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INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 37, May 2016

162004_Spatium_37_2016_(001_020).indd 1 19.04.16 07:32 Editorial

A quiet glance at the starry night rushes right through the entire sky may give you the impression . Impressum that nothing is happening out there: the are still at the very Curious to learn more? Okay, let same spot you saw them in your us enter the store of astrophysics to early youth. Their light is flicker- rummage around in what they ISSN 2297-5888 (Print) ing a bit, yes, but you know, it is have on offer! Follow Professor ISSN 2297-590X (Online) the turbulent atmosphere that Thierry Courvoisier from the Uni- causes the illusion. So, business as versity of Geneva and Head of the usual in the heavens! INTEGRAL Science Data Centre SPATIUM in Versoix: he will present you a Published by the Beware! Your conclusion might be rich assortment of , gamma Association Pro ISSI a bit over-hasty. Rather read first ray bursts and the like, and when this Spatium to learn why. You are you feel familiar with all that vio- blessed with living in an amazingly lent stuff, you will gladly lean back, clement corner of the Universe, happy to live in your quiet corner where things remain more or less of the Universe! Association Pro ISSI stable over periods that exceed by Hallerstrasse 6, CH-3012 Bern far an ephemeral human’s life span. We are thankful to Professor Cour- Phone +41 (0)31 631 48 96 The Universe has other measures voisier for the kind support he see of time. Our , for instance, granted in publishing the current www.issibern.ch/pro-issi.html started shining 4.6 billion issue of Spatium, which summa- for the whole Spatium series ago, and it will do so for a further rizes his lecture for the Pro ISSI as- appeasing 5 billion years. But sud- sociation in March 2014. President denly then, your calm corner will Prof. Adrian Jäggi, turn into an apocalyptic scene: our Hansjörg Schlaepfer University of Bern merciful daytime will explode, Brissago, May 2016 shed parts of its matter out to space Layout and Publisher to form a beautiful planetary neb- Dr. Hansjörg Schlaepfer ula similar to those you may know CH-6614 Brissago from the skyrockets on Swiss Na- tional Holiday. That is what phys- Printing ics requires stars to do at the end Stämpf li AG of their lives. CH-3001 Bern

Yet, has much more in stock. Take for example a black hole. It is a kind of a handy star, perhaps a mere 20 km across. Yet, it is so densely packed and hence possesses such a tremendous grav- ity that nothing can escape, not even a flimsy beam of light. The Front Cover hole is there, but you cannot see it. This is an artist’s rendering of Cygnus X-1, a black hole 10,000 light years And every now and then, two such away from . Its tremendous gravitational field pulls matter away monsters collide and produce a rip- from its companion star. As the gas spirals towards the black hole, it heats ple in the fabric of space-time that up and gives off x-rays and gamma rays. (Based on an ESA image)

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162004_Spatium_37_2016_(001_020).indd 2 19.04.16 07:32 The Violent Universe1 by Professor Thierry Courvoisier, University of Geneva and INTEGRAL Science Data Centre, Versoix, Switzerland.

Traditional Earth-bound telescopes country, the INTEGRAL Science Prologue portray the sky in the waveband of Data Centre (ISDC) at Versoix visible light, for which the atmos- plays a pivotal role when it comes phere is transparent. This radiation to exploiting the science data col- The space age was just dawning comes mainly from objects in the lected by INTEGRAL. The sig- when a few scientists at the AS&E2 temperature range of a few thou- nals gathered by the spacecraft ar- conducted an experiment that was sand K showing , stars and rive at Versoix a mere six seconds ­going to change our view of the planets in their eternal quietness. later, where they are then pro- Universe radically. On 12 June In contrast, x-rays and the even cessed, archived and distributed for 1962, they launched an experimen- more energetic gamma rays4 stem the benefit of the international sci- tal sensor on a sounding rocket to from objects heated up to several ence community. probe the Moon’s x-ray radiation. million K and therefore reveal Even though no lunar signal was them in a phase of fervid turmoil. Altogether, high-energy astro- observed, incidental findings This is why high energy astrophys- physics has evolved to become a turned out so revolutionary that ics is so important for understand- staggering and prolific research they earned the programme man- ing the most powerful processes oc- field to which the current issue of ager Riccardo Giacconi3 (Fig. 1) the curring in the Universe and other Spatium intends to guide our in Physics forty years events that are at the origin of in- readers. later. tense non-thermal phenomena.

Unfortunately for astrophysicists, Unsurprisingly, astrophysicists Fig. 1: won the fortunately, however, for all other grasped the potential of this new in 2002 for pio- neering contributions to astrophysics, beings, the Earth’s atmosphere ab- window quickly by commission- which have led to the discovery of cos- sorbs x-rays. Therefore, to probe ing an entire flotilla of spacecraft mic x-ray sources. the x-ray sky, sensors must be placed of ever-increasing sophistication. high enough beyond the atmos- Among the most successful mis- phere. This was known to Giac- sions stands the European Space coni. Yet, he could not know that Agency’s International Gamma- his instrument’s sensitivity was in- Ray Astrophysics Laboratory (IN- sufficient for registering the Moon’s TEGRAL). Launched in 2002, it weak x-ray radiation he was head- continues to gather the most ener- ing for. It was, however, sensitive getic radiation from deep space in enough to stumble across a bright 2016 telling us stories far beyond x-ray source in the of any imagination. It offers scientists Scorpius and mysterious diffuse insights in spectacular cosmic background ­radiation. These dis- events from within our Milky Way coveries opened an entirely new out to the very edge of the observ- window to the Universe to able Universe. On the other hand, astronomy. inside the borders of our small

1 This text is based on a lecture by Prof. Thierry Courvoisier for the Pro ISSI audience in March 2014 as well as on sev- eral of his publications. It was prepared by Dr. Hansjörg Schlaepfer and reviewed by Prof. Courvoisier. 2 American Science and Engineering, Inc., Billerica, Massachusetts, USA, a US manufacturer of x-ray equipment and ­related technologies. 3 Riccardo Giacconi, 1931, Genova, Italy, Italian astrophysicist, Nobel Prize laureate in physics in 2002. 4 Gamma rays represent the most energetic part of the electromagnetic spectrum.

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162004_Spatium_37_2016_(001_020).indd 3 19.04.16 07:32 Introduction The protostar attracts matter from the current orbits of Mercury, Ve- its environment thereby gaining nus, and possibly Earth as well. further mass. Its core gets increas- ingly hotter and denser, due to the What happens next depends on the Virtually all types of mass-rich growing strength of its gravity. At mass of the star. compact cosmic objects are signif- some point, it is hot enough and icant sources of high-energy emis- dense enough for hydrogen to start sion because of the enormous fusing into helium. This nuclear Small Stars strength of their gravitational reaction releases huge amounts of fields. Here, gravity can accelerate energy heating the core further up A small star in the range of 0.05 … particles to extreme velocities, to several million K. This makes 1 equivalents reaches the which then emit x-rays and gamma the protostar become a veritable density and temperature required rays. In order to familiarize our- shining star. to fuse hydrogen to helium. Mod- selves with those compact objects, els suggest that such a dwarf star’s we are now going to sketch the Of the various chapters of a star’s hydrogen stock may last for some processes that make them come biography, hydrogen fusion is by 1012 years, which is considerably into being. far the longest. Its duration de- longer than the actual age of the pends on the star’s size: the larger Universe. This is why the evolu- the mass, the faster it consumes the tion of such small stars is beyond hydrogen supplies. Our Sun for ex- any observational reach. ample is a relatively small star; its Stars are born; stars live, stars die, initial hydrogen stock grants it a much like everything else in lifetime of 10 billion years (of Medium-sized Stars nature. which 4.6 billion are gone). In contrast, a more massive star with A medium-size star holds between Stars are born during the collapse several times the mass of the Sun one to seven solar mass equivalents. of giant nebulae that are large in- has a life expectancy in the order Similar to a small star, its core starts terstellar clouds of dust, hydrogen, of some hundred million years with hydrogen fusing; yet, in con- helium and other ionized gases, only. Anyway, when the hydrogen trast, when the hydrogen stock is (Fig.) 2 . These clouds are really is consumed, the star’s core can no gone, its larger core still has enough huge, they may measure several more balance gravitational attrac- heat and pressure to advance to a light years across. They are not sta- tion and radiation pressure: its in- second mode of . ble in the long run; rather, inter- ner layers collapse thereby squeez- The helium produced in the first nal turbulences cause knots to form ing the core, increasing its pressure fusion step now starts to fuse into which then collapse under their and temperature even more. While carbon. This gives the core a short own gravitational attraction. As the core collapses, the star’s outer reprieve from further collapse. the knots condense, the material at layers expand outward to a size Once the helium inventory is their core heats up giving rise to a never reached before. This is a Red spent, the star contracts, leaving protostar. The cloud as a whole Giant. At this point, our Sun will behind a small, hot and dense ball does not collapse into just one sin- become sufficiently large to engulf called a . The shock gle protostar, but each different knot produces an individual pro- tostar. This is why these nebulae Fig. 2: Star nursery in the Great Nebula in Orion. Also known as M42, it is are often referred to as stellar nurs- one of the most famous nebulae in the sky, about 1,500 light-years away. In the star eries, the places where myriads of forming region’s glowing gas clouds many hot young stars can be seen. Within this well-studied stellar nursery, have identified what appear to be stars are born. ­numerous infant planetary systems. (Credit: Terry Hancock, Down Under Observatory)

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162004_Spatium_37_2016_(001_020).indd 4 19.04.16 07:32 162004_Spatium_37_2016_(001_020).indd 5 19.04.16 07:32 waves from the collapse shed the Once again, the star’s future evo- Most stars, irrespective of their star’s outer layers out to space, lution depends on its initial mass: a size, belong to a . The Milky forming a short, but showy cloud star with seven to twenty solar mass Way, our home galaxy for exam- of ionized gas called a ­Planetary equivalents ends up as a Neu­ tron7 ple, may contain 400 billion stars. Nebula5. This will be the ultimate Star, while stars even more massive This fantastic number leads us to a fate of our Sun five to seven bil- collapse into a Black Hole8. A Neu- further group of high-energy ra- lion years ahead from now. tron Star consists of an extremely diation sources. There is strong dense package of neutrons: it may ­evidence that Black Holes exist at compress an entire star’s mass in a the centres of all, even small gal- Massive Stars volume measuring a mere 10 km axies; some of these Black Holes across. Even more striking is the are embedded in material that is A massive star with more than fact that Neutron Stars may rotate attracted violently by their enor- seven times the mass of the Sun is very rapidly with periods of sec- mous gravity. These central re- fated to end more spectacularly. onds or even fractions thereof. Such gions are called Active Galactic Such a high-mass star goes initially a speedy may become ­Nuclei (AGN). Powered by the ac- through the same two steps as me- a emitting radiation observ- cretion of mass by supermassive dium-sized stars: upon consump- able as short pulses of electromag- Black Holes at the centre of the tion of its hydrogen supply, the netic waves. host galaxy, these AGN emit elec- star’s outer layers swell out into a tromagnetic radiation in many if giant star, but even bigger, form- Every now and then two Neutron not all wavebands. Of those Active ing a Red Supergiant6 while the Stars collide or a new Black Hole Galactic Nuclei, Quasars are the core will shrink becoming very forms. Such crashes produce short, most energetic representatives: one hot and dense. Then, fusion of he- yet extremely energetic flashes, single may be as bright as lium into carbon begins just as in which are probably at the origin of 100 Milky Ways. the case of medium-sized stars. Gamma Ray Bursts. When the supply of helium runs Stellar evolution started early in out, the core contracts again, but If a protostar emerges in the stellar the emerging Universe. Neverthe- now in contrast to medium size nursery close enough to another less, the great era of star formation stars, it becomes hot and dense protostar, the pair may commence is now over. Galaxies evolved and enough to ignite a further series of circling one another becoming a eventually merged to very large nuclear fusion processes gradually system of Binary Stars. In fact, galaxies with huge star formation reaching a state where its core con- more than 50% of the stars in the clouds producing numerous stars. sists mostly of iron nuclei. Universe may have stellar compan- That was some 4 billion years af- ions. In this respect, our lonely Sun ter the . Smaller galaxies is a notable exception9. produced star nurseries as well, but

5 William Herschel (1738–1822) coined the term planetary nebula. When he saw these beautiful objects in his telescopes, they resembled the rounded shapes of planets. Yet, planetary nebulae are not related to planets, rather, they are expand- ing, glowing shells of ionized gas ejected from old Red Giants. 6 Red Supergiants are large stars typically several hundred to over a thousand times the radius of the Sun with relatively cool surfaces (below 4,100 K as compared to the Sun’s surface of 5,780 K). 7 Neutrons are subatomic particles with a specific mass but no electric charge. Together with protons, neutrons constitute the nuclei of atoms. 8 See Spatium no. 28: How Black are Black Holes? By Maurizio Falanga, December 2011. 9 If, during solar system formation, planet had managed to get much more mass, say fifty times more, its core would have started fusing hydrogen like the Sun did and hence would have entered the league of stars and become a stellar companion to our Sun.

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162004_Spatium_37_2016_(001_020).indd 6 19.04.16 07:32 generally later and smaller, peak- distant future, star formation will This is the menagerie of compact ing at about 7 billion years after the cease completely and the only sur- cosmic objects, which we are go- Big Bang. Now, at an age of 13.8 vivors will be the longest-lived ing now to examine in some more billion years, star formation goes stars, the Red Dwarfs. detail. on at a relatively slow pace10. In the

Fig. 3: The lifecycle of a star. The life matter from the environment. The re- nebula while the rest contracts to a of all stars begins in large star-forming sult is a star with its planetary system. White Dwarf (cycle to the left). A more nebulae, shown here in the centre. The Mass then determines the star’s future massive star’s life expectancy is much nebula’s dust and gas eventually collapse fate: if it is below a certain threshold, it shorter; after some hundred million under their own gravitational force will become a Sun-like star, which af- years, it will become a Red Supergiant forming knots of higher matter density. ter some 10 billion years will turn into and eventually explode as a This initiates a runaway process whereby a Red Giant. Upon consumption of its ending up as a Neutron Star or even a the increasing mass leads to higher grav- helium inventory, it will shed part of its Black Hole (cycle to the right). (Credit: itational forces that in turn collect more matter out to space forming a planetary NASA)

10 See Spatium no. 2: Birth, Age and the Future of the Universe by G. A. Tammann, May 1999.

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162004_Spatium_37_2016_(001_020).indd 7 19.04.16 07:32 Cosmic Sources Sun. Neutron Stars are supported the source of a strong flux of against further collapse by the phe- ­neutrinos14 and radiation in the of High-Energy nomenon described by the Pauli13 gamma- and x-ray range. This loss Exclusion Principle. This concept of energy, however, causes the Radiation stipulates that no two neutrons can Neutron Star to cool down rapidly occupy the same place and quan- in the order of a few years. tum, posing the lower limit for the Neutron Stars packaging density. Such a densely Some Neutron Stars rotate very packed object generates of course rapidly, up to several hundred Early astrophysicists such as Wal- an unimaginably strong gravity times per second. This leads to ter Baade11 and Fritz Zwicky12 sug- field: a hypothetical object released an incredible circumferential ve- gested the existence of Neutron one meter above a Neutron Star’s locity of about 10 % of the speed of Stars as early as 1934. In seeking an surface would accelerate so fast that light. Some Neutron Stars also explanation for the phenomenon it would hit the surface at a speed emit beams of electromagnetic ra- of supernovae, they proposed the of some 2,000 km/s! This com- diation as . Neutron Stars theory that ordinary stars would pares with the snail’s pace reached can only be easily detected in cer- evolve into compact objects dur- under the same conditions on tain instances, such as if they are ing supernova explosions consist- Earth of about 4.5 m/s. a Pulsar or a part of a binary sys- tem. Non-rotating and non-ac- creting Neutron Stars are virtually Neutron Stars are the remains of undetectable. collapsed massive stars.

ing of extremely closely packed Neutron Stars are very hot: mod- neutrons. They called these hypo- els suggest initial temperatures in- thetical stars Neutron Stars. side a newly formed Neutron Star Today, we know that Neutron Stars of 1012 K. At this stage, it will be evolve from the collapse of massive stars following a supernova explo- Fig. 4: Chinese astronomers observed an outstandingly bright object in the con- sion. Neutron Stars are the densest stellation of Taurus in 1054, which was visible even during daytime. They called and smallest stars known to exist the newcomer a guest star. After two years, the guest disappeared. Today, we know in the Universe: with a radius in that it was a supernova leaving us the beautiful Crab nebula at a distance of 6,500 light-years. In its centre lies the Crab Pulsar, a Neutron Star 30 km across with the the order of 10 km, they can have incredible spin rate of 30 times per second. It emits pulses of radiation spanning a a mass of about twice that of the spectrum from gamma rays to radio waves. (Credit: NASA, ESA)

11 Wilhelm Heinrich Walter Baade, 1893, Schröttinghausen, Germany – 1960, Göttingen, German and astrophysicist. 12 Fritz Zwicky, 1898, Varna, Bulgaria – 1974, Pasadena, USA, Swiss astronomer. 13 Wolfgang Ernst Pauli, 1900, Vienna – 1958, Zurich, Austrian scientist and Nobel Prize Laureate in Physics 1945. 14 Neutrinos are fundamental particles which are similar to electrons but without an electric charge and an ­extremely small mass.

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162004_Spatium_37_2016_(001_020).indd 8 19.04.16 07:32 162004_Spatium_37_2016_(001_020).indd 9 19.04.16 07:32 Pulsars confirmed the emission from the radio waves. Their signals have the same spot in the sky, which elim- form of short radio waves with The first pulsating radio star (Pul- inated any sort of instrumental ef- very precise intervals in between sar) was found by chance by Joce- fects. Another hypothesis inter- that may last from a few millisec- lyn Bell Burnell15 and Antony preted the signal as coming from a onds to seconds. Appropriate elec- Hewish16 in 1967 when they ob- distant civilization prompting Bell tronics can convert the radio emis- served sequences of strange regu- and Hewish to call it LGM-1, for sions of Pulsars into audible signals lar radio pulses separated by 1.3 s. “Little Green Men”. Yet, when in producing a thrilling sign of life of No one would have expected such a different part of the sky a second an entity billions of light-years signals to come from deep space: pulsating source made its appear- away. Our readers are warmly en- the pulses’ short period eliminated ance, they quickly abandoned the couraged to enjoy some of the lat- most astrophysical sources of radi- LGM hypothesis and continued est Pulsar hits by downloading ation, such as stars. Further, since looking for further explanations … from http://www.radiosky.com/ the pulses followed sidereal time17, rspplsr.html. it could not be a man-made radio Today, Pulsars are interpreted as a frequency interference. Later ob- rare subcategory of Neutron Stars, servations with other telescopes as most Neutron Stars do not emit

Pulsars are rapidly spinning ­Neutron Stars.

Fig. 5: The principle of a Pulsar. A lighthouse models the principle of a pul- sar: the lighthouse to the left emits a nar- row beam of light in a horizontal plane. This light can be seen from afar in the moments when the beam is exactly di- rected ­towards the observer. This gives rise to seemingly short light pulses. The Pulsar to the right emits a radio beam along its magnetic axis, which rotates around its spin axis. From Earth, this signal can be observed as short radio pulses during the mo­ ments that the ra- dio beam directs exactly towards Earth.

15 Susan , 1943, Lurgan, Northern Ireland, Northern Irish astrophysicist. 16 , 1924, Fowey, Cornwall, British radio astronomer, Nobel Prize Laureate in Physics, 1974. 17 Sidereal time is a time scale based on the Earth’s rate of rotation measured relative to the fixed stars.

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162004_Spatium_37_2016_(001_020).indd 10 19.04.16 07:32 Binary Stars plosion does not kick the second interesting case of recycling as it star away, the binary survives. The returns the Neutron Star to a fast The term binary (star) was first Neutron Star will emit electro- spinning mode again. The matter coined by Sir William Herschel18 magnetic radiation powered by its falling on the Neutron Star piles in 1802. He discovered hundreds rotational speed, which thereby up on its surface, and when the of Binary Stars and even multiple decreases. On the other hand, it coating reaches a height of some systems. His outstanding theoreti- may attract and accrete matter 10 metres, it ignites a thermo­ cal and observational work pro- from the second star by its ex- nuclear explosion releasing huge vides the foundation of modern Bi- tremely strong gravity field. If this flashes of x-rays lasting from se­ veral nary Star astronomy. matter falls onto the Neutron Star, seconds up to several minutes. it can spin it up again. This is an When two stars emerge in a star nursery not too far apart, they can form a binary system whereby each Binary Stars are pairs of stars star orbits a common barycentre ­orbiting each other. (Fig. 6). Systems of two, three, four, or even more stars (multiple star systems) have been found. The stars in such multiple systems share a common fate. If they are in the appropriate mass range, their life will end in a supernova explosion. As the more massive star reaches its end earlier, the greater star will blast first, producing a Neutron Star (or a Black Hole). If the ex-

Fig. 6: A supernova in the southerly constellation of Lupus. The expand- ing cloud originates from a stellar ex- plosion in the 1006 AD some 7,000 light-years away producing a cosmic light show across the entire electromag- netic spectrum. This image combines x-ray data in blue, optical data in yel- lowish hues, and radio image data in red. The cloud is now 60 light-years across and constitutes the remains of a White Dwarf. Part of a sys- tem, the dwarf gradually captured ma- terial from its companion star. The build-up in mass finally triggered a thermonuclear explosion that destroyed the dwarf star. (Credit: ESA, NASA, Zolt Levay, STScI)

18 Sir Frederick William Herschel, 1738, Hannover, Germany – 1822, Slough, Berkshire, Britain, German-born British ­astronomer and composer.

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162004_Spatium_37_2016_(001_020).indd 11 19.04.16 07:32 Black Holes a massless wave such as light. This Indeed, the reality of Black Holes changed only when Albert Ein- exceeds any imagination; this in- The concept of Black Holes is as- stein developed his theory of Gen- duced US-American Kip tonishingly old: it was in 1783, eral Relativity in 1915. That gave S. Thorne to hallmark them the when John Michell19, one of the the Black Holes a theoretical justi- brightest objects in the Universe that most brilliant and original scien- fication even though the same Al- emit no light. This lack of light tists of his time and virtually for- bert Einstein rated them a theoret- makes them invisible, and they are gotten today, stipulated the exist- ical oddity rather than a serious not just black, but also tiny, some ence of cosmic objects so massive reality some 25 years later. tens of kilometres across holding that even light could not escape. Still, many solar mass equivalents. Earth, science ignored Mitchell’s dark stars Black Holes are certainly among for instance, squeezed enough to for a long time, since it was not the most alluring entities of which become a Black Hole, would fea- clear how gravity could influence the Universe is so incredibly rich. ture the size of a grape.

Fig. 7: An artist’s interpretation of dous velocities causing it to emit x-rays axis with nearly the speed of light. The Black Hole Cygnus X-1 feeding on the and gamma rays that reveal the presence underlying mechanism remains one of Blue Giant companion star’s ­matter. of the Black Hole at left. In addition, this the great mysteries of ­modern physics. Gravity accelerates the gas to tremen- Black Hole emits jets along its rotation (Credit: NASA/CXC/M. Weiss)

19 John Michell, 1724, Eakring, Nottinghamshire, UK – 1793, Thornhill, Yorkshire, English clergyman and natural philosopher.

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162004_Spatium_37_2016_(001_020).indd 12 19.04.16 07:32 Nevertheless, astronomers would of our galaxy. Indeed, most Black fraction of a second before fully like to get hold of them. This in- Holes in the recent Universe are fusing together, they emit power- spired the Russian physicist Yakov quiescent, because there is simply ful gravitational waves. The merger Zel’dovich20 to suggest an indirect no longer enough matter falling product is again a massive Black way of observation: if a Black Hole into them. It is assumed that some Hole, whose mass, however, is less would orbit a visible companion unknown mechanism links the than the sum of the masses of the star in a binary system, the visible formation of the galaxy to that of individual Black Holes as a signif- star could betray the Black Hole’s its Black Hole and vice versa reg- icant part of their mass is converted presence by the Doppler effect on ulating each other’s growth. into energy in the form of gravita- its radiation due to the periodic The lower size limit of Black Holes tional waves according to Ein- variation of its speed relative to is the object of ongoing scientific stein’s famous law E=mc2. It was Earth. Making a further step, he debates. While there is consensus also who predicted argued that a Black Hole might heat up the surrounding in-falling Black Holes are the remains of matter to extremely high temper- ­collapsed extremely massive stars. atures, which in turn would emit x-rays, see Fig. 7. Armed with this insight, astronomers started look- ing for a binary star with the re- regarding the minimal mass re- the existence of gravitational waves quired properties and found it in- quired for a star to collapse into a in 1915 in the frame of General deed with Cygnus X-1. It consists Black Hole, some hypotheses pre- Relativity. This miraculous vibra- of a Blue Giant along with an in- dict that micro Black Holes could tion of the space-time fabric had to visible companion that is bright in exist as well and could form at en- wait a hundred years for its obser- the x-rays. That was in 1978, and ergy levels available in modern vational confirmation: in August in the meantime, after many more particle accelerators. This prompted 2015, scientists at the LIGO Ob- observations, astronomers rate the concerns about the Large Hadron servatory22 witnessed the first re- probability that the dark compan- Collider (LHC) at CERN21 be- cord of gravitational waves passing ion is really a Black Hole, at a con- cause of fears that it could gener- through our planet. They origi- vincing 95%. ate micro Black Holes with un- nated from the merger of two known consequences for Earth. Black Holes some 1.3 billion years The key feature of Black Holes is The LHC has been operational ago. One of them had 29 times the their mass. They come in a great since 2008 and no micro Black solar mass while the other was even variety of sizes ranging from a few Hole has made an appearance so more massive with 36 solar mass times the solar mass ( far. equivalents. The resulting Black Black Holes) up to millions of so- Hole now has a mass of 62 solar lar masses (supermassive Black Like any celestial body, two Black mass units while, within a mere Holes). The latter are found in the Holes may constitute a binary sys- 0.25 s, three solar masses were con- centre of galaxies, where they can tem orbiting each other. After bil- verted into the energy carried away be very bright (Active Galactic lions of years, these Black Holes by gravitational waves. Nuclei) or dormant as is the case may eventually merge into one sin- with the Black Hole in the centre gle Black Hole. During the final

20 Yakov Borisovich Zel’dovich, 1914, Minsk – 1987, Moscow, Russian physicist. 21 European Organization for Nuclear Research, Meyrin, Switzerland. 22 LIGO stands for the Laser Interferometry Gravitational Wave Observatory. It is a twin system operating in Hanford, Washington and in the 3,000 km distant Livingston, Louisiana, USA.

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162004_Spatium_37_2016_(001_020).indd 13 19.04.16 07:32 Quasars

Quasars are the most distant and most energetic members of a class of objects called Active Galactic Nuclei (AGN), Fig. 8. Hypotheti- cally, they formed approximately 12 billion years ago when the first galaxies collided and their central Black Holes merged to form either a or a bi- nary Black Hole system.

The first Quasar was discovered in the late 1950s as a radio source lacking any corresponding visible counterpart. It was only in 1963 when such a radio source could be collocated with an object discern- ible in the optical waveband: as- tronomers detected a faint blue star at the very location of the radio source. Yet, analysing the blue star’s emission spectrum brought up a great surprise: it contained many so-far unknown broad emis- sion lines. This weird finding de- fied interpretation for many years

Fig. 8: Portraits of Quasar 3C 273. Its light has taken some 2.5 billion years to reach us. Despite this great distance, it is still one of the closest Quasars. Dis- covered in the early 1960s, it was the first Quasar ever identified (top table, Credit: ESA/NASA.). Quasars are the enor- mously violent centres of distant, active galaxies, powered by a huge disc of par- ticles surrounding a supermassive Black Hole. As material from this disc spirals inwards, this Quasar fires off super-fast jets of matter into surrounding space. One of these jets appears as a cloudy streak, measuring a staggering 200,000 light-years in length (bottom table, Credit: R.C. Thomson, IoA, Cam- bridge, UK; C.D. Mackay, IoA, Cam- bridge, UK; A.E. Wright, ATNF, Parkes, Australia)

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162004_Spatium_37_2016_(001_020).indd 14 19.04.16 07:32 and the initial claim of an ex- The of Quasars is vari- of power from a relatively small re- tremely large was not gen- able with time scales ranging from gion requires a power source far erally accepted. Later, it could be hours to months. This in turn more efficient than the nuclear fu- shown by Maarten Schmidt23 that means that Quasars generate and sion process that powers ordinary this spectrum originates indeed emit their energy from an amaz- stars. The release of gravitational from hydrogen redshifted by a ingly small region, more specifi- energy by matter falling towards a breath-taking receding speed of cally a Quasar varying on a time massive Black Hole is the only pro- 47,000 km/s, which, due to the ex- scale of a few weeks cannot be cess known to produce such high pansion of the Universe, betrays an larger than a few light-weeks across. power levels continuously. incredibly great distance. Other The emission of enormous amounts Quasars came up with even higher speed making them objects at the edge of the . Quasars are compact regions in Because of their enormous distance the centre of galaxies surrounding and the finite velocity of light, we see them today as they existed in a supermassive Black Hole. the very early Universe.

Fig. 9: Final greetings from a dead the filaments glows brightly at green in the meantime, the green clouds will Quasar. This image shows green fila- wavelengths. These structures are so far continue to glow for much longer be- ments around the galaxy 2MASX away from the galaxy’s centre that light fore they too will fade away. (Credit: J22014163+1151237 illuminated by a fi- travelled tens of thousands of years to NASA, ESA, W. Keel, University of Al- nal blast of radiation from the Quasar in reach the filaments and light them up. abama, USA) the galaxy’s centre. Ionized oxygen in Even though the Quasar has turned off

23 Maarten Schmidt, 1929, Groningen, Dutch astronomer.

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162004_Spatium_37_2016_(001_020).indd 15 19.04.16 07:32 Gamma Ray Bursts the energy of a Gamma Ray Burst could probably cause a mass ex- produced within our home galaxy tinction event on our planet. Gamma Ray Bursts (GRBs) are extremely energetic short flashes of gamma rays from galaxies bil- lions of light-years away (Fig. 10). Such a flash of a few seconds may Gamma Ray Bursts are flashes contain the same amount of energy of gamma rays from very distant as the Sun releases over its entire 10-billion-year lifetime. GRBs are galaxies. hence the brightest electromag- netic events in the Universe. While the shortest bursts are thought to come from colliding Neutron Stars, the longer bursts (up to 100s seconds) may result from super- Fig. 10: Farther than any known galaxy, the Gamma Ray Burst GRB 090423 nova explosions of the very first recorded in April 2009 had a redshift of 8.2 indicating its occurrence at a time, when the Universe had a mere 4% of its present age. A few minutes after dis­covery, generation of supermassive stars in large ground telescopes registered its faint infrared afterglow (within the circle). the Universe. An exciting possibility is that this GRB happened in one of the very first genera- tions of stars announcing the birth of an early Black Hole. (Credit: Gemini Obser- Gamma Ray Bursts were detected vatory/NSF/AURA, D. Fox & A. Cucchiara (Penn State U.) and E. Berger, Har- vard Univ.) incidentally in 1967 by US VELA intelligence satellites that aimed at revealing secret nuclear weapons tests. During the following years, the science community was busy with putting forward theoretical models to explain GRBs. Yet, they kept their secrets until 1997, when a GRB could be collocated with an object in the x-ray band and then in the optical waveband. This allowed an estimation of their redshift to be made and hence their distance and energy outputs. These observations placed them among the most dis- tant observable galaxies.

On the other hand, such flashes are extremely rare: scientists estimate their frequency at a few flashes per galaxy per million years. ESA’s IN- TEGRAL satellite observing the entire sky comes across GRBs at a rate of about one per day. Fortu- nately, all GRBs observed so far originated beyond the Milky Way:

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162004_Spatium_37_2016_(001_020).indd 16 19.04.16 07:32 Cosmic Background time. The background light is the the major confirmations of the Big relict of the hot Big Bang; it car- Bang theory. We owe the father of x-ray astron- ries the signature of the early his- omy, Riccardo Giacconi, not only tory of the Universe. Due to the Distorted background radi- the discovery of a bright x-ray expansion of the Universe, we see ation: Photons emanating from the source, but also the finding of a it strongly redshifted appearing cosmic microwave background diffuse x-ray component coming now in the microwave region. This may interact with energetic elec- from all parts of the sky. This adds radiation is considered to be one of trons distributed between distant to other types of background radi- ation known in different regions of the electromagnetic spectrum, Cosmic Background radiation is each disclosing the tale of an im- electromagnetic radiation from no portant part of the history of the Universe, Fig. 11. optically discernible source.

X-ray background radiation: The x- ray background results from a combination of many yet unre- Fig. 11: Cosmic Background Radiation spans over the entire electromagnetic solved x-ray sources outside the spectrum. Starting with the green line in the radio waveband, the red line contin- ues in the infrared, visible and ultraviolet regions to reach the x-ray part of the spec- Milky Way. With increasing reso- trum (blue line) discovered by R. Giacconi in 1962. The graph ends with the grey lution of x-ray space telescopes, sci- line in the gamma ray waveband. (Credit: R. Gilli, Bologna Astronomical entists can also collocate the sources Observatory) with objects discernible in the vis- ible range, which are mostly Ac- tive Galactic Nuclei. This radiation is therefore generated by matter falling into supermassive Black Holes at the edge of the observa- ble Universe. The diffuse extraga- lactic x-ray background is hence the sum of individual faint sources.

Microwave background radiation. Discovered unintentionally in 1965 by Arnold Penzias23 and Rob- ert Wilson24 this type of radiation comes from photons produced by the Big Bang 14 billion years ago, that have streamed from an when the Universe became trans- parent to radiation25 for the first

23 Arnold Allan Penzias, 1933, Munich, German physicist and astronomer, Nobel Prize Laureate in Physics, 1978. 24 Robert Woodrow Wilson, 1936, , Texas, US-American physicist, Nobel Prize Laureate in Physics. 25 See Spatium no. 1: Entstehung des Universums by Johannes Geiss, April 1998.

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162004_Spatium_37_2016_(001_020).indd 17 19.04.16 07:32 clusters of galaxies. Thereby, they Infrared/optical background radiation: nuclear fusion reactions taking receive an energetic boost result- In the infrared and optical do- place as stars evolve. Another ing in a slight shift towards shorter mains, the background radiation is source of infrared background ra- wavelengths (blue shift). This is the superposition of the light of diation is Quasars. Their x-ray called the Sunyaev26-Zel’dovich faint galaxies. It summarizes the emission is partly absorbed by the effect, which is currently used to cosmic history of starlight and its dust in the accretion disk and then detect extremely distant clusters of components reprocessed by dust. re-emitted in the infrared. galaxies. This light therefore emanates from

Fig. 12: The Moon in x-rays. The Ger- background, which can be seen outside because of interactions of the solar wind man ROSAT Observatory gathered this the Moon’s disk. Some residual radia- with the Moon’s surface. (Credit: Max- image of the Moon. Interestingly, its tion stems from the Earth’s extended at- -Institut für extraterrestrische dark side is darker than the background, mosphere, which surrounds the orbit- Physik, Garching, Germany) yet not completely dark. Obviously, the ing ROSAT observatory. The bright Moon shields radiation coming from the hemisphere to the right shines in x-rays

26 Rashid Alievich Sunyaev, 1943, Tashkent, Russia, Russian astrophysicist.

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162004_Spatium_37_2016_(001_020).indd 18 19.04.16 07:32 two separate spacecraft flying in Outlook tandem to accommodate the build- ing blocks of the observatory: one will hold the telescope system More than half a century ago, Ric- while some 500 m behind a second cardo Giacconi opened the win- spacecraft will carry the detector dow to the x-ray universe. High- system. energy astrophysics has made tremendous progress since provid- Such complex missions require ing surprising insights into the new technologies, not least the ex- emergence and the evolution of the pertise for precise formation-fly- Universe. Yet, we are only at the ing spacecraft. And above all, a beginning. Many mysteries are still new generation of scientists and unexplained and many questions engineers will take over the helm remain unanswered: How did the to advance our knowledge towards Universe originate and what is it made the mesmerizing realm of the of? These topics establish one of the unknown. four major strategic directions de- fined by the European Space Agen- cy’s Cosmic Vision programme for Recommended further reading: the 2015–2025 period. This long- T. Courvoisier: High Energy Astro- physics, Springer Verlag, Berlin term programme aims at defining Heidelberg. the basic programmatic objectives to which the science community is invited to respond and to allow the industrial partners to prepare the required technological skills.

Space technologies will continue to evolve further and provide novel means to build new space probes with ever-increasing capabilities. Based thereon, scientists can ad- dress new frontiers as for example, observing the earliest structures in Fig. 13: Beauty meets Violence. Zeta the Universe. From here onwards, Ophiuchi, a star about 20 times more the subsequent co-evolution of massive than the Sun some 460 light- years away, produces an arcing interstel- galaxies and super-massive Black lar seen in this infrared por- Holes, and the accretion process of trait. The star rushes at a velocity of 24 matter falling into Black Holes can km/s to the right. The interstellar me- be observed, the powerful source dium compresses and heats its stellar wind causing it to glow in a variety of of the most energetic radiation hues. Zeta Ophiuchi was likely once a reaching us from space. These ob- member of a binary star system, its com- jectives will require new missions panion star was more massive and hence such as for instance a new gamma- shorter lived. When the companion ex- ploded as a supernova, it ejected Zeta ray imaging observatory with an Ophiuchi out of the system. (Credit: incredible focal length calling for NASA, JPL-Caltech)

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The Author

Thierry J.-L. Courvoisier con­ cluded his studies in theoretical physics with a thesis on in 1977 at the Swiss ­Federal Institute of Technology in Zurich (ETHZ) and a Ph. D. the- sis thesis at the University of Zu- rich on the transport of neutrinos in ­supernovae. Then, he moved to the European Space Operations Centre in Darm­stadt as an EXO- SAT Duty Scientist and ­later to the European Southern Observatory in Garching, Germany. From 1988 onwards, he held several positions at the University of Geneva, where he became full professor in astro- physics in 1999. During that pe- riod, he served also as Principal ­Investigator and Director of the INTEGRAL Science Data Centre in Versoix, Switzerland. From July 2009 to June 2010 he was skipper on his sailing boat Cérès for a ­cruise around the Atlantic Ocean together with his wife Barbara.

T. Courvoisier is the author or co-author of more than 400 papers. The advisory career in numerous international scientific boards cul- minated with his Presidency of the Swiss Academy of Sciences from 2012 to 2015 giving him an excel- lent opportunity to foster scienti- fic reasoning in the political deci- sion-making process.

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