocities). cal system moving under its own grav- On the other hand there were nu- ity. Qij is the the quadrupole moment merous technical issues, as well as of the system, while the triple dots de- note a triple derivative with respect to time. The G, c constants are the usu- al gravitational constant and the speed of light respectively. This formula is the analogue of dipole radiation in electro- magnetism emitted from accelerating charges. The factor in front of the expres- sion G/c5 is so small, of the order of 10–53(joules/s)(–1) that nobody would ever thought of ever being able to ob- serve any effect on such systems due to gravitational radiation. The rest part of the formula, which is related to the physical characteristics of the system itself, should bring forward the rest dimensions, (joules/s)2, to get the final answer. What if the phys- ical parameters of the systems were normalized in such a way that the fi- nal formula had the factor G/c5 invert- ed with the right dimensions of pow- er, and the rest were simple dimen- sionless numbers characterizing how Figure 3: On the first line of diagrams the raw data of GW150914 signal are shown (the left is relativistic is the system, namely v/c from Hanford LIGO, while the right is from Livingston LIGO). To emphasize the coincidence, the (the velocity of the parts of the sys- left diagram is superimposed on the right one after a suitable time shift accounting for the signal tem) and R/RS (how large compared to travel from one site to the other. The second line of diagrams are theoretical waveforms of the to its Schwarzschild radius RS is the signal itself that matches the raw data of each detector. The signal has the characteristic chirp for system). Working out the details of both amplitude and frequency, representing the inspiral orbit of the two black holes, up to the point this magical inversion, we finally ar- where the two horizons of the black holes merge into a single one. Then the final, highly deformed, rive at an order of magnitude for the new black hole oscillates violently and, in a few hundredths of a second, it finally rests in peace as a quiescent spinning black hole, by radiating away all its deformations. The third line presents the rate of energy emitted difference between the actual signals and the best fitting theoretical waveforms of line two. The 5 2 6 fourth line is a spectrogram showing up the chirping evolution of the frequency, along with the in- dEGW c R v = . (2) tensity of the signal. dt G RS c

HIPPARCHOS | Volume 3, Issue 4 6 funding ones (of the order of a few hun- with a new type of device to sense the 6. The future dred million US dollars). The Nation- cosmic, violent events that are not ac- of Gravitational-Wave­ al Science Foundation of US was con- cessible through electromagnetic radi- Astronomy vinced that the endeavour was worth ation. the money, since gravitational waves During the last decade new gravitational­ would open up a new window to ob- -wave detectors are build around the serve the cosmos. After a long experi- 5. The current status globe (IndIgo in India, KAGRA and mentation in small laboratories both in of the LIGO-VIRGO­ TAMA in Japan, AIGO in Australia), Caltech and MIT, two four by four ki- observatory of the while further technological advances lometers interferometers were built in dark violent universe are planned for the existing detectors. the two corners of USA; the Hanford Also new, much longer, much more LIGO, and the Livingston LIGO [6]. Since the first successful detection of sensitive ground based detectors are The locations were chosen so that the gravitational wave signals, around 50 currently under consideration (Einstein two sites were as far apart as possible candidate signals have already been re- telescope, Cosmic explorer). Finally in order to obtain some useful infor- ported; most of them are black hole– there are plans to fly spaceborne de- mation about the direction of the sig- black hole binaries, with masses in the tectors (like LISA), that would be able nal, by measuring the small differences range of a few ≃ 10 to a ≃ 100 solar to detect gravitational waves for mas- in arrival time of the detected signals. masses. There are also a couple of neu- sive black holes, since these detectors Also the arms between the two sites tron star–neutron star binaries or neu- are most sensitive at frequencies two are forming an angle close to 45° in or- tron star–black hole binaries among orders of magnitude lower than their der to be able to pick up the two differ- the detection events. One of them, the terrestial counterparts. ent polarizations encoded in a signal. GW170817, consisted of two neutron In a few years the gravitational­- A similar third interferometric de- stars, that emitted strong electromag- wave detectors will offer scientists tector VIRGO, named after the VIRGO netic radiation a couple of seconds af- a new point of view of the Universe, cluster, with three kilometers arms was ter their collision. Such combined de- “hearing” the dark corners that cannot built in Pisa, Italy and after 2007 they tection by gravitational wave and elec- be seen, and enriching our view about have agreed to share their data with LI- tromagnetic wave detectors was ex- the cosmos, its constituents, and its GO, and publish jointly their results. tremely fruitful. It gave gravitational­- history. A new branch of Astronomy The operation of LIGO­-VIRGO wave astronomers the opportunity to has been born. lead to the first detection of gravita- pinpoint the source with high accura- cy. Furthermore the evolution of the tional waves (actually VIRGO was not Acknowledgement operating at the time), the so called signal, when the tidal effects arising on GW150914, a few days before the for- each neutron star from its compan- Most of the stories written here are mal beginning of the research phase. ion are strong, depends highly on the taken from the great book “Travel- Although the signal was quite strong, details of the actual equation­-of-­state ing at the speed of thought” of Daniel it took the LIGO team five months to governing the behavior of neutron­-star Kenefick [8]. Daniel was a very gentle ensure that the detector output was material. A greek young scientist, Kat- graduate colleague of mine with whom clean and no other physical explana- erina Hatziioannou, is one of the lead- I had the chance to spend 4 years being tion could justify this characteristic ers in the quest of putting constraints both members of Kip Thorne’s group. chirping signal, lasting a few tenths of in the equation­-of-­state of neutron I would like to thank him for offering a second. As is well known, this great stars, based on the tidal deformabili- me his book and made me feel that we event drew international publicity, and ty of neutron stars which is encoded in were both small parts of this great ad- marked the commencement of a new the gravitational wave signals from neu- venture. era, where scientists were equipped tron star–neutron star mergers [7].

References

1. L. D. Landau and E. M. Lifshitz, Classical 3. J. H. Taylor and J. M. Weisberg ApJ, 253 by A. Eddington. Theory of Fields Addison­-Wesley, Cam- 908 (1982). 6. LIGO is an acronym for Laser Interfer- bridge, Mass. (1951). 4. J. M. Weisberg and Y. Huang 2016 ApJ ometric Gravitational­-Wave Observa- 2. The strain of gravitational waves mea- 829, 55 (2016). tory. sures the relative change in the distance 5. A. Einstein Koniglich Preussische Akad- 7. K. Hatziioannou, GRG 52 109 (2020). between objects flying in space when a emie der Wissenschaften Zu Berlin, Sit- 8. D. Kennefick, Traveling at the Speed of gravitational wave hits the line joining zungsberichte, 154 (1918). The corre- Thought Princeton University Press the two objects perpendicularly. sponding formula was later corrected (2007).

HIPPARCHOS | Volume 3, Issue 4 7 A revolution in transient astrophysics by Giorgos Leloudas DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark

t is not an exaggeration to state that tion scheme of Filippenko 1997). These The switch to modern wide-field I transient astrophysics has overgone a are fundamentally different explosions: surveys that target the field rath- revolution during the last decade. As a Type Ia SNe are the thermonuclear ex- er than specific galaxies has proven a matter of fact, even the term transient plosions of white dwarfs in a binary sys- game changer for transient astronomy. astrophysics was not widely used before tem. It is these supernovae that are Thanks to wide-field cameras, several then. So what is transient astrophysics? used as standardizable candles to meas- sky surveys operating in the time do- It is difficult to make a strict definition ure distances across the Universe. The main are able to observe large parts of but, generally speaking, it is the study other supernovae result from the core- the sky and employ image subtraction of astrophysical phenomena of transient collapse of a massive star after it has ex- techniques to find new transients, ir- nature. From this definition we implic- hausted its nuclear fuel and can no long- respective of their association to host itly exclude periodical events (such as er sustain its own gravity. An important galaxies. As a result, the number of variable stars) or objects in the solar point to note is that, except the super- transient discoveries has exploded system (e.g. asteroids, comets). Usually, novae discovered in cosmological sur- since 2000 (Figure 1). The large num- transients are implied to be catastroph- veys searching for high-redshift Type bers have allowed for intrinsically rare ic events, such as supernova explosions Ia supernovae, the majority of super- transients to be uncovered, i.e. phe- or tidal disruptions of stars, although novae were still discovered with the nomena whose rate can be as low as this is not always the case. same method that Fritz Zwicky intro- 1/1000 compared to common Type Ia A few years ago, this field was al- duced in the 1930s : by repeated obser- SNe. At the same time, the galaxy-un- most synonymous with the study of su- vations of galaxies and searching for the biased nature of these surveys has re- pernova explosions. At the turn of the appearance of “new” stars. The prob- vealed a wealth of transient phenom- century, supernovae (SNe) were a con- lem is that this method, which has pro- ena that were previously unknown, solidated field of study that was experi- duced extremely significant results and simply because they tend to occur in encing a rapid increase of interest due which is still widely used by amateur different types of galaxies (or even at to the discovery of the acceleration of astronomers, is biased towards bright large distances from any galaxy!) than the Universe. By that point, almost all and nearby galaxies, which are typically those monitored before. Finally, due to supernovae belonged to well-estab- rather massive and chemically evolved the rolling nature of these surveys, it is lished classes, including both thermo- (metal-rich). Therefore, rare types of now possible to find and characterize nuclear (Type Ia), as well as core-col- explosions, or explosions that favor dif- supernovae within the first days from lapse supernovae (Types II, Ib, Ic) and ferent kinds of environments, were not explosion, allowing for novel science to their subtypes (see e.g. the classifica- probed by this method. be conducted.

Figure 1: The evolution of supernova discoveries through the years with a few key events highlighted. The number of confirmed supernovae has increased dramatically in the last few years (notice the y axis is logarithmic). The acronyms and arrows on the right hand side stand for different transient surveys and their years of operation (Credit: Miika Pursiainen).

HIPPARCHOS | Volume 3, Issue 4 8 as SLSNe I, in analogy to SNe I where “I” signifies a lack of hydrogen in their spectra. The discovery of these events sparked a huge interest as it quickly be- came apparent that the standard para- digm of Fe-core collapse cannot explain these events: their light curves cannot be powered by radioactive Ni56 as or- dinary supernovae. For this reason, al- ternative models have been proposed, such as the powering by the spin down of a newly-born magnetar, or the colli- sions of H-free massive shells expelled by the progenitor star, possibly via the pulsational pair-instability mechanism (for a review on the possible luminosity sources, see Moriya et al. 2018). During the last decade, significant observational progress was achieved in the field of SLSNe I (see Gal-Yam 2019 for a review). A first peak (a “bump”) was observed before the main light curve maximum and it was later shown that such bumps are very common in SLSNe. It has been possible to perform studies of samples and show that the Figure 2: Transient phase space with typical transient luminosity (absolute magnitude) on the y ejected masses are large, spanning a axis and characteristic evolution timescale on the x axis. Different types of transients occupy differ- range of 3-30 M☉ (solar masses) point- ent regions of the phase space, e.g. the brightest events appear on the top of the graph, the faster ing to massive progenitors, although events to the left, etc. The acronyms for different classes of transients can be found in the main suspicions exist that there could be text. Individual unique events are marked with a star. two different sub-classes within SLSNe I (“fast” and “slow”-evolving events). In this review, I will introduce and space diagram as they are significant- Mounting evidence for the presence of give a brief summary of phenomena ly brighter (by a factor of 10-100) than circumstellar shells around SLSNe I has been inferred either by late-time inter- that were discovered during this “tran- typical core-collapse or thermonuclear action or indirectly by resonant echoes. sient revolution” and that go beyond SNe. Up to a few years ago, the fidu- Intriguingly, it has been suggested that the traditional supernova classes as cial limit M = –21 mag was used to sep- SLSNe I obey a brightness - light curve these were known a few years ago. To arate SLSNe from “ordinary” SNe but width relation that can be standardized this end, I will use the concept of the this hard limit is no longer in use, as it (Inserra & Smartt 2014), similar to SNe transient phase space, which character- has been shown that this was just oper- Ia. If true, this means that they can be izes transients depending on their lu- ational but without any physical mean- used as distance indicators to measure minosity and duration (Figure 2). Dif- ing. Exceptionally bright SNe were oc- the expansion of the Universe at much ferent types of transients occupy differ- casionally discovered in the past but it higher redshifts than SNe Ia (the cur- ent regions of this phase space. I will 11 was not until 20 , when Quimby et al. rent record holder is at z = 4; Cooke thus focus on transients that are bright- combined a number of objects discov- et al. 2012). Nevertheless, there is still er, fainter or faster than the main types ered by the Palomar Transient Factory no consensus on what powers SLSNe, of supernovae (Type Ia and core-col- (PTF) with the enigmatic hostless tran- which can be a hurdle in using them as lapse). I will also address transients oc- sient SCP06F6, and determined their tools. curring in the nuclei of their host galax- distances, that it was safely demon- Important clues to the nature of ies (tidal disruptions of stars) and I will strated that a distinct class of SLSNe SLSNe can be derived from the en- highlight a couple of spectacular unique existed showing common properties. vironments they are found in, which events. Finally, I will address a few cur- These objects had unprecedented pre- are very different from those of reg- rent challenges in the field as well as fu- maximum spectra dominated by ab- ular SNe. Several studies have shown ture prospects. sorption lines of O II that later evolved that SLSNe I show a strong preference 9 to SNe Ic (i.e. SNe without any evi- for dwarf galaxies (< 10 M☉), which The bright – Superluminous dence of H or He), they had broad light have low metallicities and are often curves (often bell-shaped), blue colors extremely star-forming and interact- Supernovae and they were found in very faint host ing (Figure 3). One of the most com- Superluminous supernovae (SLSNe) are galaxies (often not detected). These prehensive studies has been produced found on the top of the transient phase objects are now collectively known by the SUSHIES team (Leloudas et al.

HIPPARCHOS | Volume 3, Issue 4 9 last two properties immediately indi- cate that the progenitors of these ex- plosions must be related to an old stel- lar population, making it very unlikely that they can be related to the collapse of a massive star. The favored progeni- tor scenario therefore involves a bina- ry system of white dwarfs which was likely ejected from the host (or a glob- ular cluster) and had sufficient time (many Gyr) to travel to a remote lo- cation. The exact explosion mechanism is however debated although different suggestions have been proposed such as He shell detonations on the surface of the white dwarfs. De et al. (2020) estimate that the true rate of these events is about 15% of regular Type Ia supernovae (which are also thermonu- clear explosions of white dwarfs). Other transients that are found in the gap between novae and superno- vae are the eruptions of Luminous Blue Figure 3: The host galaxy properties for different types of transients can provide important clues Variable (LBV) stars, such as eta Car- for their nature. The host galaxies of typical core-collapse SNe (Type II) are consistent with the inae, a class of intermediate luminosi- main sequence of star forming galaxies in star formation rate versus stellar mass (taken from the ty red transients (ILRTs), which might UltraVISTA survey for z < 0.4). Dashed lines mark equal specific star formation rate. SLSNe I are be related to weak electron-capture typically found in starbursting (dwarf) galaxies pointing to massive progenitors. In the contrary, TDEs occur in more passive galaxies below the main sequence. There are only deep limits for the star supernovae, and luminous red novae formation at the location of the extreme event ASASSN-15lh (modified from Leloudas et al. 2016). (LRNe) that may be associated with the results of stellar mergers and the ejec- tion of a common envelope (see Pas- 2015, Schulze et al. 2018) where it is transients are called “gap transients” as torello & Fraser 2019 for a review of suggested that SLSNe represent the they are found in the luminosity “gap” these classes). Finally, in the faint re- first explosions after a starbursting ep- between supernovae and regular novae gime, we also find the famous kilono- isode, indicating very high progenitor (Figure 2). va AT 2017gfo, which was associated masses. Particularly interesting is a class of with the source of gravitational waves Except the mysterious SLSNe I, transients that have been termed “Ca- GW170817 (Abott et al. 2017). A kil- there also exist their H-rich siblings: rich”, due to the strong Ca lines in their onova occurs when two neutron stars SLSNe II. These have spectra domi- nebular spectra. These objects have merge and produce radioactive materi- nated by narrow Balmer lines indicat- typical luminosities of M = –16 mag, al of heavy isotopes. Due to the small ing that they are powered by circum- short rise times of 10 days, and spectra ejected masses (Mej ~ 0.04 M☉) and stellar interaction with H-rich circum- reminiscent to those of Type Ib/Ic su- high opacities the resulting transient is stellar material. It is therefore consid- pernovae around peak. They enter the faint (the name kilonova actually means ered that they are “just” the high-lumi- nebular phase (i.e. their ejecta become that it is 1000 times brighter than a no- nosity tail of their low-luminosity cous- optically thin) unusually early (typical- va!) and very red. Neutron star merg- ins, SNe IIn. For this reason, they have ly at 30 days versus > 100 days for nor- ers and kilonovae might be responsible perhaps attracted less attention than mal supernovae). The first object to for the production of the heaviest el- SLSNe I. However, future systematic be noticed with these unusual proper- ements in the Universe, including gold studies might shed more light to these ties was SN 2005E (Perets et al. 2010), (although the presence of gold has not exciting phenomena. but it quickly became apparent that this been confirmed in AT 2017gfo). is a distinct class with more members The faint (Kasliwal et al. 2012). What is especial- The fast ly striking about these transients is the – a diverse class of objects – Rapidly evolving transients large offsets that they are found from We usually classify as “faint”, transients their host galaxies (often > 20-30 kpc)! The majority of transients in the tran- less luminous than M = –16 mag. These In fact, these offsets sometimes make it sient phase space evolve with charac- include both the faint tail of the ordi- hard to even associate them with a spe- teristic time scales > 2 weeks (Figure nary core-collapse supernova distribu- cific galaxy as they can appear “host- 2). For example, the rise time of Type tion, especially of Type II, but also phe- less”. At the same time, for most Ca- Ia SNe is 15-20 days, while SNe II have nomena that can be of fundamental- rich transients, there is no evidence for characteristic plateaus of 100 days. In ly different nature. Sometimes, these any underlying star formation. These fact, most searches in the past were

HIPPARCHOS | Volume 3, Issue 4 10 tailored to discover such supernovae cooling emission from extended stel- Tidal disruptions of stars (by employing e.g. a typical cadence of lar envelopes. Perhaps the most fa- Tidal disruption events (TDEs) mark 4 days), while faster events could be mous RET is AT 2018cow (also known the gravitational disruption of stars easily missed. In addition, even if such a as “the cow”!) as it is the most near- that pass close enough to super-mas- fast transient was discovered, an equal- by RET that has been discovered (z = sive black holes and get torn apart by ly fast reaction was required to follow 0.014) and it has been followed exten- their gravitational field. Half of the mass it up (e.g. by means of spectroscopy) sively, producing a number of studies of the disrupted star becomes unbound making its study operationally difficult. (e.g. Prentice et al. 2018, Margutti et but the other half is bound in highly ec- Nevertheless, modern surveys now al. 2019). This event had a rise time of centrical orbits and it eventually forms possess the necessary infrastructure 2.5 days and reached a luminosity of an accretion disc around the black hole and motivation to explore this region M = –20 mag, bringing it easily in the where it gets heated before getting ac- of the phase space and it has been pos- SLSN regime, before declining rapidly creted onto the black hole. This event sible to uncover a population of rapidly at a rate of 0.4 mag / day! It was also manifests with a transient burst of radi- evolving transients (RETs). detected in the radio and the X-rays, ation. The existence of TDEs was the- More than 100 of these objects but despite the exquisite quality of da- oretically predicted many years ago have now been discovered in archi- ta, no consensus has been reached on (Rees 1988) and some first candidates val searches by the PS1 survey (Drout its origins with suggestions spanning were discovered later in the X-rays and et al. 2014) and the DES survey (Pur- phenomena as different as a central shorter wavelengths. But it was again siainen et al. 2018). They have typical engine or a tidal disruption by an inter- thanks to the advent of wide-field op- rise times between 1-10 days, decline mediate mass black hole! In the recent tical surveys that this field has gained rates that can be as fast as 0.4 mag / years, a few more transients that could a new momentum, as there is now a fit the RET description have been dis- day (5 times faster than a Type Ia SN), well-established class of optically-dis- covered and studied, in particular by their spectra (when available) are typ- covered TDE candidates (van Velzen the Zwicky Transient Facility (ZTF). A ically featureless, and they have initial et al. 2020), numbering more than 40 study of 12 nearby events (albeit not blue colors that can be modelled by a events discovered after 2010. The loca- the most extreme RETs, all with du- black body and that get redder with tion of these events is consistent with rations > 5 days) found in a systemat- time. For this reason, they are often al- the nucleus of their host galaxy, where ic search, showed that they all evolved so encountered under different names, the a supermassive black hole is nor- spectroscopically to generally ordinary such as Fast Blue Optical Transients mally expected to be, and they show supernova types (Types II, IIb, Ibn, IIn, (FBOTs). In fact, there is no broad con- little temperature (colour) evolution, Ic-BL; Perley et al. 2020). It is still not sensus on how exactly to define a spe- unlike supernovae that normally cool clear, however, whether these nearby cific class of rapid transients, or even down with time. Spectroscopically, op- events belong to the same population if one single class can be defined at all. tical TDEs are dominated by H and/ as the RETs from PS1 and DES. RETs span a very large range of abso- or He broad emission lines at differ- lute luminosities (–15 > M > –22 mag), which means that they are likely a het- erogeneous class, as it would be very difficult for one single mechanism to give rise to this diversity. In any case, under RETs we do not include here a number of other transients discussed Figure 4: Spectra of previously that could fit some of the optical TDEs showing criteria presented above, such as Ca- a continuum of spec- rich transients or kilonovae. Once we tral properties. In blue, correct for the fact that we are in- we show example TDEs trinsically biased against finding these that show lines of He events, it turns out that RETs are rela- II without any evidence for H. In green, we tively rare, but not that rare: their true show TDEs with both rate can be as high as 30% of Type Ib/ He II, H and Bowen flu- Ic supernovae. They are typically found orescence lines (N III, in star-forming galaxies, so likely relat- O III). Finally, the TDEs ed to massive stars, but not as extreme coloured in red are as SLSNe (Wiseman et al. 2020). dominated by Balmer lines, while He is absent A number of physical mechanisms (modified from Lelou- has been proposed for RETs. What das et al. 2019). is certain is that canonical radioac- tive decay of Ni56 cannot explain these events as it cannot reproduce their light curves. Alternative scenaria that have been proposed include shock breakout in a stellar wind or shock-

HIPPARCHOS | Volume 3, Issue 4 11 ent strengths and ratios (Arcavi et al. are dissimilar to anything we have wit- eruption observed in 1954) via the pul- 2014). In addition, it has now become nessed before. Such transients defy a sational pair instability mechanism, but apparent that a significant fraction of categorical classification and puzzle us even this cannot explain all the observ- TDEs show Bowen fluorescence lines with their peculiar nature. Although in- ables and many other alternatives have (Figure 4). trinsically rare, due to the large num- been proposed in the literature. Similar to other accretion-relat- bers of transients found today, it is not ed phenomena (such as X-ray binaries, infrequent to find a few peculiar cas- cataclysmic variables and AGNs) TDEs es. It is not the purpose of this review Challenges are prime laboratories for studying to make an extensive list of all unique black holes at different scales of grav- and peculiar transients. The purpose is and future outlook ity and time. The vast majority of su- rather to show that the Universe does The rapid progress in transient astro- permassive black holes in the Universe not cease to surprise us with exotic physics has demonstrated the richness are invisible to us (with the exception phenomena! I will here highlight two of the transient sky and has revealed a of those in very nearby or active gal- such transients that have attracted sig- wealth of phenomena that await to be axies) and they can only be probed nificant attention. elucidated. Their impact is important when they are momentarily “lit up” by ASASSN-15lh reached an immense in many domains, extending beyond as- a TDE. For this reason, it is imperative luminosity of M = –23.5 mag being at trophysics. Who can argue that the dis- to understand the physics behind TDEs least twice as bright as any supernova covery of the kilonova associated with and their emission mechanisms. De- discovered before. Some initial spec- a gravitational wave was one of the big- spite the progress achieved, there are troscopic similarity with SLSNe I, led gest scientific breakthroughs of the last many open questions. Perhaps the big- Dong et al. (2015) to classify ASAS- decade? But together with opportunity, gest puzzle is the very existence of op- SN-15lh as the brightest SN ever re- come great challenges. The large num- tical TDEs itself: if powered by accre- corded, even if truly extreme models bers of transients discovered today re- tion, then the predicted temperatures would have to be invoked, pushing even quire a change in the way we do tran- of TDEs should peak at 105-106 ° K a n d models for SLSNe to their limit. An al- sient astronomy. the radiation should come out in the ternative explanation (Leloudas et al. One of the biggest challenges pre- X-rays. So how is it possible to even 2016) is that ASASSN-15lh was an ex- sented together with the advent of find TDEs that emit in the optical/UV, treme TDE, consistent with the pas- wide-field cameras is the lack of cor- in most cases without any X-rays at all, sive nature of its host galaxy (Figure responding spectroscopic time. In plain and with typical black-body tempera- 3) and its location exactly in the nucle- words, this means that we do not have tures of 104 ° K? One possible expla- us. The temperature evolution is also the resources to follow-up and study all nation is that the accretion region is more consistent with a TDE. The prob- these transients that we discover. The surrounded by an optically thick phot- lem with this interpretation is that the transient name server (TNS1), a facil- osphere, likely originating in materi- mass of the supermassive black hole of ity that was introduced exactly in or- al from the disrupted star itself, which ASASSN-15lh is too large (> 108 Msun) der to manage and bookkeep transient reprocesses the radiation and re-emits to produce an observable TDE. For so discoveries, reports that in 2020 there it in longer wavelengths (Guillochon et massive black holes, the tidal radius lies were 21664 transients reported by dif- al. 2014). An alternative explanation is inside the event horizon and therefore ferent surveys. Out of these, only 2091 that the observed energy may be lib- no signal from any stellar disruption (i.e. less than 10%) have a spectroscop- erated by collisions between debris should be expected! The solution pro- ic classification! This means that for all streams during the formation of the ac- posed for this is that the black hole is the rest we do not know their nature. cretion disc, i.e. before the actual ac- rapidly spinning which would bring the This trend is only getting worse with cretion (Piran et al. 2015). The discov- event horizon closer to the singulari- time, as the number of transient discov- ery of Bowen fluorescence lines seems ty allowing for the disruption to take eries steadily increases (Figure 1), and to favour the hidden accretion scenario place outside it. At the same time, the is expected to culminate in the near fu- as a strong source of X-ray/EUV pho- assumption that the black hole is spin- ture, after the employment of the Leg- tons able to excite the He II Lyα line is ning rapidly, elegantly explains the en- acy Survey of Space and Time (LSST) at required (Leloudas et al. 2019). ergetics of ASASSN-15lh. the Vera C. Rubin Observatory. LSST Another true “weirdo” was the su- is expected to survey the entire south- pernova iPTF14hls (Arcavi et al 2017). ern sky at a depth of 24 mag, and dis- This was an event that was spectro- cover many hundreds of thousands of Truly unique events scopically identical to normal Type II transients! To fully exploit this poten- Except the “old” traditional tran- supernovae but its light curve had a du- tial, a large amount of telescope follow- sient classes, such as ordinary Type Ia ration of 600 days (versus 100 days), up time would be necessary, with spec- and core-collapse supernovae, which demonstrating at least 5 episodes of troscopy in particular. This would re- number many thousand members, and rebrightening during this time! The ex- quire a large number of dedicated ro- the more recent transient classes, such act nature of this event remains debat- botic telescopes but even in the most as the ones described above, number- ed as no explanation is entirely satis- optimistic scenario this is not going to ing hundreds (e.g. SLSNe, RETs) or tens factory. The most favoured scenario in- suffice. (e.g. Ca-rich, TDEs) of objects, occa- volves the ejection of multiple shells sionally, truly unique events appear that (possibly consistent with an archival 1. https://www.wis-tns.org/

HIPPARCHOS | Volume 3, Issue 4 12 Therefore, together with acceler- will need to be equipped with knowl- est has already progressively shifted to- ating our efforts to optimally exploit edge of advanced analysis tools, such as wards finding very young transients, i.e. already existing as well as developing machine learning. Furthermore, rath- just a few hours from explosion, which new facilities, we will need to reconsid- er than following-up just any superno- allows to constrain important physi- er our methodologies. We are entering va (which is already impossible today) cal parameters of the progenitor stars, the era of big datasets and the analysis it is imperative to perform a strict se- while finding the same transient a few will need to be targeted towards large lection of the objects that are most in- days later is too late. The transient rev- samples of events (even for what are teresting to answer individual research olution is just maturing. Exciting times today considered to be rare transients) questions. For example, a scientist aim- lie ahead! rather than producing studies on indi- ing to study the acceleration of the vidual events. For example, TDEs will Universe with SLSNe at high redshift, be discovered in large numbers and will first need to develop a method to Acknowledgements over a large redshift range and they find these objects and efficiently select can be used to map the formation and them as real needles in the haystack of I thank Miika Pursiainen for providing mass evolution of supermassive black transients. The same is true for many a modified version of Figure 1 from his holes across cosmic time. The analysis other kinds of transients many of which PhD thesis and for providing comments of such large datasets borders compu- present different operational challeng- on the manuscript. ter science and the future astronomers es. For instance, the community inter-

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HIPPARCHOS | Volume 3, Issue 4 13 Predicting Solar Energetic Particle (SEP) events: current status and future perspectives by A. Anastasiadis and A. Papaioannou Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Greece e-mail: [email protected]

onal mass ejections (CMEs) (Reames, I. Abstract: II. Introduction 1999; 2021; Papaioannou et al., 2016; Space Weather effects of Solar En- Solar energetic particle (SEP) events Vlahos et al., 2019). SEP events are clas- ergetic Particle (SEP) events range are recorded as clear enhancements sified as impulsive and gradual, with the from the direct radiation hazard above the background response of de- former being of short duration (≤1 day), that those impose to crews and tectors orbiting within the near-Earth, lower intensity and numerous – presum- equipment primarily in the in- the inner heliosphere and the inter- ably associated to flares; while the latter terplanetary space to ramifica- planetary (IP) space (Reames, 2021). being of longer duration (lasting up to tions within the Earth's magneto- These particle fluxes are consisted of several days), relatively rare and cover- sphere and atmosphere. Hence, protons, electrons and heavier ions ing many orders of magnitude in their SEP events are of particular impor- and their energy spans over many or- achieved intensity – presumably asso- tance for the near future planned ders of magnitude, from a few keV to ciated to CMEs (see e.g. Reames, 1999; manned missions to the Moon and hundreds of MeV extending even to the 2021). The transport of SEPs is governed Mars, as well as, for the un-obsta- GeV range, in some cases (see e.g. An- by the propagating conditions met at the cle daily living. In this review, we astasiadis et al., 2019). SEP events last IP space (Kahler, Burkepile & Reames, present key findings that have been from a few hours to several days and 1999, Kahler and Vourlidas, 2014a, Lar- utilized and/or explored by the sci- can adversely affect space and ground- io and Karlitz, 2014); the presence of entific community over the last de- based systems. seed population (Tylka et al., 2005; De- cades in order to establish predic- Space Weather effects associat- sai et al., 2006) and the interaction of tion schemes of SEP events. We ed with SEP events include communi- multiple CMEs (Kahler and Vourlidas, first discuss how the parameters cation and navigation systems interfer- 2014b). Thereby, complex environment of the parent solar events (i.e. solar ence, failures in spacecraft electronics, and physical processes dominate the or- flares and coronal mass ejections - space power systems damages (Iucci et igin, acceleration, injection and trans- CMEs) are related to the probabil- al., 2005), enhanced radiation risk for port of particles in the IP space, mask- ity of occurrence and critical char- manned space missions (Cucinotta et ing the connection between the proper- acteristics (i.e. peak proton flux, al., 2002), and aircrews in commercial ties of SEP events and their progenitors fluence) of SEP events in the near- flights (see e.g. Schwadron et al., 2017). at or near the Sun. Earth environment and beyond. Based on the new planned manned mis- Moreover, the temporal scale of 1 Next, we review modeling efforts sions like Artemis and their vital role SEP prediction further adds to the dif- of SEP events that are geared to- in today’s modern technological depen- ficulty of the prediction efforts. How- wards operational prediction, par- dent society there is a clear need for ever, based on a few decades of avail- ticularly focusing on transport ef- the development of a scheme which able measurements, linkages between fects and multi-spacecraft obser- will provide advanced warning of SEP parent solar and SEP events have been vations. We, further, explore the events and their expected characteris- revealed and as a consequence several applicability of higher order multi- tics. In particular, it is of paramount im- dependencies have been identified (see variate, machine learning and arti- portance to predict when and where an e.g. Klein and Dalla, 2017). Hence, the ficial intelligence methods and high- SEP event will (or will not) take place, wealth of statistical and modeling stud- light the particular value and limita- what would the maximum peak inten- ies underpins the science related to the tions of such advances. Finally, the sity of this event be and what the du- acceleration of the SEPs and provides most recent current operational ration and ultimately the time profile outstanding scientific understanding approaches in the prediction of SEP of the SEP event will be. All these in- which in turn is required in order to events, together with future chal- formation are difficult to elucidate due assess and predict SEP events. lenges that need to be addressed to the inherent complexity of the sub- by the scientific community are put ject, the variability of SEP events and forth and discussed. the current gap of knowledge. After of more than 50 years of space III. Key dependencies Keywords: Sun: particle emission • observations we can conclude that SEPs It has been established that both fast Sun: solar flares • Sun: coronal mass are accelerated by solar flares and cor- and halo CMEs form favorable con- ejections-CMEs • solar-terrestrial re- ditions for the acceleration of parti- lations • space weather 1. https://www.nasa.gov/specials/artemis/ cles that will result in SEP events since

HIPPARCHOS | Volume 3, Issue 4 14 Figure 1: A schematics of geospace and the inner heliosphere, for context. A solar flare observed by SDO and a CME recorded by LASCO are included in the figure to underline the parent solar events of SEPs. Missions presented span from radial distances very close to the Sun (i.e. the current missions of Solar Orbiter, Parker Solar Probe & BepiColombo) to 1 AU (STEREO) and to L1 watchdogs (SOHO & ACE). A mutli-spacecraft SEP event denotes the lon- gitudinal evolution of particles in the inner heliopshere (adapted and combined from images available by SOHO, ACE, LASCO, SDO, PSP, SolO, STEREO and the Lario et al., 2013 paper). these are a prerequisite for the estab- parent flare (e.g. Belov et al. 2005) dence that distinguishes flares as- lishment of a shock, driven by the cor- pointing to the magnetic connection sociated with SEPs from flares that responding CME event. Hence, a fair- that needs to be established, so that are not. Moreover, another catego- ly reasonable correlation between the particles shall be routed to the ob- ry of flares are also associated to SEP peak of the SEP flux and the speed of server site. In other words, western event: those with progressive spectral the CME has been identified (see e.g. locations on the solar disk have a high- hardening in hard X-rays that usually Kahler 2001; Cane et al., 2010; Papa- er potential to result in an SEP event follow the flare’s impulsive phase by ioannou et al., 2016). This is expect- at (e.g.) Earth, due to the curvature of significant time intervals (Kiplinger, ed since particle acceleration models the Arhemidean spiral IMF (Desai and 1995). These two types of flares over- at CME driven shocks point to the de- Giacalone, 2016). Yet, the legacy of the lap at the upper M-class and lower X- pendence of the acceleration rate on Solar Terrestrial Relations Observato- class flare intensities (Garcia, 1994a). the speed of the shock travelling in the ry (STEREO) mission has shown that Building on this observational evi- upstream region (see Lee et al., 2012; wide-spread SEP events are detected dence Garcia (1994b) demonstrated Kouloumvakos et al., 2019). Nonethe- at locations broadly separated in longi- that the ratio R of the two nominal less, such a dependency in not definite tude (see e.g. Papaioannou et al., 2014; 0.05-0.4 nm and the 0.1-0.8 nm bands since many different factors can affect Richardson et al., 2014) from the site of GOES soft X-rays can be used in the SEP intensity (see e.g. Lario and of the parent flare and particles spread order to compute the flare plasma Kaleritz, 2014, Richardson, Mays and even over almost 360° (Gomez-Herre- temperature and emission measure. Thompson, 2018), including the im- ro et al., 2015). For these multi-space- The work by Garcia was recently up- portance of shock geometry and seed craft SEP events, CME-driven shocks dated for the 1998–2016 time peri- particle populations for shock acceler- were the apparent explanation (Rouil- od in which SEP events were separat- ation (see Tylka et al., 2005), as well lard et al., 2011; Lario et al., 2013), ed based on their achieved peak flux as, the usage of a crude proxy as the pointing to gradual SEPs. Neverthe- at an integral energy of E>10 MeV, by projected speed of the CME which’s less, it has been shown that even im- Kahler and Ling (2018). The authors value varies with respect to the cat- pulsive SEP events are registered over found that flares that led to small SEP alogue employed each time (Richard- wide longitudinal separations (Wie- events (peak flux < 10 pfu) were not son et al., 2015). denbeck et al. 2013). well separated from the flares that In addition, the SEP occurrence Finally, Garcia (1994a) showed led to non-SEPs, in contrast to typi- depends on the heliolongitude of the that there is a temperature depen- cal NOAA SEP events, whereas large

HIPPARCHOS | Volume 3, Issue 4 15 (≥300 pfu) SEP events originating of SEP events. However, the prediction have been proposed by the scientific from flares located in the western so- of SEP events is based on different time community. In particular: lar hemisphere are much better sepa- scales directly driven by the input em- rated from non-SEP flares. ployed. Therefore there are long-term Solar flares and the short term predictions with the Initially (in the 70’s and 80’s), and up un- former termed as forecasts and the lat- til today, flare characteristics were the IV. Predicting SEP events ter as nowcasts (see details and defini- main (and only) contributor to such SEP a. Proxies tions in Anastasiadis et al., 2019). The prediction concepts. This is because majority of the operational concepts flares: (a) were routinely delivered by | cause and effect relations today offer nowcasts, while intensive ef- GOES patrol measurements since 1976 Such (and other) key observational de- forts of the scientific community seek – long before the SOHO era in 1997 pendencies, listed in Section III, have to find a solution to the delivery of ac- that made CME identifications possi- been utilized as indicators/classifiers for curate forecasts, well ahead of time ble in the space era; (b) are available at the development and the implementa- (Falconer et al., 2011). Such efforts, ex- every minute (i.e. 1-min or 5-min) and tion of concepts that offer forewarning ploiting flare or CME characteristics, thus perfectly suit operations and (c) a

Figure 2: The maximum temperature (in MK) versus the maximum SXR peak flux (in W/m2). Flares associated to SEP events are depicted as diamonds whereas flares not associated to SEP events as dots (from Garcia 1994a) (left hand side). The Peak Flux Ratio (R) as a function of maximum SXR peak flux (in W/m2) for flares associated (red dots) and not associated or achieving a peak proton flux < 10 pfu (blue dots) to SEP events (adapted from Kahl- er and Ling, 2018).

Figure 3: Panel on the left hand side: SEPSTER Predicted versus Observed peak fluxes at E=14-24 MeV. Each color and shape corresponds to a different ob- server (i.e. Earth, STA, STB). The plot serves as a contingency table since it shows the false alarms, hits, misses and correct rejections based on a threshold of 0.0001 pfu/MeV which is approximately the instrument’s threshold (from Richardson, Lays and Thompson, 2018). (Panel on the right hand side) The predict- ed SEP spectra for the event on 19/07/2012. Red lines are the actual predicted spectra between 10-130 MeV; the dashed-blue lines are the extrapolations to lower/higher energies. The gray band denotes the one-σ uncertainty associated with the model prediction. The experimental data for comparison include GOES (EPEAD and HEPEAD) as well as the high-energy observations of the PAMELA experiment, marked by empty stars (from Bruno and Richardson, 2021).

HIPPARCHOS | Volume 3, Issue 4 16 wealth of observational evidence (see (flares, CMEs), Posner (2007) demon- processes occur at shock waves, the Section III) pointed to their close con- strated that near relativistic electron shock-drift mechanism at quasi-perpen- nection to the underlying acceleration, fluxes can be successfully used for the dicular shocks and the diffusive shock injection and propagation of SEPs. That nowcasting of 30–50 MeV protons. acceleration (DSA) process at quasi- said, two of the very first nowcasting The Relativistic Electron Alert System parallel and oblique shocks are prima- systems were put forth by the National for Exploration (REleASE) concept is ry candidates together with shock surf- Oceanic Atmospheric Administration based on a matrix that maps the regis- ing acceleration (e.g Vainio & Afanasiev, (NOAA) and the Air Force Research tered electron intensity to the expect- 2018). (3) SEP emission; the emis- Laboratory (AFRL), namely PROTONS ed intensity of the protons and thus sion of particles at the shock depends (Balch, 2008) and the Proton Predic- provides a deterministic nowcasting of on the conditions around the shock; i.e. tion System (PPS) (Kahler et al., 2017). the expected proton flux at each mo- the presence of a turbulent wavy re- Adding to these, concepts that use lag- ment of time. Due to the fact that it gion upstream of the shock; (4) SEP correlation between SXR and parti- only relies on in-situ electron data (and transport; Scattering in the IP medi- cle (differential and/or integral) fluxes not parent solar eruptive events) RE- um plays a critical role in determining near Earth have been proposed under leASE can be used also in the absent of SEP observations. Studies showed ev- the UMASEP scheme which was initial- solar signatures (i.e. backsided flares). idence of particle scattering by Alfvén ly trained for E>10 MeV (Nunez, 2011) waves generated by streaming protons and later extended to higher energies b. Modeling accelerated at shocks, indicating that IP (E>100 MeV and E>500 MeV) (Nunez, | physical mechanisms scattering is sometimes dominated by 2015). A mounting body of work has been de- a dynamic wave spectrum rather than voted to understand the origins, accel- a universal background one (e.g., Tylka CMEs eration, injection, transport, and prop- et al. 2005). Rigidity-dependent scat- Although gradual SEP events have direct erties of SEPs – mainly at 1 AU. These tering of particles has been used in or- Space Weather relevance (see Section constitute primary open issues that der to explain SEP particle intensity I), the fact that: (a) routinely obtained have not yet been resolved. Namely, (1) temporal profiles and spectral variabil- data onboard SOHO by LASCO were SEP origin; Observations of rare so- ity (Desai and Giacalone, 2016). How- made available only since 1997 and (b) lar wind (SW) elements in SEP events ever, it becomes apparent that in or- the telemetry of the data imposes a has provided compelling evidence that der to improve the quality of predicted ~6 hour time delay between data re- CME-driven IP shocks accelerate ma- SEP properties it is essential to intro- ception and data retrieval the usage of terial preferentially out of a suprather- duce physical mechanisms into the pre- CME identifications has not been fully mal pool (Desai et al. 2006). This pool diction chain. comprises contributions from heated explored by the scientific community. c. Multivariate statistical & Nonetheless, recently, simple 2D prob- solar wind, coronal material, and rem- machine learning methods ability functions for the probability of nants of solar transient events, among SEP occurrence based on the width and others. Quantifying the relative con- With the current availability of large the speed of CMEs, as well as, linear tributions of seed population to SEPs datasets that continue to grow (e.g. regressions of the SEP peak flux to the is critical to understanding the event- GOES SXR and particle data; SOHO/ CME speed were derived (Papaioannou to-event variability. (2) SEP acceler- LASCO), and given the complexity of et al., 2018) which were then integrat- ation; different particle acceleration underpinning the relation of the parent ed into the Forecasting Solar Particle Events and Flares (FORSPEF) tool (An- astasiadis et al., 2017). Another simple formula, i.e. SEPSTER (SEP predictions based on STEREO observations), was put forth (i.e. Richardson et al., 2014) showing promise in the prediction of the SEP peak flux at 14-24 MeV, at dif- ferent vantage points within the helio- sphere (i.e. Earth, STEREO-A & B) uti- lizing the CME speed and the connec- tion angle Φ of the observer (Richard- son, Lays and Thompson, 2018). This concept was further expanded in or- der to provide the expected proton peak flux particle spectra ranging from 10-130 MeV at 1 AU (Bruno and Rich- ardson, 2021). Figure 4: Colormap of feature permutation importance scores. The x-axis depicts the eight (8) features that were investigated while the y-axis presents the eight (8) ML models that were applied In-situ particles in the study. The colorbar overlays the achieved permutation importance score for each case (i.e. model & feature). CEM speed, width and SXR Fluence stand out in several of the employed mod- Apart from the signatures of the par- els (from Lavasa et al., 2021) ent solar eruptive events of SEP events

HIPPARCHOS | Volume 3, Issue 4 17 solar events to SEPs and the different applied a Principal Component Analysis ed SEP event nowcasting systems that variables that give rise to their charac- (PCA) for SEP prediction. Most recently, will make use of the state-of-the-art teristics, predictive efforts have focused Lavasa et al., (2021) performed a thor- implementations. This has led, for ex- on higher dimensional order correla- ough analysis and applied a set of dif- ample, to the realization of ensemble tions and have increasingly been turning ferent techniques including logistic re- solutions, which are designed in a way to multivariate statistical and machine gression (LR), support vector machines that incorporate many different mod- learning (ML) methods (see Campore- (SVM), neural networks (NN), random els and set ups, like the novel modular ale, 2019 for a review). Such complex al- forests (RF), decision trees (DTs), ex- Advanced Solar Particle Events Cast- gorithms have the advantage of resolv- tremely randomized trees (XT) and ex- ing System (ASPECS) tool. Specifical- ing problems, which cannot be easily treme gradient boosting (XGB), further ly, the ASPECS tool is a new automat- approached by more traditional math- evaluating the importance of each fea- ed modular advanced warning system ematical and algorithmic tools. For ex- ture employed and thus it’s usage in the of SEP events that couples data-driven ample, time series analysis of SXRs and binary (i.e. yes or no) predictability of concepts and physics-based models. It protons recorded by GOES were used SEPs. provides for the first time the expect- to develop successful decision tree (DT) ed SEP event time profile for a set of models in order to nowcast SEPs at high integral energies (E>10-; >30-; >100-; V. Current status energies (i.e. E>100 MeV) (Boubrahimi >300 MeV) in near real-time mode at et al., 2017). These efforts were preced- SEP prediction poses many challenges 1 AU. In order to do that, ASPECS in- ed by Winter and Ledbetter (2015) who and there is a clear need for integrat- corporates many different models in

Figure 5: A composite output from the ASPECS tool (http://phobos-srv.space.noa.gr/). From top to bottom: the SXR flux (red line) and CMEs identified by CACTus (blue vertical lines) as well as the proton peak flux for different integral energies of interest. The following 3 panels provide the prediction of the SEP time profile scaled at the 50% and 90% confidence level for each energy. The first prediction is delivered at the time of the solar flare occurrence ~50 minutes prior to the arrival of the E>10 MeV particles, then the prediction constantly evolves with time and the last two panels show the obtained SEP time profiles latter during the event.

HIPPARCHOS | Volume 3, Issue 4 18 sequential modules that provide pre- it still constitutes the way forward. SEP propagation from 0.3 to 1 AU. dictions of: (a) the SEP occurrence; (b) Furthermore, enhance modeling ef- This knowledge could be then used the expected proton peak flux at re- forts that quantify the escaping of in order to provide SEP prediction spective energies of interest spanning; ions at the shock on the appropri- across the inner heliosphere. (c) the expected SEP time profile using ate field lines connected to the ob- a combination of simulated time pro- server. The obtained SEP intensity Human space exploration is the new files based on SOLPENCO2 (Aran et time profiles depend on how the frontier and its right around the cor- al., 2006; Pomoell et al., 2015; Aran et magnetic connection of the space- ner. We need to address fundamen- al., 2017) and semi-analytical solutions craft (the observer) is established tal questions about the dominance of based on a modified Weibul fitting pro- with the particle source; on how the Sun to the heliosphere and to be cedure (Kalher and Ling, 2017). One of efficiently protons are accelerated able to predict when and where an SEP the advances of ASPECS is that it pro- and injected by the shock into the event will take place. In doing so, inte- vides the SEP profile time evolution connecting magnetic flux tube(s), grated solutions are the way forward and and thus it directly translates the con- and on how the IMF irregularities currently the scientific community has ditions of the near Earth space into us- modulate this population during the capability to make new and decisive able information during the total dura- its journey in interplanetary space. steps towards this exciting journey! tion of the SEP event. Hence, all these should be further explored. • Exploit data from heritage and Acknowledgements VI. Future perspectives current missions at a range of This work was partially supported by radial distances from the Sun. As noted in Anastasiadis et al., 2019: the European Space Agency (ESA) un- An early milestone in our under- “An integrated system that mimics (dif- der contract 4000120480/17/NL/LF/ standing of the interplanetary ferent energies, thresholds, needs) ter- hh, "Solar Energetic Particle (SEP) Ad- space was achieved back in the 70s restrial weather forecasting is the im- vanced Warning System (SAWS)" when the twin spacecraft Helios 1 mediate future step”. This is especial- and Helios 2 explored the inner http://tromos.space.noa.gr/aspecs/ ly true and urgent. In order to accom- heliosphere from about 0.29 AU to plish this goal the scientific communi- 1 AU. The in situ instruments mea- The present work benefited from dis- ty needs to: sured, among other properties, cussions held at the International Space Science Institute (ISSI, Bern, Switzer- • Perform focused scientific work the solar wind, the heliospheric land) within the frame of the interna- on SEP emission & transport for magnetic field, and energetic par- tional teams: High EneRgy sOlar partI- gradual SEP event that are di- ticles. Given the proximity of the Cle events analysis (HEROIC) rectly Space Weather relevant. Helios spacecraft to the Sun, and The focused transport mathemat- the continuous observations of https://www.issibern.ch/teams/heroic/ ical formalism includes almost all SEPs at Earth, revisiting these da- important particle transport ef- ta and utilizing the recordings at We would further like to thank A. fects, such as particle streaming both heliocentric distances is ide- Vourlidas, M. K. Georgoulis, R. Vainio, along the field line, adiabatic cool- ally suited for stating the expect- A. Aran, B. Heber, M. Laurenza, S. W. ing in the expanding solar wind, ed environmental context for the Kahler, I.G. Richardson and P. Jiggens pitch angle diffusion of particles, current ongoing missions toward for useful discussions. and magnetic focusing in the di- the inner heliosphere (Solar Orbit- verging IMF, although, the latter is er – SolO and Parker Solar Probe typically assumed as a Parker field – PSP) and for the unfolding of the

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The conference will celebrate the life and Hellenic Astronomical Society's Colloquium. Dr. Jason Spyromilio, Tuesday 13 October 2020 1 Ελληνική Αστρονομική Εταιρεία the scientific achievements of Alvio Renzi- 1:01:04 ni, who turned 80 in July 2020. Alvio’s ex- traordinary career spans over nearly 6 de- Hellenic Astronomical Society's Colloquium. Dr. Manos Chatzopoulos, Tuesday 8 December 2 2020 1:10:06 cades of amazing productivity and covers PLAY ALL Ελληνική Αστρονομική Εταιρεία a great diversity of scientific topics, includ- ing stellar evolution, stellar populations in Ομιλίες Hellenic Astronomical Society's Colloquium. Dr. Angelos Vourlidas, Tuesday 10 November 2020 11 videos• Updated 2 days ago 3 Ελληνική Αστρονομική Εταιρεία globular clusters, the Milky Way and near- 57:12 by galaxies, elliptical galaxies, high-redshift star formation, theoretical modeling, ob- Hellenic Astronomical Society's Colloquium. Prof. Dimitra Rigopoulou, Tuesday 12 January servations. 4 2021 1:09:10 Ελληνική Αστρονομική Εταιρεία

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HIPPARCHOS | Volume 3, Issue 4 20 Characterising exoplanetary atmospheres: the legacy of HST/WFC3 by Angelos Tsiaras Department of Physics & Astronomy, UCL, Gower Street, WC1E6BT London, UK e-mail: [email protected]

1. Introduction - Exoplanet • planets with radii below three Earth outside our Solar System, the diversi- demographics radii are the most common planets ty found in masses, radii and orbital pa- in the galaxy and they are clearly rameters indicates that these distant The existence of planets beyond our separated into two populations, the worlds can vary dramatically. The key Solar System is not a new idea. Camp- smaller and the larger ones, usually to understand more about these plan- bell et al. (1988) cautiously claimed to referred as super-Earths and mini- ets is to study the chemistry and the have detected the first planet in orbit Neptunes, respectively (Fulton et dynamics of their atmospheres. around γ Cephei A, while Wolszczan al., 2017). and Frail (1992) reported the discovery 1.1. Gas giants Current and future survey missions, of two planets orbiting the 6.2ms pul- such as GAIA (Perryman et al., 2001), Exoplanets with masses larger than ten sar PSR B1257+12. However, the first TESS (Ricker et al., 2014), and PLATO Earth masses and radii larger than four detection of a planet orbiting a main- (Rauer et al., 2014) are expected to Earth radii are expected to host a sig- sequence star came in 1995. It was find tens of thousands of new planets. nificant amount of hydrogen and helium, 51Pegb, a planet with mass compara- These new populations will improve similarly to Jupiter, Saturn, Uranus and ble to that of Jupiter, orbiting 0.05 AU our understanding of exoplanets and Neptune. Despite their simpler compo- away from its parent star (Mayor and will provide a huge sample of planets sition compared to rocky planets, mass- Queloz, 1995). In 2019, M. Mayor and available for further characterisation. radius diagrams for the giant planets D. Queloz shared half of the Nobel For many planets we can measure (Figure 1) do not put clear constrains on Prize for their revolutionary discovery. the radius, the mass and the temper- their overall internal structure. Today, the most efficient method ature. However, as it can be deduced To be noticed first from the mass- for detecting exoplanets is the transit from studying the planets within our radius diagram is that most of the gas- method. The first transiting exoplanet Solar System, the bulk characteristics eous planets are inflated compared to was discovered at the end of the pre- alone are not enough to infer the na- models based on hydrogen composition vious millennium (Charbonneau et al., ture of a planet. The most distinct ex- (Bodenheimer et al., 2001; Guillot and 2000; Henry et al., 2000), but it was ample is the comparison between Earth Showman, 2002). It has been suggested only after the results of large-scale and Venus, two extremely different en- that this inflation is the result of atmos- surveys from space and ground – e.g. vironments on two planets with almost pheric heating through tidal orbital cir- OGLE (Udalski et al., 2002), HATNet, identical masses and radii. For planets cularisation (Bodenheimer et al., 2001), (Bakos et al., 2002), WASP (Pollacco et al., 2006), CoRoT, Kepler (Borucki et al., 2010), K2 (Howell et al., 2014), TESS (Ricker et al., 2014) – that we fi- nally had a more clear picture of the planetary populations in the Galaxy. From a sample of more than 4000 con- firmed exoplanets, we currently know that: • there are at least as many planets as stars in the galaxy (Cassan et al., 2012), • 1 6.5% of the main-sequence FGK stars have at least one planet be- tween 0.8 and 1.25 Earth radii, or- biting in less than 85 days (Fressin et al., 2013), • there is one Earth-like planet in the habitable zone for every four M type stars (Dressing and Charbon- neau, 2015), and Figure 1: Mass-radius diagram for the gas giants (Fulton et al., 2015).

HIPPARCHOS | Volume 3, Issue 4 21 winds (Guillot and Showman, 2002), or Ohmic dissipation (Batygin and Steven- son, 2010). Concerning the chemical compo- sition of giant planets, due to the high temperatures of the planets detected so far (>1000 K), molecules like H2O, CH4, CO, CO2 and NH3 are expected to be in the gas phase. Determining the pres- ence and the abundances of such mole- cules can answer questions like: can we explain the chemistry of the planetary atmosphere by thermal equilibrium or is non-equilibrium chemistry affecting its composition? (Venot et al., 2012), and which are the metallicity and the C/O ratio of the atmosphere? These funda- mental properties are related to the formation and the evolution of the plan- etary systems that they belong (Matter et al., 2009) hence they provide a win- dow to the past of these systems. Final- ly, another important aspect that needs Figure 2: Mass-radius diagram for all the currently known super-Earths with masses below 20 to be investigated is the dynamics of the Earth masses, known within 20% uncertainty (Lopez-Morales et al., 2016). atmospheres of giant planets. The ther- mal structure, both horizontal and ver- From a formation perspective, Iko- of both giants and super-Earths, and tical, as well as atmospheric escape are ma and Hori (2012) suggested that in c) the thermal structure of the atmos- important pieces of information con- situ accretion of H/He rich atmos- phere probing the atmospheric dynam- cerning the kinetics (Showman et al., pheres without disk migration is the- ics, and heat redistribution. 2010) and the evolution (Lammer et al., oretically possible, but only under very 2003) of these planets. specific conditions, while Ogihara et al. (2015) initial mass of the planet (Lopez 1.2. Super-Earths 2 Transits, Eclipses and Fortney, 2013). Hansen and Zink & Phase Curves The term super-Earth refers to the (2015) also proposed that a super- class of planets with masses between Earth can be the result of atmospheric There is a number of techniques that those of the Earth and Neptune. De- loss from a Neptune-mass gas giant due are currently used to characterise the spite being absent in our Solar System, to tidal heating. Such a mechanism can atmosphere of a planet and extract in- they are the most common planets in be triggered by evolution through secu- formation on its composition and ther- our Galaxy (Fulton et al., 2017). For lar resonances, leading to atmospheric mal structure. All these methods, these planets, mass and radius meas- heating and Roche lobe overflow. Nev- which are described below, aim to sep- urements cannot constrain their bulk ertheless, Lammer et al. (2013) provid- arate the planetary spectroscopic sig- composition (Figure, 2). ed more detailed calculations on at- nal from the stellar one. Apart from the assumption that mospheric escape of hydrogen and heli- 1 super-Earths are scaled-up Earth an- um due to XUV illumination and found 2. . Transit spectroscopy alogues, two other explanations for that if the inflated super-Earths host H/ During a transit event, the stellar and their mass-radius relationship have He envelopes, they will not lose them the planetary projected discs are over- been proposed. The first, suggests that during the remaining of their lifetimes. lapping, and the planet is blocking a they are “ocean-worlds”, containing a An important piece of informa- small portion of the stellar light. As- substantial amount of water (Kuchner, tion concerning the conditions of the suming that the planet is surrounded 2003; Leger et al., 2004), while the sec- two classes of planets discussed above, by an atmosphere, this atmosphere is ond, suggests that they are “mini-Nep- comes from the composition of their also blocking a part of the stellar light. tunes”, having large rocky iron cores atmospheres. More specifically, atmos- However, the amount of light blocked surrounded by a hydrogen/helium enve- pheric studies can provide information by the atmosphere differs at different lope (Adams et al., 2008). Current ob- on: a) the size of the gaseous envelope wavelengths, depending on its compo- servations indicate that planets up to constraining the structure and evolu- sition. If we observe a transit at differ- six Earth masses can be described by tion of super-Earths, and investigating ent wavelengths, we can measure the small atmospheric envelopes on top of the escape rates and hence the surviv- modulations in the apparent planetary a core with an Earth-like composition al of the atmosphere, b) the composi- radius and infer the existence of cer- (Dressing et al., 2015), while more mas- tion of the gaseous envelope inferring tain atoms, ions, molecules, hazes or sive planets must host a larger, volatile- the chemical and photochemical proc- clouds (Tinetti et al., 2007). rich, envelope (Valencia et al., 2013). esses, the formation, and the evolution For an atmospheric envelope of

HIPPARCHOS | Volume 3, Issue 4 22 temperature T, and mean molecular such signatures can imply the presence depth of an eclipse is equal to the total weight μ, the scale height, H, is: of clouds (Tinetti et al., 2007). Infrared light reflected and emitted by the day transit spectroscopy is now the most side of the planet. Hence, by measur- kT H = µg , efficient technique used to character- ing the eclipse depth we can infer the ise exoplanet atmospheres. albedo and the brightness temperature where k is the Boltzmann constant, and of the day-side of the planet, under the g is the gravity of the planet. The total 2.2. Eclipse spectroscopy assumption that it is emitting as a black- height of the atmosphere that affects The light received from a star-planet body (Charbonneau et al., 2005). The the transit depth measurements is ap- system is the sum of three components: emission component can be described proximately equal to five scale heights, a) the stellar light, b) the stellar light re- as follows: hence the total atmospheric absorption flected by the planet (reflected light), Rp 2 B (λ,Tp) – i.e. the maximum additional transit δ(λ) = . and c) the light emitted by the planet (R ) B (λ,T ) depth caused by a completely opaque * * (emitted light). In the infrared, the con- atmosphere is: tribution of the emitted light to the total Infrared eclipse observations in spec- 2RpH troscopic mode measure the plane- A ~ 5 , flux increases with the temperature of R2 tary emission at different depths inside * the parent star, and, for solar type stars, it dominates over the reflected light. the planetary atmosphere. Such meas- where R and R are the radii of the p * While the planet is orbiting around urements allow us to map the vertical planet and the star, respectively. This the star and spinning around its axis on- temperature-pressure profile of the at- quantity is not negligible for atmos- ly one of its hemispheres is facing the mosphere, along with its chemical com- pheres of low μ and high T , making hot star (day-side). The other hemisphere position (e.g. Fortney et al., 2005). Jupiters the best possible targets, as will be referred to as the nightside. The their atmospheres consist mainly of H/ reflected light comes only from the day- 2.3. Phase Curve spectroscopy He and have a μ close to 2.3 amu. On side and depends on the albedo of the Transits and eclipses are happening at the other hand, super-Earths are more planetary atmosphere. The emitted light very specific times during the planet’s challenging, firstly, because we do not comes from both sides and depends on orbit. However, during the orbit differ- know whether their atmospheres have the temperature and the composition of ent parts of the planetary sphere are a substantial amount of H/He or they the planetary atmosphere. All the above projected towards the Earth (phases). are heavier, and, secondly, because they parameters – albedo, temperature, and As explained above, the reflection and are smaller in size. composition – can deviate from being emission from the planetary sphere are Transit spectroscopy at UV wave- uniform across the planetary sphere. not uniform. Hence, the varying phases lengths can give us information about Especially for close-in planets, which are of the planet are causing modulations the upper atmosphere of a planet, and usually tidally locked, the difference in the light received from the star-planet in particular about hydrodynamic es- temperature and composition between system – known as phase-variations or cape (Vidal-Madjar et al., 2003). The the illuminated and the non-illuminated phase-curves – which can be convert- hot Jupiter HD 209458 b was the first sides can be significant. ed to 2D longitudinal maps (Cowan and planet to be observed at these wave- For transiting planets, the orbital Agol, 2008). The total amplitude of the lengths and found to host a variable at- configuration is such that the planet al- phase-variations originates from the mosphere extending beyond the Ro- so goes behind the star (eclipse). The temperature difference between the che limit, suggesting that it is escaping (Vidal-Madjar et al., 2003). In addition, heavier atoms and ions have been de- tected, suggesting that such elements can be dragged up by the evaporation process (Linsky et al., 2010). Shifting towards longer wave- lengths, optical transit spectroscopy can provide information on the pres- ence of alkali metals in the atmosphere of exoplanets. The absorption lines of Figure 3: sodium and potassium, in particular, Summary are shaped by the temperature-pres- of observational techniques sure profile of the planetary atmos- (Hecht, 2016). phere. In addition, the amplitude of the lines and the structure of the spectrum out of the lines are representative of the high-altitude hazes and clouds that may exist (Seager and Sasselov, 2000). Signatures from molecules such as water, methane, carbon monoxide and dioxide, are more prominent at infra- red wavelengths, while the absence of

HIPPARCHOS | Volume 3, Issue 4 23 dayside and the night-side of the plan- latter is equipped with an HgCdTe de- ing atmospheric characterisation of ex- et, while the time difference between tector, which is actively cooled down oplanets in the near infrared. Below, the maximum of the phase-curve and to 145K and has a quantum efficien- some of the most important studies the eclipse indicates a longitudinal shift cy close to 90% at wavelengths longer with the WFC3 camera are discussed. between the hottest area on the ob- than 0.8 μm, and two grisms suitable served surface of the planet and the for slitless spectroscopy, the G102 (0.8- 3.1. Gas giants substellar point – the most highly illu- 1.15 μm, R=210 at 1.0 μm) and the G141 Since the wavelength range covered by minated area of the planet which is ex- (1.075-1.7 μm, R=130 at 1.4 μm). the WFC3 camera is limited (0.8-1.7 pected to be the hottest one. Both ob- Since 2012, the WFC3 camera has μm), molecular signatures from H2O, servational constrains are providing in- been used in two different observ- CO2, CH4, CO, HCN, NH3, TiO, and formation on the non-uniform distribu- ing modes, the normal (staring) mode, VO are the only that can be detected. tion of temperature in the planetary at- where the telescope pointing is fixed However, water vapour is more abun- mosphere, which is a combined result on the target, and the spatial scanning dant in the atmospheres of exoplanets, of dynamics, irradiation, heat redistri- mode, where the telescope is slewing and it also absorbs much more strong- bution, and radiative cooling. during an exposure, causing the image ly than the other molecules at these Spectroscopic observations of or the spectrum of the target to move wavelengths. Consequently, the major- phase-variations are an invaluable tool on the detector. The spatial scanning ity of results so far can be summarised for probing the atmospheres of exo- technique allows for a larger number of in two main categories: detections and planets as they provide, simultaneous photons to be collected in a single ex- non-detections of water vapour. information on the vertical and hori- posure without the risk of saturation. The first results using the WFC3 zontal thermal structure, as well as on As a result, overheads are reduced, and scanning mode were presented by De- the chemical composition (Stevenson the achieved signal-to-noise ratio (S/N) ming et al. (2013) with the detection of et al., 2014). While such observations is increased. With the implementation water vapour in the atmospheres of the are extremely long – even short orbits the spatial scanning, WFC3 is today the hot Jupiters HD 209458 b and XO-1 b. of exoplanets are of the order of a few most successful instrument in perform- Tens of additional such observations days – the wealth of information pro- vided in spectroscopic observations of phase-curves, makes them an impor- tant objective for future instruments.

3. The legacy of HST/WFC3 The Wide Field Camera 3 (WFC3) is one of the instruments currently on- board the Hubble Space Telescope (HST), and it was installed in 2009 dur- ing the 4th servicing mission of HST. It consists of two independent channels: the ultraviolet/optical channel (UVIS) and the near infrared channel (IR). The Figure 4: WASP-39 b: one of the 30 planets analysed in Tsiaras et al. (2018).

Figure 5: Phase Curve of WASP-43 b as observed by WFC3 (Stevenson et al., 2014).

HIPPARCHOS | Volume 3, Issue 4 24 have been made until today reaching PIST-1 – planets b, c, d, e, f (de Wit et Analysis of WFC3 spectra by Tsiar- more than 50 in total. The largest uni- al., 2018), and g (Wakeford et al., 2019) as et al. (2016) (Figure 6) came to the form catalogue produced to date in- – have not shown any molecular sig- conclusion that the atmosphere of 55 cludes 30 hot planets in which 16 have natures and have excluded the pres- Cancri e is light-weighted with a signif- been found to have statistically signifi- ence of cloud-free, H/He atmospheres icant H/He component. Furthermore, cant atmospheric detections (Tsiaras et around them. no evidence of water vapour has been al., 2018). found while a potentially high C/O ra- From the giant planets observed 3.2.1. 55 Cancri e tion was reported. This detection was so far, interesting cases are those of At a distance of only 12 pc, 55 Cancri the first of an atmosphere around a su- WASP-121 b (Mikal-Evans et al., 2019) is a Sun-like star (G8V type) hosting an per-Earth and its presence is also sup- and KELT-9 b (Changeat and Edwards, extremely interesting planetary system ported by the dynamics necessary to 2021) which are tow of the hottest plan- with five planets, all discovered via ra- explain the temperature difference be- ets observed so far, where metal oxides dial velocity measurements. The fourth tween its day and night side (Kite et al., (TiO, VO) have been detected. More- planet to be discovered, 55 Cancri e 2016). However, other studies were not over, WFC3 has been used to observe (McArthur et al., 2004), was the least able to detect the atmosphere using dif- spectroscopically the benchmark planet massive planet discovered until then, ferent methods (Deibert et al., 2021), WASP-43 b (Stevenson et al., 2014). with a minimum mass of 14.21 Earth leaving the case of 55 Cancri e as an masses, orbiting the parent star at 0.04 open question for future instruments. 3.2. Super-Earths AU every 2.808 days. However, Daw- 3.2.2. K2-18 b The WFC3 camera is the first instru- son and Fabrycky (2010) revised these ment to provide atmospheric measure- values, reporting a period of 0.7365 K2-18 b was discovered in 2015 by ments for planets with masses lower days and a minimum mass of 8.3 Earth the Kepler spacecraft, and it is orbit- than 10 Earth mass. The first attempts masses, giving 55 Cancri e a high chance ing around an M2.5 dwarf star, 34 pc to study the atmospheres of the inflat- (25%) of causing transit events. 55 Can- away from the Earth. Based on the plan- ed super-Earths HD 97658 b (Knutson cri e is an “exotic” example of a super- et’s characteristics, it orbits within the et al., 2014) and GJ 1214 b (Berta et al., Earth as it orbits very close to the host star’s habitable zone, with an effective 2012; Kreidberg et al., 2014) resulted in star and consequently the temperature temperature between 200 K and 320 K. flat spectra, suggesting that their atmo- on its surface is high – i.e. hotter than HST/WFC3 observed eight transits of spheres are either dominated by clouds 2000 K. Transits caused by 55 Can- K2-18 b and multiple studies that analy- or are heavier than expected. Further- cri e were observed from space, with sed those data (Tsiaras et al., 2019; Ben- more, WFC3 has been used to observe the Spitzer Space Telescope (Demory neke et al., 2019; Madhusudhan et al., the atmospheres of the planets in the et al., 2011; Gillon et al., 2012) and the 2020) came to the same conclusion: the TRAPPIST-1 system. Ttransit observa- MOST Space Telescope (Winn et al., atmosphere of K2-18 b hosts water va- tions of six temperate Earth-size plan- 2011; Dragomir et al., 2014), confirm- pour (Figure 7). This result makes K2- ets around the ultra-cool dwarf TRAP- ing its small size. 18 b the first super-Earth with a detect-

Figure 6: WFC3 spectrum of 55 Cancri e and best-fit, compared to ab-initio esti- mates (Tsiaras et al., 2016).

HIPPARCHOS | Volume 3, Issue 4 25 Webb Space Telescope (JWST) and ESA’s M4 mission, Ariel (Tinetti et al., 2018, 2021). In addition, the TESS mis- sion, which is currently in operation, is already delivering and will continue to deliver planets that will be within the reach of these telescopes, as TESS (in contrast to Kepler) is mainly observing nearby, bright, stars. JWST is planned to be launched in October 2021 and one of its key science goals will be to explore the atmospheres of exoplanets. With a six-meter-long mirror, a wavelength coverage (not si- multaneous) between 0.6 and 12 μm (for Figure 7: WFC3 and best-fit models for the three different scenarios (Tsiaras et al., 2019). exoplanet-related observations) and the ability to deliver continuous, long-dura- able atmosphere and with a molecular al., 2020), reconnaissance with Hub- tion observations, JWST will bring a new detection. However, while the water va- ble WFC3 (Edwards et al., 2021) shows window into the atmospheric composi- pour feature has been confirmed, it is modulation in the transit depth over the tion and dynamics for exoplanets. Dur- not possible to constrain the exact wa- 1.1-1.7 μm wavelength range. This mod- ing the lifetime of the mission, JWST will 1 ter abundance (relative to H/He) due to ulation can be explained by the presence be able to observe up to 50 exoplan- 1 the narrow wavelength coverage of the of water vapour, but the S/N provid- ets (Cowan et al., 20 5). JWST will be WFC3 camera. In Tsiaras et al. (2019) ed by HST is not enough to come to a the first instrument to provide detailed thee possible scenarios are identified: strong conclusion (2.6σ). However, this transmission, emission and phase spec- tra of both gas giants and super-Earths, • A secondary atmosphere with a this the first rocky habitable-zone plan- due to the increase S/N. With such da- mean molecular weight explained et for which there is a hint towards the ta it will be possible to put better con- by water vapour (up to 50%) addi- presence of water vapour and will be a straint on the planetary C/O ratios and tionally to H/He. prime target for future instruments. metallicities, to construct 3D chemical • A secondary atmosphere with trac- 11 and dynamical maps of the atmospheres, es of water vapour additionally to 3.2.4 GJ 32 b study the nature of clouds and hazes and one or multiple undetectable ab- Based on WFC3 observation, Swain probe the molecular composition of sorbers (e.g. N2) and H/He. et al. (2021) reported the detection habitable-zone planets. of a secondary atmosphere around an • A cloudy primary atmosphere com- Further in the future, Ariel is planned posed mainly by H/He and traces of Earth-sized planet, GJ 1132 b, which has to be launched in 2028/9 and will be the water vapour. a mass of 1.66 Earth masses and a ra- first space telescope dedicated to the dius of 1.16 Earth radii, making it a ter- Most importantly, the thickness of the characterisation of exoplanetary atmo- restrial planet (Bonfils et al., 2018). The atmosphere cannot be inferred from spheres. With a one-meter-long mir- transmission spectrum suggested the the WFC3 observations. This informa- ror and simultaneous wavelength cov- presence of aerosols, HCN and CH tion is critical to constrain the bulk na- 4 erage between 0.5 and 7.8 μm, Ariel in a low mean molecular weight atmo- ture of the planet – i.e., whether K2-18 will observe 1000 exoplanets. The ob- sphere, inferring the presence of hydro- b is an Ocean planet with a liquid surface servations of Ariel will be organised as gen. Since GJ 1132 b has most likely lost or there is a thick H/He atmosphere. a survey, in 4 Tiers: all the planets will his primordial H/He envelope through However, the updated mass and radius be studied at low resolution to exam- photoevaporation the study suggests of the planet Benneke et al. (2019) place ine the detectability of their atmosphere that the atmospheric signal can be ex- the planet in the category of the larger (to check if there is an extended envel- plained by mantle out-gassing. On the low-mass planets, known as “mini-Nep- op or if there are clouds covering the contrary, Mugnai et al. (2021) analysed tunes”, reducing substantially the possi- molecular signal), from them about half the same dataset, finding no evidence bilities of a surface being present. will be further monitored to extract for the presence of an atmosphere. their molecular composition while for 3.2.3 LHS 1140 b a smaller sample (~50) further observa- With a radius of 1.7 Earth radii and 4. James Webb tions will aim at higher precision chemis- –3 a density of 7.5 gcm , LLHS 1140 b is Space Telescope and Ariel try and variability. Finally, there will also likely to be a rocky world (Ment et al., be a special group of planets to be mon- 2019) and, with an equilibrium temper- Despite the large number of first dis- itored throughout their full orbit (phase ature of –235 K, it is within the conser- coveries, HST is limited as far as the curves). With the large numbers provid- vative habitable-zone of its star. While wavelength coverage is concerned. In ed by Ariel, it will give us the opportuni- recent ground-based observations were the near future, new space facilities will ty to study the planets as celestial bod- not precise enough to constrain atmo- be able to deliver data of much better ies from a population perspective for spheric scenarios (Diamond-Lowe et quality. These facilities are the James the first time in history.

HIPPARCHOS | Volume 3, Issue 4 26 Figure 8: Performance Comparison between HST and JWST (Bean et al., 2018).

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