PCA ETR:INTRODUCTION FEATURE: SPECIAL

Chandra’s first decade of discovery

Douglas A. Swartza,1, Scott J. Wolkb, and Antonella Fruscioneb aUniversities Space Research Association, National Aeronautics and Space Administration Marshall Space Flight Center, Huntsville, AL 35805; and bHarvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138

Edited by Harvey D. Tananbaum, Smithsonian Astrophysical Observatory, Cambridge, MA, and approved February 4, 2010 (received for review December 16, 2009)

We review some of the many scientific results reported at a symposium held in September 2009 celebrating the 10th anniversary of National Aeronautics and Space Administration’s (NASA’s) Chandra X-ray Observatory. These results were contributed by scientists who were among the more than 300 symposium participants. We highlight those results that most emphasize the unique imaging and spectroscopic capabilities of Chandra.

review | X-rays

fter many years in the making regions of cosmic ray acceleration. Inves- Compact Objects (1), the Chandra X-ray Obser- tigators can also take advantage of the A SNR often contain central compact A vatory, one of the National strong broad emission lines from SNR to objects (CCOs). These are thought to be Aeronautics and Space Admin- perform spatially resolved spectroscopy neutron stars formed in the core collapse istration’s Great Observatories, was de- with Chandra and, thereby, map the dis- supernova explosions of massive stars. ployed by the STS-93 crew of the space tribution and measure the mass of the These stars are often just tiny dots em- shuttle Columbia on July 23, 1999. abundant elements from C to Fe in the bedded in the extensive X-ray glow of their Chandra’sofficial first light image was re- ejecta. As explained in detail by invited accompanying remnants. Unless they corded less than a month later on August speaker Carles Badenes (3), this mapping pulse, or are extremely bright, or radiate 19, 1999. The ensuing decade has wit- helps researchers achieve the ultimate at dissimilar energies, CCOs can be dif- nessed a profound revolution in our un- goals of constraining the supernova pro- ficult to detect in young SNRs. derstanding of the X-ray universe as a genitors (particularly of Type Ia events) direct consequence of the subarcsecond and understanding fundamentals of the Neutron Stars. The CCO in the Cas A SNR imaging capabilities afforded by Chandra’s explosion mechanism (particularly for was first identified in the Chandra first unique focusing optics. The symposium core-collapse events). light image and was one of Chandra’s first Chandra’s First Decade of Discovery was Jae-Joon Lee (4) described observations scientific discoveries (7). The spectrum held in Boston from September 22–25, of G292.0+1.8, the remnant of a core- is consistent with a blackbody but the 2009, to celebrate this revolution. collapse supernovae with a pulsar in its inferred luminosity of the object implies Here we review some of the many sci- midst. The Chandra image reveals spec- aradiusof<0.5 km, suggesting the entific results contributed by the more than tacular substructures including a torus, jet, emission is perhaps from a hot spot on 300 symposium participants. We try to and an extended central compact nebula a larger compact object. Hot spots should highlight those results that most emphasize or pulsar wind nebula associated with the produce pulsations as the star rotates the unique imaging and spectroscopic embedded pulsar. but pulsations have not been detected. capabilities of Chandra. The review is A much younger remnant, in fact the The Cas A CCO is now used as a pro- organized in a manner similar to the topical youngest remnant known in our , was totype of a class of likely neutron stars sessions of the symposium. Each topical recently imaged by using Chandra as with no radio pulsations. It has been session had one invited speaker who reported by Kazimierz Borkowski (5). shown (8) that these could be low mag- reviewed the broader aspects of the topic. Borkowski and his colleagues determined netic field objects with hydrogen atmos- These invited speakers have also made the young age of this remnant by measur- pheres if they had radii of ≈5kmbut written contributions to this volume ing its expansion with time. It was first that would require they be (very small) and are cross-referenced throughout this discovered in radio observations in 1985. quark stars of very low mass. Craig review. When Chandra observed it some 22 years Heinke (9) reported an intriguing alter- Unfortunately, not all of the many ex- later it had expanded by ≈16%, a change native model for the neutron star atmo- cellent contributions can be summarized consistent with an age of only about 140 sphere composed of carbon rather than within this short review. The abstracts years (compared with ≈300 years for Cas H or He. This model fitsthedatawell of all of the contributions along with most A and older ages for the historical SNRs and gives a radius consistent with theo- of the slide presentations and videos of like Tycho and Kepler, whose supernova retical predictions for neutron stars. the oral presentations are available at events were observed as bright “guest Heinke suggests the carbon atmosphere http://cxc.harvard.edu/ChandraDecade/ stars” in the days before telescopes were is a signature of youth in that H and proceedings. invented). Chandra has since observed the He accreted from the remnant can burn remnant again to confirm the expansion to C during the first 1,000 years or so Supernova Remnants and young age. before a thick H layer accumulates at We begin with the subject of Chandra’s first There was also a detailed report on the later times. light image, the supernova remnant (SNR) Chandra monitoring campaign of SN Cassiopeia A (Fig. 1; ref. 2). Galactic 1987A in the Large Magellanic Cloud SNRs like Cas A are rich sources of in- presented by Sangwook Park (6). The SN Author contributions: D.A.S., S.J.W., and A.F. wrote the formation because Chandra can resolve so 1987A remnant has expanded to an an- paper. much of the structure and hence map out gular size of only 1.6 inches during its 22- The authors declare no conflict of interest. forward and reverse shocks, separate year lifetime; Chandra is the only X-ray This article is a PNAS Direct Submission. observatory that can measure radial ex- ejecta from presupernova circumstellar 1To whom correspondence should be addressed. E-mail: and interstellar material, and delineate pansion on such miniscule scales. [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.0914464107 PNAS | April 20, 2010 | vol. 107 | no. 16 | 7127–7134 Downloaded by guest on September 24, 2021 before dispersing on ≈100 Myr timescales. Young clusters in the are favorite targets of X-ray study because of the many high energy processes connect- ing stars and their natal environments. Several investigators reported on surveys of young star clusters at the meeting. The first of these was an interim report on a wide field survey of the Eta Carina re- gion led by Leisa Townsley (15). Carina is a starburst cluster, an example of the large-scale star formation seen in starburst . The Carina complex is com- posed of at least 10 young stellar clusters covering a range of about 0.1–3 Myr in age. Carina contains at least 60 massive O and early-B stars, including an O2 star of initial mass well over 100 M⊙. The lu- minous blue variable η Car and bright diffuse X-rays hint that the Carina com- plex might have seen cavity supernovae. These supernovae are often faint in optical light because they occur within hot tenu- ous bubbles or cavities formed by earlier stellar winds and supernovae. Kenji Ha- maguchi (16, 17) has also confirmed a Fig. 1. Energy-colored photon counts image of the Cassiopeia A supernova remnant observed by neutron star exists in the field, suggesting Chandra. Red (0.5–1.5 keV) green, (1.5–2.5 keV), and blue (4.0–6.0 keV) are individually logarithmically that at least one supernova has already scaled to bring out the fine structure. Red and green are thermal radiation from the ejecta that has occurred in this young star complex. passed through the hot reverse shock. The blue outer filamentary structure is dominated by high-energy The Carina program, which involved synchrotron emission from electrons accelerated at the forward shock. archival data and 20 dedicated pointings, resolved >14,350 point sources, making Compact objects can radiate for periods picture (see ref. 13 for details) that in- it, by far, the largest collection of point of time far in excess of the lifetime of cludes two arcs, inner jets along the di- sources detected for a single Chandra their progenitors—and much longer than rection of proper motion, a faint hook program. Because young stars remain the time it takes for their expanding ahead of the compact PWN, and asym- X-ray bright for a relatively short time, remnants to fade. Thus, compact objects metric diffuse emission. most of the sources are expected to be are found in a wide range of environments, Pavlov reported in the meeting on new premain sequence stars associated with not just within the SNRs in which they observations taken as recently as July the multiple generations of star formation were formed. 2009. The observations show a significant in this region. The evidence for young Isolated neutron stars (INSs) are often change of the compact pulsar wind nebula. stellar ages is that 87% of the X-ray point difficult to identify because they are The system is clearly highly dynamic with sources have counterparts in deep infrared small and hot, making them faint at X-ray-emitting structures varying in images. The detections include 117 mas- other wavelengths. Derek Fox (10) re- brightness on timescales of a week and sive stars, including OB stars, Wolf-Rayet ported on a campaign to discover isolated moving outward at greater than half the stars, and luminous blue variables. neutron stars. The campaign has id- speed of light. Using simulations and other observed entified the eighth known INS and has source properties to ascertain a statistical the potential to double or triple the total Stars and Star Formation sense of the populations, Townsley’s group number of known INSs. SNRs and their CCOs represent, along has developed a Bayesian source classifier. Invited speaker Victoria Kaspi (11) with stellar-mass black holes and white Their main result is that it appears that the discussed how the diverse observational dwarf stars, the X-ray-emitting endpoints stars are clustered in distinct groupings, behavior exhibited by neutron stars can be of stellar evolution. Chandra has also suggesting that the bulk of the star for- unified by a basic physical picture that contributed to profound insights into the mation is occurring sporadically and re- attributes most of the diversity to differ- physics of stellar birth and planet forma- sulting in these smaller clusters. ences in age and magnetic field. tion. These contributions are possible be- There is significant diffuse X-ray emis- cause X-rays penetrate the thick dust that sion from the Carina complex. The pre- Pulsar Wind Nebulae. Kaspi pointed to envelops surrounding protostars and trace cision of the Chandra optics allows the studies of rotation-powered pulsars as one the energetic interactions between stars group to remove the 14,000-plus point of Chandra’s greatest legacies because of and their surroundings. Invited speaker sources and still find structured excess Chandra’s unique ability to resolve the Eric Feigelson (14) presented a thorough emission. In general, this emission is softer complex structures within their pulsar overview of the many exciting phenomena than the expected temperature of stellar wind nebulae (PWN); many of which are revealed by X-ray studies of stars, the winds or coronae. The low temperature is just a few arcseconds across. star-formation process, and the effects evidence that this is indeed real diffuse The Vela pulsar is one of the best studied of X-ray irradiation from stars on their emission and not unresolved faint sources. examples of a PWN. George Pavlov (12) environment. The substructure within the emission shows reported on recent Chandra observations that while the emission is generally con- of this source. The first Chandra observa- Young Star Clusters. Most stars form in sistent with cool thermal plasma, there is tion of the Vela pulsar showed a stunning clusters within giant molecular clouds a narrow band centered near η Carina that

7128 | www.pnas.org/cgi/doi/10.1073/pnas.0914464107 Swartz et al. Downloaded by guest on September 24, 2021 X-radiation when they interact with solar system bodies. Carey Lisse (24) summarized the many X-ray discoveries within our solar system made with Chandra and other X-ray sat- ellites. The discovery of high energy X-ray emission in 1996 from Comet Hyakutake revealed a new class of solar system X-ray emitting object (25). Subsequent de- tections of the morphology, spectra, and time dependence of the X-rays from >20 comets have shown that the very soft (energy < 1 keV) emission is due to a charge–exchange interaction between highly charged solar wind minor ions and the comet’s extended neutral atmosphere (26, 27). Recently it has been demon- strated that the spectra of comets corre- lates well with the ionization state of the solar wind (27). Several other solar system objects are now known to shine in the X-ray, including Venus, Mars, the Moon, the Earth, Jupiter, and Saturn (28). Like comets, the X-ray emission from the Earth’s geo-corona, the Fig. 2. Chandra image of the Perseus cluster of galaxies. Color denotes relative X-ray intensity ranging Jovian aurora, and the Martian halo are from yellow (high) to dark blue (low). Contours denote Very Large Array observations at 1.4 GHz. Contour all driven by charge exchange between levels start at 1 mJy per beam (5-inch beam) and increase by a factor of two to just below the peak of 21.7 fi highly charged minor (heavy) ions in the Jy per beam. At the center is the cluster central radio galaxy NGC 1275. Radio emission lls the X-ray solar wind and gaseous neutral species cavities. Note the steepest part of the radio contours abuts the brightest part of the eastern patch of X-ray ’ fi emission. [Reproduced with permission from ref. 60 (Copyright 2002, John Wiley and Sons).] in the bodies atmosphere. The rst soft X-ray observation of Earth’s aurora by Chandra shows that it is highly variable, is rich in iron. This enrichment could be ever achieved in X-ray astronomy, Chandra and the Jovian aurora is a fascinating attributable to O star winds or to one or also boasts two transmission grating spec- puzzle that is just beginning to yield its more supernovae. trometers with exceptional resolving secrets. The nonauroral X-ray emissions power. The gratings have been used ex- from Jupiter, Saturn, and Earth, and Stellar Jets. Many X-ray jets have been tensively to study emission and absorption those from disks of Mars, Venus, and discovered thanks to the unprecedented lines and edges in sources as diverse as the Moon are mainly produced by scat- fi resolving power of Chandra. The rst bright nearby stars and powerful tering of solar X-rays. However, the convincing evidence of X-ray jets from distant AGN. mechanisms are far from completely un- stars came from the objects HH2 (18) and One of the early highlights of the derstood and there are several ongoing HH154 (19). Although more sources were Chandra mission was the discovey of X-ray projects. One is a study of the emission detected later, HH154 remains unique, emission from accretion of protoplanetary from the comet 17P/2007 Holmes, which being the nearest and the most luminous disk material onto the young T Tauri star experienced an optical outburst of nearly among the stellar jets: The details of the TW Hydra (21). The key evidence for the 10 magnitudes becoming one of the relevant jet morphology can be studied to process was a suppressed forbidden line brightest optical comets in the Chandra a level impossible with more distant ob- that indicated that the X-rays were being era and yet was nearly invisible in X-rays. jects such as AGN. emitted by a high density plasma but other Meanwhile, 8P/2008 Tuttle, observed Rosaria Bonito (20) reported in- explanations were possible. Nancy Brick- near the minimum of the solar cycle ranks vestigating the mechanisms of X-ray house (22) reported on a deep observation among the brightest of the X-ray comets. emission from jets through analysis of of TW Hydra that resolved many X-ray Further, the Jovian system as a whole is multiepoch Chandra data of HH154 emission lines from the source. Brickhouse a puzzle with hard X-rays seen from the spanning four years. used detailed modeling of the accretion poles of Jupiter and with detections of Io, Bonito and colleagues find that the flow (following Cranmer; ref. 23), to match Europa, Ganymede, and the Io plasma X-ray source consists of an unresolved, all of the observed emission line temper- torus. The latter links Io to Jupiter and point-like component with no detectable ature and density diagnostics confirming may provide the anomalous sulfur-oxygen- proper motion and an elongated compo- that one is observing the accretion shock. carbon ratios observed in Jupiter’s polar nent with a proper motion, consistent with X-radiation. a shock moving away from the parent star. Bradford Wargelin (29) and Jonathan The Solar System They have developed numerical models of Slavin (30) each presented studies of jets where X-rays are generated by jet Like most stars, the Sun was also likely X-ray emission generated by charge ex- impacts onto the circumstellar medium. A part of a group or cluster of stars in its change between solar wind ions and in- hydrodynamic model of a randomly ejected youth. The planets and their moons, flowing neutral H and He from the Local pulsed jet reproduces the knotty mor- comets, and other rocks and stones are Interstellar Cloud that surrounds the solar phology observed. the most tangible remains of those for- system. They compared Chandra Deep mative years. The Sun currently emits Field-South observations collected Accretion and High-Resolution Spectroscopy. X-rays, although weakly, as well as a wind during solar maximum and solar min- In addition to the highest spatial resolution of energetic charged particles that cause imum that also sample different lines of

Swartz et al. PNAS | April 20, 2010 | vol. 107 | no. 16 | 7129 Downloaded by guest on September 24, 2021 is part of the later gravothermal oscillation phase; see ref. 32 for details). Tom Maccarone (33) described many of the important reasons for studying the large and diverse population of extraga- lactic globular clusters accessible through Chandra. An important and surprising result of this work is that metal-rich glob- ular clusters are significantly more likely to contain X-ray sources than metal poor clusters. Numerous very luminous X-ray sources have been found in extragalactic clusters, providing observational evidence that black holes (rather than only neutron stars as in the Milky Way) are far more common in globular clusters than was once imagined. Nearby Galaxies X-ray sources have been discovered throughout all nearby galaxies imaged with Chandra. Although some sources are associated with globular clusters, many others are aligned with spiral arms, for instance, or are clustered in the nuclear regions where densities are high and high star formation rates are common. These Fig. 3. Chandra X-ray image of the nearby galaxy Centaurus A. North is up, and east is to the left. A different associations with galactic struc- strong dust lane in the galaxy is viewed nearly edge-on and runs across the middle of the image from southeast to northwest. A supermassive black hole at the center of the galaxy powers the narrow (white) ture imply different types of X-ray sources. jet beaming toward the northeast. Colors depict X-ray energy ranges red (0.5–1.0 keV), green (1.0–1.5 For example, an association with a star- keV), and blue (1.5–2.0 keV). Absorption due to the dust lane creates the blue-green band. Thermal forming region suggests a recently for- emission from hot gas appears red. Nonthermal emission from the jet to the northeast and the shocked med compact object accreting from a short- region surrounding the radio lobe to the southwest appears yellow-green to white. The X-ray jet extends lived massive companion star. Chandra for ≈13,000 light years away from the central black hole. The distance from the AGN to the shock front can usually detect ≈100 of the most lumi- in the southwest is ≈17,600 light years. nous sources in a typical observation of a normal galaxy located less than ≈100 Mpc distant. That is sufficient to perform sight through the solar helium focusing lar objects in globular clusters, X-ray studies of the demographics of these cone (into which the Sun’s gravity focuses studies have historically led to some of populations. However, what is needed is neutral He as it moves relative to the the most important breakthroughs in identification of (optical) counterparts or surrounding medium) with data on solar our understanding of the dynamical pro- at least some details about source envi- wind composition from the ACE satellite. cesses unique to these dense stellar ronments. This is a difficult task because They used this comparison to constrain environments. the 100 or so detected X-ray sources are the level of hot-gas emission from the X-ray sources have long been known embedded in a field of some 1010 stars Local Bubble and to study the solar wind to be orders of magnitude more abun- of all makes and models. Andreas Zezas spatial and temporal variability. These dant per unit mass in globular clusters than (34) reported on such a multiwavelength studies of nearby gas can have important in the rest of the galaxy. Invited speaker study of the nearby M81. implications for the interpretation of dif- David Pooley (31) pointed out that, thanks Similar studies of Local Group galaxies fuse warm and hot gas observed at to the discovery of >1,500 X-ray sources were presented in posters by Vallia An- larger distances. in Chandra observations of >80 galactic toniou (35) (on the SMC), Roy Kilgard globular clusters, it is now clear that the (36) (IC 10), and several by a group Globular Clusters number of X-ray sources in a globular led by Paul Plucinsky (37–40) (on the Most young star clusters are unbound cluster correlates very well with its en- galaxy M33). and will disperse after only one or so counter frequency. This correlation points There is one class of X-ray object found orbits around the Galaxy. However, there to dynamical formation scenarios for the in nearby galaxies, termed the ultra- are hundreds of massive, old, bound X-ray sources and shows them to luminous X-ray sources (ULXs), that has clusters in the Milky Way and in other be excellent tracers of the complicated become a favorite subject of study in the large galaxies, particularly the early type internal dynamics of globular clusters. Chandra era. By definition, their X-ray ellipticals. These globular clusters are The relation between the encounter fre- luminosities, assuming they radiate iso- composed of some 1 million stars and quency and the number of X-ray sources tropically, greatly exceed the Eddington contain hundreds of faint point-like X-ray has been used to suggest that we have luminosity of any known stellar-mass black sources. The clusters are so dense—most misunderstood the dynamical states of hole. Thus, they are of interest because of their stars are contained within a re- globular clusters and that most of them they could be the observational signatures gion only a few parsecs across—that only are in the relatively early core contraction of the elusive intermediate-mass black the high angular resolution of Chandra phase (which is now estimated to last holes that are sought to bridge the gap can resolve all their X-ray emitters. In- several billion years) and only 20% are between single-star remnants of 10–20 M⊙ terestingly, although X-ray sources make in the binary burning phase (with few if known throughout the Local Group gal- up only a tiny fraction of the many stel- any in a deep core collapse state which axies and the ubiquitous supermassive

7130 | www.pnas.org/cgi/doi/10.1073/pnas.0914464107 Swartz et al. Downloaded by guest on September 24, 2021 often show cavities, bubbles, and related structures. These structures are believed to be caused by energetic bursts of matter and light ejected by a central engine— an or AGN—and are strong signatures of feedback. Galaxy Groups Just up the hierarchical ladder from gal- axies are galaxy groups that contain up to hundreds of gravitationally bound gal- axies and are the main reservior of baryons in the universe. As pointed out by Myriam Gitti (49) (and similarly in Fig. 4. Mosaic of Chandra images of the central region of our Milky Way galaxy. The image is 400 × 900 a poster presentation by Jan Vrtilek; ref. light years. It reveals thousands of white dwarf stars, neutron stars, and black holes within diffuse 50), groups are an excellent target for multimillion-degree gas. The supermassive black hole at the center of the galaxy is located inside the studies aimed at a better understanding bright white patch in the center of the image (along with many other sources resolved by Chandra but of the feedback process, the mechanisms not visible in this smoothed image). The colors indicate X-ray energy bands—red (low), green (medium), and timescales of energy injection, and and blue (high). the effects on galaxy and group evolution. Groups are good targets because they have shallow potentials relative to the more > 5 black holes ( 10 M⊙ or so) thought to ferred by theorists because it practically massive galaxy clusters so that the hot gas populate the centers of almost all galaxies. guarantees a near Chandrasekhar mass is distribution is more extended and feed- fi Roberto Soria (41) gave a thorough review reached and, hence, a xed amount of back can operate over a relatively larger of what we have learned about ULXs in supernova fuel resulting in the proverbial area, which helps make the gas inter- the past decade. He pointed out that Type Ia standard candle. Gilfanov showed, actions easily apparent even at large dis- roughly 200 ULX candidates have now however, that the long cycle of classical tances. In particular, Gitti reported strong been discovered. Statistically, they display novae that accompanies the gradual shock structures visible in the surface many of the properties of their lower- buildup through H accretion along with brightness and temperature maps of the luminosity cousins, the X-ray binaries. the usual hard X-ray emission from the Hickson Compact Group 62 observed X-ray spectra and timing initially sug- accretion process itself would leave with Chandra. gested intermediate-mass black holes for a clear residual hot X-ray signature lin- Several other Chandra observations of a few of these sources because of an ap- gering within (primarily massive elliptical) galaxy groups, coupled with radio obser- parent soft spectral component. However, galaxies that is not observed. In fact, vations, were reported. For example, Ewan these spectra are dominated by a power Gilfanov finds only ≈5–10% of Type Ia O’Sullivan’s (51) Chandra and GMRT law component; ULXs do not seem to events can have occurred by this path in study of Stephan’s Quintet shows a ridge exist in the standard thermal-dominated nearby elliptical galaxies. Therefore, of X-ray emission in a region where high/soft state and so the analogy to (and most must be due to rapid merging of a member galaxy is colliding with a fila- extension from) Galactic stellar-mass white dwarf pairs. ment of tidally stripped HI gas. This spiral- black hole candidates must be viewed There is another form of warm/hot dominated system appears to be building carefully. Instead, Soria notes ULXs gas that is predicted to surround massive up its hot gaseous halo by shock-heating seem to be predominantly in a new spec- galaxies and to trace the cosmic web of HI rather than by accreting primordial tral state he referred to as the Ultra- large-scale structure. The search for this material or by stellar winds and super- luminous State (42) where most of the gas through absorption-line spectroscopy novae. Similar investigations include those power comes from an inner disk region and by other means was the main topic reported by Nazirah Jetha (52), Simona that is modified by a warm-thick Comp- discussed by invited speaker Daniel Giacintucci (53), Eric Miller (54), and tonizing corona. Using this model, most Wang (45). Wang also pointed out that Laurence David (55). In some of these ULXs can be explained by a 30–100 solar the current failure to locate this gas com- systems, a central AGN plays a dominant mass compact accretor although a few ponent may be because it is in a hotter role in shaping the hot gas halos. ULXs are found in a hard, possibly sub- galactic halo or has been somehow pushed Eddington, state. Some of these latter away. Both of these arguments must in- Feedback in Galaxy Clusters sources remain good intermediate-mass voke some form of energy feedback on Revealing the critical role of AGN feed- black hole candidates (see also ref. 43). galactic scales. One form of feedback Wang back heating in the cores of clusters of Marat Gilfanov (44) reported the results noted is the cumulative effect from winds galaxies—solving the cooling flow of a kind of indirect population study; and supernovae from many massive stars problem—ranks among modern X-ray a very interesting and novel X-ray look at resulting in a superwind as observed in the astronomy’s greatest contributions to the progenitors of Type Ia supernovae. nearby M82. The X-ray science so far. As reviewed by invited These supernovae are known to result manifestation of less powerful star- speaker Elizabeth Blanton (56), the rela- from the thermonuclear explosion of forming activity is a diffuse component of tively high gas densities at the centers of a white dwarf star near the Chandrasekhar hot gas prevading the disks of spiral clusters of galaxies allow that gas to cool limiting mass. What is not known is galaxies. K. D. Kuntz (46) and Zhiyuan quickly thereby decreasing the gas pres- whether these stars reach such a high mass Li (47) presented studies of such hot gas sure support against gravity resulting in through slow buildup via accretion of components in several nearby galaxies. an inward gas flow. The problem is that (mainly) H from a main sequence com- Christine Jones (48) discussed nuclear large amounts of cooling gas is not ob- panion or catastrophically through the X-ray emission from hot gas-rich (but served either as optical emission from merger of two white dwarfs in a close bi- otherwise normal) elliptical galaxies. new stars or as Fe XVII X-ray line nary system. The former has been pre- The X-ray morphology of these galaxies emission from gas cooling to below ≈107 K

Swartz et al. PNAS | April 20, 2010 | vol. 107 | no. 16 | 7131 Downloaded by guest on September 24, 2021 (57–59). Instead, high-resolution Chandra clusters to constrain w0 to <3% statistical (69), and AEGIS (70) surveys were sum- imaging and broadband X-ray spectro- uncertainty compared with the thousands marized. Paul Green (71) described how photometry have led to detailed surface of clusters needed to do so by using important it is for theories of coevolution brightness and temperature maps that optical observations) due to the fact that of supermassive black holes and galaxies reveal a variety of structures in clusters. X-radiation probes the dominant baryon to know what fraction of galaxies host an These images show AGNs in the centers of content of clusters. Vikhlinin finds struc- actively accreting nucleus—as defined by cool core clusters that inflate bubbles that ture formation is suppressed at redshifts their X-ray luminosity. Combining 323 X- rise buoyantly through the intercluster z < 1, which is a strong signature of the ray fields analyzed by the Chandra Multi- medium and can produce shocks and influence of dark energy. wavelength Project (ChaMP) together sound waves and heat the surrounding gas. Thomas Culverhouse (63) described with Sloan Digital Sky Survey data, Green Unlike the heating due to strong shocks, preliminary results from X-ray ob- derived the AGN fraction in local galaxies the gas surrounding these cavities is often servations combined with new Sunyaev- as a function of absolute optical magni- rather cool. The cavities are radio emit- Zeldovich Array radio interferometer tude, X-ray luminosity, and redshift (z). ters. These radio lobes transport energy measurements of clusters at high redshift He also demonstrated the existence of and matter outward from the central AGN (z > 1). Resulting mass measurements will a correlation between the Eddington ratio and its host galaxy. Fig. 2 (60) shows an be used to compare the properties of the of low luminosity AGN and their X-ray example of the radio lobes and X-ray high redshift sample to scaling relations power-law spectral slope that is remark- cavities in the Perseus galaxy cluster. The derived at lower redshift to test for evi- ably similar to the same relation for stellar patterns of the systems of X-ray cavities dence of cluster evolution. In another mass X-ray binaries (72). suggests the radio outbursts are cyclic, work in progress, Nicole Hasler (64) re- Deeper than ChaMP, the C-COSMOS their repetition timescales can be calcu- ported using a joint X-ray and Sunyaev- survey is nearly 1/10th as sensitive as and lated, and the energy released by the radio Zeldovich model to do cosmology-in- samples a factor-of-10 larger field area sources can be calibrated. In most cases, dependent measurements of cluster gas than the deepest Chandra surveys. This the energy injected into the cluster can fractions in a systematic search to detect combination of depth and area produces counteract cooling losses. gas fraction evolution with redshift. large numbers of sources, allows for Using a sample of nearby galaxy clusters, Adam Mantz (65) reported his studies on good statistics on previously sparse samples Rupal Mittal (61) showed that cool cores constraints on galaxy cluster X-ray scaling (e.g., high z quasars), and enables the in galaxy clusters must develop late and relations by using a large sample of flux- discovery of rare classes of objects (e.g., that the central cooling time of a cluster is selected clusters at moderate redshifts. double AGNs). The depth of the survey the strongest indication of whether it is Mantz argued that the large scatter ob- matches the depth of the many comple- a cool core cluster. Mittal finds that all served in the LX-Mcluster relation is caused mentary imaging bands from radio to strong cool core clusters have a radio by excess heating in the central regions of UV, which enabled > 95% identification source at their center. Her study clearly many clusters. He showed that the scatter rates, spectral energy distributions, and demonstrates the need for a heating can be significantly reduced if the innermost accurate photometric redshifts to be found mechanism and provides statistical and regions of cluster cores are ignored in the rapidly. The wealth of multiwavelength quantitative evidence supporting the AGN analysis, which results in a relation consis- data on many sources already allows heating scenario. tent with self-similar scaling. a variety of science investigations, and There were also several poster pre- Martin Elvis (73) presented the latest re- sentations on aspects of cluster merging, Active Galactic Nuclei sults on coeval growth of SMBH and a process that can also offset cooling in Intimately connected with the growth of host galaxies, on dust-obscured galaxies, cluster cores. As pointed out by John structure are the supermassive black on the logN-logS relation of z > 3 ZuHone, Chandra has discovered many holes (SMBHs) growing in the nuclei of quasars, on off-nuclear ULX objects and “cold fronts” in the hot intracluster me- galaxies. The finding that the mass of on double AGNs. The interaction between dium that appear as sharp discontinuities SMBHs scales with that of their host galaxy the SMBHs and the large scale environ- in the X-ray surface brightness. The spheroids establishes this close link be- ment and, in particular, the cross-correla- temperature is colder on the brighter, tween AGN and star formation. As argued tion between AGNs and clusters, was denser side of the front suggesting the by invited speaker Neil Brandt (66), the explored by Nico Cappelluti (74), who cool gas is sloshing back and forth in the Chandra X-ray bandpass is the ideal win- showed how AGN seem to cluster like cluster potential minimum. ZuHone pre- dow for the detection and study of AGN massive late type galaxies. sented simulations where such sloshing is locally as well as in the high red- A survey of nuclear X-ray activity on initiated by gravitational disturbances shift universe. early type galaxies in the cluster was from passing subclusters and showing illustrated by Elena Gallo (75). These are how sloshing is an additional form of AGN Evolution. Extragalactic X-ray surveys galaxies in which supermassive black holes feedback similar in effect to interactions have immensely improved our general and nuclear star clusters may be equally among galaxies in groups. understanding of how the populations of important. Gallo showed how ≈30% of the active galactic nuclei (which represent the galaxies host an accreting black hole with Galaxy Cluster Cosmology majority of the sources in X-ray surveys) no evidence for an increase in the active Galaxy clusters have long been a favorite evolve across the history of the universe. fraction with host galaxy stellar mass for subject among cosmologists because they Shallow, wide, and deep surveys comple- Eddington-limited samples. are the most massive objects tugging on ment each other by covering different Finally, scientific results from the AE- the fabric of the universe. Invited speaker regions in the Flux vs. Survey Area dia- GIS survey, with an aim to study co- Alexey Vikhlinin (62) explained how gram. Brandt (66) highlighted the spec- evolution of black holes and host galaxies, Chandra observations of clusters place tacular results from several surveys were presented by Elisa Laird (76) for useful and tight constraints on the dark conducted by Chandra and showed some the AEGIS team. Multiwavelength data energy equation of state and on cosmo- of the key insights gained from AGN de- are suggesting that typical AGNs at z ∼ 1 logical density fluctuations. These meas- mography and physics. In other con- are in massive red host galaxies, that urements can be done efficiently with tributions, specific results from the AGN hosts are bulge-dominated, and are clusters (Vikhlinin used only ≈90 X-ray ChaMP (67), C-COSMOS (68), AMUSE found predominantly in dense

7132 | www.pnas.org/cgi/doi/10.1073/pnas.0914464107 Swartz et al. Downloaded by guest on September 24, 2021 environments. Several posters highlighted SMBH accreting at or near its Eddington 3; ref. 82). Nulsen showed how the X-ray results from other surveys or illustrated limit, is an ideal laboratory to examine emission surrounding the radio lobe is specific studies from groups of objects in the role of AGN outflows and feedback. predominantly synchrotron and similarly the surveys mentioned above. In particu- Multiwavelength imaging of this nearby for the radio jet, and how a moderate or lar, the poster of Heng Hao (77) was the AGN shows that the radio jet, [OIII] transient asymmetry in the environment cowinner of the best conference poster emission and X-ray emission are spatially can make the difference between a jet by a student. She studies the evolution of related. Spatially resolved, high-resolution and a lobe. the Spectral Energy Distribution shapes Chandra high-energy transmission grating High-energy transmission grating near 1 μm for Type 1 AGNs in the COS- spectra point to an entirely photoionized observations of the X-ray binary Cyg X-1 MOS survey as a function of luminosity, narrow line region (NLR), with no in- presented by Manfred Hanke (83) in- redshift and Eddington ratio. dication of collisional ionization dicated clearly how high-resolution X-ray spectroscopy, combined with timing in- fl from the jet, which means that the AGN Jets and Out ows. AGN are interesting radiation field dominates the energetics. formation and multisatellite campaigns, is objects of study in their own right. They Outflows are detected along the NLR crucial in understanding the behavior of display a range of phenomena that reveals ionization cone, with velocities > 500 this binary system and, in particular, for their complex inner workings. The superb − km·s 1 extending up to several kpc from the study of the very focused wind out- spatial and high spectral resolution of the nucleus. flowing from the Roche lobe. The wind’s Chandra have allowed the detection and The disk-jet connection was discussed study of resolved X-ray jets and are giving photoionization structure in Cyg X-1 is by Joseph Neilsen (80) based on high- the first indications of the composition and constrained by X-ray measurements, and dynamics of winds and outflows in a variety resolution grating observations of the mi- the wind absorption needs to be taken into of astronomical systems. Invited speaker croquasar stellar-mass black hole, GRS account in soft X-ray model spectra for Dan Schwartz (78) described how Chandra 1915 + 105. Chandra grating observations this source. observations have revealed X-ray jets in all show that spectrally hard states (known for sorts of galactic (protostars, binaries, ro- their production of optically thick jets) The Galactic Center. Our own Milky Way tation-powered pulsars, and black hole exhibit broad iron emission lines, while galaxy also hosts a supermassive black hole binaries) and extragalactic sources (radio softer states produce an accretion disk at its center. However, it is extremely sources and quasars). X-ray observations wind. Comptonization is proposed as underluminous. In fact, it required careful have proven essential for the study of the the mediating effect in the disk–jet work with Chandra observations to be fi emission mechanisms of the jets, in re- interaction. identi ed as an X-ray emitter. Invited vealing the interaction of jets with the in- One of the extragalactic sources that speaker Sera Markoff (84) reviewed the tergalactic or intracluster media, and in even more spectacularly shows the in- X-ray studies of the Galactic center re- studying the energy generation budget. terconnection among jets and their envi- gion. The region is rich in a large variety of Many aspects of jets remain uncertain, ronments is Centaurus A, the nearest X-ray sources as shown in Fig. 4 (85). including the composition of the jets, the AGN and extragalactic radio source. There are other low-luminosity super- means of jet confinement, and the particle Paul Nulsen (81) presented new analyses massive black holes in the local universe. It acceleration mechanisms. of Cen A that shows several components is thought many of these, including ours, Dan Evans (79) showed how the nearby including hot gas, dust lanes, a radio lo- go through quiescent phases for reasons 7 active galaxy NGC 1068—with a 10 M⊙ be surrounded by a shock, and a jet (Fig. that are still debated.

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