TY - JOUR
AU1 - Wilkes, Belinda, J
AB - Abstract Belinda J Wilkes looks back at 20 years of science from the Chandra X-ray Observatory – and forward towards more discoveries 1 Open in new tabDownload slide An accurate reproduction of Chandra in orbit, minted to celebrate the 20th anniversary in 2019. (NASA/CXC) 1 Open in new tabDownload slide An accurate reproduction of Chandra in orbit, minted to celebrate the 20th anniversary in 2019. (NASA/CXC) In 2019, NASA's Chandra X-ray Observatory is celebrating 20 years of observation and discovery since its launch on 23 July 1999. Chandra has uniquely high spatial resolution (∼0.5″), matching that of ground-based optical telescopes for the first time, and an order of magnitude better than any other existing or planned X-ray observatory. Chandra has revolutionized X-ray astronomy, vastly expanding our knowledge of X-ray emission from a full range of celestial sources, from planets to the most distant supermassive black holes and the structure of the universe. Chandra's broad energy range (0.5–10 keV) combines traditionally soft and hard X-rays for the first time (matched by ESA's XMM-Newton, launched later in the same year). Transmission gratings provide high spectral resolution data, E/ΔE ∼ 100–1000, over this full energy range (0.5–10 keV, 1–175 Å), and variable sources can be studied with high time resolution (16 μs) data. Chandra was launched from Cape Canaveral on the space shuttle Columbia, the only shuttle in the fleet with a sufficiently large cargo bay to carry the 10 000 lb, 45 ft long (4.5 t, 14 m) payload, the largest and heaviest lifted. The commander, Eileen Collins, was the first woman to command a shuttle. After release, Chandra, under the control of the operations teams in Cambridge, MA, fired its own engines to start the two-week journey to its final orbit. The orbit began with an apogee one-third of the way to the Moon (140 000 km) and a period of 63.5 hours, ∼16–18 hours of which are spent in the Earth's radiation belts with detectors turned off to protect them from high-energy particles. While the orbit precesses, changing its ellipticity and position angle over time, the period is very stable. Chandra is a general observatory which provides multiple ways in which to observe cosmic sources and their varied properties. Observing time is awarded via the standard process of peer review of proposals submitted by the worldwide astronomical community in response to an annual call for proposals. The observing time is consistently oversubscribed by a factor of 5.5. The user community numbers more than 4000 astronomers from 43 countries, not accounting for archival users, which are not tracked. Chandra data were downloaded to sites in about 108 countries over the past three years. Chandra is extraordinarily productive, with more than 8000 highly cited (35 citations after six years) science papers to date, an average of about 480 per year, and more than 90% of the archived data is published in one or more papers. Over the next six pages you can read about key areas where Chandra has driven advances in astronomy. The birth and death of stars Chandra has been a game-changer for observations in crowded fields, with star-forming regions being an excellent example. Stars form in the midst of dense clouds of gas and dust and, being hot and unstable, emit strongly in X-rays. Because these X-rays penetrate the gas and dust, X-ray telescopes are uniquely able to find young stars. One of the first star-forming regions to be observed by Chandra was the Orion Nebula, for which ROSAT observations had detected about 250 sources, with the cluster centre fully confusion-limited (figure 2; Gagne et al. 1995). Chandra's much higher spatial resolution revealed about 1400 stars, including faint stars in the wings of the brightest cluster members. Thus Chandra X-ray observations pinpoint the young stars among the large amounts of obscuring gas and dust in which they form, allowing the stellar populations to be studied in detail for the first time in the Orion Nebula and other star-forming regions. X-ray observations trace previously unseen populations, such as those with no protostellar discs. In consort with the Spitzer Space Telescope, the fraction of stars with discs was found to increase from 20% to 50% as stellar mass decreases below ∼2 M⊙ (Ribas et al. 2015), with disc evaporation timescales of a few Myr, and most having dissipated by ∼10 Myr. Star-formation triggering mechanisms can be probed by mapping the spatial distribution of the youngest stars within a star-forming region. Knowledge of stellar X-ray activity is key to determining whether an exoplanet can form and retain an atmosphere. X-ray monitoring can also help to distinguish candidate exoplanets from variations due to starspots or flares, which often have similar signatures. 2 Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide (Left) The Orion Nebula star-forming region as seen by the Roëntgen satellite (ROSAT). About 250 sources were resolved within the nebula in this image, with the central regions of this dense cluster heavily confused (Gagne et al. 1995). (Centre) The same field observed with Chandra showing ∼1400 sources and no confusion even in the bright wings of the central brightest stars (Feigelson et al. 2005). (Right) The X-ray data (blue, orange, yellow) combined with an optical image (red, purple) dominated by gas and dust observed by the Hubble Space Telescope. (NASA/CXC) 2 Open in new tabDownload slide Open in new tabDownload slide Open in new tabDownload slide (Left) The Orion Nebula star-forming region as seen by the Roëntgen satellite (ROSAT). About 250 sources were resolved within the nebula in this image, with the central regions of this dense cluster heavily confused (Gagne et al. 1995). (Centre) The same field observed with Chandra showing ∼1400 sources and no confusion even in the bright wings of the central brightest stars (Feigelson et al. 2005). (Right) The X-ray data (blue, orange, yellow) combined with an optical image (red, purple) dominated by gas and dust observed by the Hubble Space Telescope. (NASA/CXC) The violent explosions (supernovae, E ∼ 1051–53 erg) that result either from massive stars running out of fuel (core collapse) or thermonuclear runaway due to over-accretion onto a white dwarf, are bright X-ray sources and frequent Chandra targets. The expanding debris distributes heavy elements and energy into the host galaxies, feeding the next generation of stars, planets and life. Young supernova remnants (SNR) are some of the most picturesque and varied of Chandra's targets, as is clear from figure 3, ranging from roughly spherical (Tycho, G292.0) to very complex (3C58). The Crab Nebula has at its core a pulsar wind nebula (blue in figure 3), where non-thermal emission results from the interaction of the central, rotating pulsar's strong magnetic field with the surrounding plasma. High-quality Chandra data on multiple young SNRs, tracking the spatial distribution and velocities of the material, has revealed information on the explosion and expansion of the debris, as well as the amount and extent of pre-explosion mass loss from the progenitor star (Hwang & Laming 2012). Chandra has also located the central compact objects in multiple SNRs, facilitating their study, and found synchrotron emission in the outer parts of the forward shock, including band-like structures that provide strong evidence (confirming earlier suggestions) that cosmic rays are accelerated in SNRs (e.g. for Tycho's SNR, Slane et al. 2014). 3 Open in new tabDownload slide A montage of Chandra images of supernova remnants where the colours indicate the energy of the X-rays at each position (blue is high energy). (NASA/CXC) 3 Open in new tabDownload slide A montage of Chandra images of supernova remnants where the colours indicate the energy of the X-rays at each position (blue is high energy). (NASA/CXC) Chandra's long lifetime has also revealed how dynamic are young SNRs and their central compact objects on human timescales. The expansion of SN 1987A, observed over multiple wavebands including optical and ultraviolet (UV, HST), radio (VLA), and X-ray with Chandra, has tracked the changing structure of the debris as it interacts with circumstellar material resulting from pre-explosion mass loss of the progenitor star (Frank et al. 2016). The stellar remnant remains undetected after 30 years, leading to conclusions that it is likely to be a strongly dust-obscured, thermally emitting neutron star (Alp et al. 2018). Several young SNRs have visibly expanded over the past 20 years. A prime example is Chandra's iconic “first light” target, the ∼340-year-old Cassiopeia A (as seen on the cover of this issue), for which a movie showing the motion is available at chandra.si.edu/photo/2019/firstlight. Resolving the background Diffuse X-ray emission, known as the cosmic X-ray background, was ubiquitous in data taken by all previous X-ray missions. The desire to understand whether it is truly diffuse or made up of many, unresolved background sources drove Chandra's design (high spatial resolution, low background and deep flux limit). The deepest observation to date, collected over a period of about 12 years, is the Chandra Deep Field South with an accumulated exposure of 7 Ms (∼80 days; figure 4, Luo et al. 2011). About 1000 sources are detected down to a flux limit of 6.4 × 10−18 erg cm−2 s−1 (0.5–2 keV), seeing X-ray bright active galaxies (AGN, i.e. those with an accreting supermassive black hole [SMBH] in their nuclei) out to redshifts >4 (10% of the current age of the universe), and luminosities ∼ 1042 erg s−1 ⁠. The wide range of colours, which are mapped to the energy of the X-rays, seen in figure 4, indicate that Chandra is detecting active galaxies with a broad range of obscuration. Such highly obscured sources are difficult to find in traditional optical and near-infrared surveys because much of this emission is absorbed. At the faintest X-ray flux levels the number counts are dominated by X-ray faint, low-redshift galaxies (figure 5). 4 Open in new tabDownload slide Open in new tabDownload slide (Left) A view of the primary, 16 arcmin square region covered by the CDFS. (Above) A zoom on the central 8 arcmin, demonstrating the high source-density in the central part of the field, which has the highest spatial resolution. Colours indicate the energy of the X-rays detected in each source: red is low energy, blue is high. (From Luo et al. 2017 doi:10.3847/1538-4365/228/1/2) Reproduced by Permission of the AAS 4 Open in new tabDownload slide Open in new tabDownload slide (Left) A view of the primary, 16 arcmin square region covered by the CDFS. (Above) A zoom on the central 8 arcmin, demonstrating the high source-density in the central part of the field, which has the highest spatial resolution. Colours indicate the energy of the X-rays detected in each source: red is low energy, blue is high. (From Luo et al. 2017 doi:10.3847/1538-4365/228/1/2) Reproduced by Permission of the AAS 5 Open in new tabDownload slide The no. of sources brighter than a given flux (S) commonly known as the log N vs log S distribution of the sources in the CDFS in the (a) soft and (b) hard energy bands as labelled. The source numbers continue to increase towards lower flux levels, but the galaxies become more numerous than the SMBHs below flux ∼ 10−17 erg cm−2 s−1 ⁠. (From Luo et al. 2017 doi:10.3847/1538-4365/228/1/2) Reproduced by Permission of the AAS 5 Open in new tabDownload slide The no. of sources brighter than a given flux (S) commonly known as the log N vs log S distribution of the sources in the CDFS in the (a) soft and (b) hard energy bands as labelled. The source numbers continue to increase towards lower flux levels, but the galaxies become more numerous than the SMBHs below flux ∼ 10−17 erg cm−2 s−1 ⁠. (From Luo et al. 2017 doi:10.3847/1538-4365/228/1/2) Reproduced by Permission of the AAS The full population of active galaxies has been traced via multiple surveys covering a traditional wedding-cake structure, with the sky area coverage increasing as the flux limit decreases. These include the COSMOS field, ∼160 ks depth, 1.7 degree2, 2273 sources (Civano et al. 2016), the extended Groth Strip, 200 ks depth over 0.67 degree2, 1325 sources (Laird et al. 2009), the Bootes field, 25 ks depth over 6 degree2, 1479 sources (Hickox et al. 2007) and Stripe 82X, 31 degree2 which combines Chandra (1146 sources, 7.8 degree2) and XMM-Newton data (LaMassa et al. 2016). Multiwavelength observations of jets The first targeted source, used to focus the Chandra telescope, was a distant (z = 0.659) radio-loud AGN, PKS 0637–752. Accreting SMBHs in the nuclei of galaxies are surrounded by very hot material being pulled in towards the SMBH by their gravity. This material shines more brightly than the ∼ 1011 stars in their host galaxies. Thus, once sufficiently far away from Earth, they appear as a point source. While PKS 0637–752 did suffice to focus the telescope, the extended linear structure on the western side had engineers and scientists alike puzzled and concerned until it was realized that the emission was co-spatial with the known radio jet. Thus, even with the focus target, a new discovery was made and a new field born: multiwavelength studies of radio jets (e.g. M87, figure 6). In particular, comparing the X-ray and radio emission from the same region provides a constraint on the magnetic field of the emitting plasma unavailable from radio data alone. In general, the magnetic fields in non-relativistic plasma, such as radio lobes, are a factor of ∼2–3 lower than the previously assumed equipartion fields (Worrall 2009, Harris & Krawczynski 2006), which assumed that the magnetic and particle energies are equal, for a minimum energy of the system. 6 Open in new tabDownload slide Radio (top) optical (middle) and X-ray (bottom) images of the complex relativistic jet in M87. The various hot spots and knots are labelled in the HST (optical) image. Colours indicate the intensity, with blue being fainter. (Reproduced from Marshall et al. 2002doi.org/10.1086/160316 by permission of the AAS) 6 Open in new tabDownload slide Radio (top) optical (middle) and X-ray (bottom) images of the complex relativistic jet in M87. The various hot spots and knots are labelled in the HST (optical) image. Colours indicate the intensity, with blue being fainter. (Reproduced from Marshall et al. 2002doi.org/10.1086/160316 by permission of the AAS) The X-ray jets were initially interpreted as due to inverse-Compton emission via interaction of the intrinsic electron population with seed photons from the cosmic microwave background (Tavecchio et al. 2000). However, in the last few years significant non-detections at γ-ray energies in Fermi observations rule this out in favour of synchrotron emission from a second, higher-energy electron population in several of these sources (Meyer et al. 2015, Meyer & Georganopoulos 2014). Multiwavelength observations allow tracking of individual knots and hot spots to probe the electron energy distributions, and their acceleration sites and mechanisms. Variability data provides information on region sizes and structures (Harris et al. 2009). The resulting constraints on detailed models have led to the current spine-sheath scenario, where the core of the jet is highly relativistic while the outer layers are slowed via interaction with the surrounding medium (Sikora et al. 2016). Recent comparisons of Chandra observations of the M87 jet with a five-year spacing have detected X-ray super-luminal motion for two components: HST-1 (6.3 ± 0.4c) and Knot D (2.4 ± 0.6c), for the first time (figure 6; Snios et al. 2019). These match well with measurements in the optical and ultraviolet with HST, indicating that the emitting regions move together despite the much stronger X-ray flux variations. Clusters of galaxies The largest gravitationally bound structures in the universe, clusters of galaxies can include more than 1000 galaxies. The presence of an order-of-magnitude more mass in hot gas than in the galaxies was known from Einstein and EXOSAT observations, e.g. Hughes (1989). However, Chandra observations again resulted in a major surprise. The X-ray-emitting hot gas is highly dynamic, with complex structures including sharp cold fronts, shock fronts, and deep voids providing a “fossil” record of the activity each cluster has undergone (figure 7). The sweeping cold fronts are thought to be the result of interactions with other large clusters, while the bright shocks and dark voids are the result of jet activity from the AGN in the brightest cluster galaxies (Markevitch & Vikhlinin 2007). Radio-emitting plasma fills the voids in the X-ray gas, heating the surrounding gas and slowing down both star formation, which occurs as the gas cools, and the flow of gas towards the central SMBH. This activity forms a feedback loop that regulates the growth of the SMBH and its host galaxy, preventing runaway cooling that would otherwise occur. Figure 8 shows a classic example of this process in the nearby radio galaxy Cygnus A, where the relativistic, bipolar, radio jets (red) blast through and heat the surrounding, hot X-ray emitting gas (blue). The narrow, relativistic radio jets form characteristic lobes of emission at their ends as they interact with the surrounding medium, clearing a cavity in the X-ray emitting gas. 7 Open in new tabDownload slide Open in new tabDownload slide (Above) Matched images of the Perseus cluster in the optical (left) showing the brightest cluster galaxy NGC 1257 in the centre of the field and in the X-rays from Chandra (right) showing complex structures of bright shock fronts surrounding the outer edges of voids formed by relativistic jets launched from the central AGN (Fabian et al. 2006). (NASA/CXC) 7 Open in new tabDownload slide Open in new tabDownload slide (Above) Matched images of the Perseus cluster in the optical (left) showing the brightest cluster galaxy NGC 1257 in the centre of the field and in the X-rays from Chandra (right) showing complex structures of bright shock fronts surrounding the outer edges of voids formed by relativistic jets launched from the central AGN (Fabian et al. 2006). (NASA/CXC) 8 Open in new tabDownload slide (Right) X-ray and radio image of the nearby radio galaxy Cygnus A showing radio emission (red) filling cavities in the X-ray-emitting hot gas (blue). The structures in the inner X-ray gas are likely to be due to the break-up of the cool core of the Cygnus A cluster by the passage of the jets (Duffy et al. 2018). (NASA/CXC) 8 Open in new tabDownload slide (Right) X-ray and radio image of the nearby radio galaxy Cygnus A showing radio emission (red) filling cavities in the X-ray-emitting hot gas (blue). The structures in the inner X-ray gas are likely to be due to the break-up of the cool core of the Cygnus A cluster by the passage of the jets (Duffy et al. 2018). (NASA/CXC) Chandra grating spectroscopy Chandra's gratings have brought high-resolution X-ray spectroscopy into the mainstream for the first time, reaching well beyond the bright stars that had been observable in the X-rays in the past. Beautiful spectra, rich in the emission and absorption lines that lie in the X-ray band, have been obtained for stars, outbursting binary systems and supernovae, facilitating detailed modelling of accreting material, and outflowing winds, and ushering in a new era of understanding the physics of active stars and stellar systems. Chandra gratings have also obtained rich absorption and emission line X-ray spectra of AGN. The first such long observation was of the low-redshift Seyfert 1 galaxy NGC 3783 (figure 9) which constrained physical and dynamical models of the material around the galactic nucleus. The best fit model for NGC 3783 includes a two-component (high and low ionization) plasma, outflowing at ∼750 km s−1 with an estimated mass loss rate of ∼0.2–4 M yr−1, sufficient to impact the AGN host galaxy (Krongold et al. 2003). Variability indicates the material is only ∼6 pc from the nucleus (Krongold et al. 2005). More recently, additional transient, higher velocity, high-column-density obscuration has been reported in NGC 3783 (Kaastra et al. 2018). 9 Open in new tabDownload slide A portion of the 900 ks exposure Chandra HETG spectrum of the Seyfert 1 galaxy NGC 3783 (black). The best fit spectrum is shown in red and the elemental identity of many of the absorption and emission features is indicated above. (NASA/CXC) 9 Open in new tabDownload slide A portion of the 900 ks exposure Chandra HETG spectrum of the Seyfert 1 galaxy NGC 3783 (black). The best fit spectrum is shown in red and the elemental identity of many of the absorption and emission features is indicated above. (NASA/CXC) Binary/dual and merging compact objects Chandra has proven to be excellent at finding binary SMBHs in the cores of galaxies. The first example, in the nearby merging galaxies NGC 6240 (figure 10), demonstrated that the two bright galaxy nuclei seen in optical images are both AGN. Chandra data resolved two strong X-ray sources, only 1 kpc apart, in the galaxy centre (Komossa et al. 2003). These will eventually merge, forming a larger SMBH and generating gravitational waves (GW) at frequencies that ESA's planned Laser Interferometer Space Antenna (LISA) would be able to detect. Deeper Chandra observations revealed an extended hot X-ray halo with temperature ∼7.5 × 106 K (figure 10; Nardini et al. 2013). Multiple search methods for binary AGN, including searching for double-peaked emission line profiles and offset nuclei for optically and X-ray selected targets, have since discovered tens of objects with a wide range of separations. Closer pairs tend to be more X-ray luminous, suggesting that the merger is key in powering both AGN (Koss et al. 2012). They are more common at higher redshift, perhaps reflecting a higher merger rate in the past (Comerford et al. 2013). Infrared-based searches for merger activity in galaxies using the WISE archive have also found several new binary AGN systems, and recently a rare triple (Pfeifle et al. 2019) reported in a recent Chandra press release (bit.ly/2NfIk3t): three accreting SMBHs, in a system of three merging galaxies, SDSS J084905.51+111447.2, all <10 kpc apart. It is expected that gravitationally bound triple systems will experience a shorter inspiral and thus merging timescale than binary systems. Studying the population provides constraints on galaxy merger rates as well as predicting rates for SMBH mergers likely to be detected in gravitational waves by LISA. 10 Open in new tabDownload slide A multiwavelength image of the merging galaxies NGC 6240 showing the binary nucleus and hot X-ray gas in red, yellow and white and the optical emission from HST in blue. (NASA/CXC) 10 Open in new tabDownload slide A multiwavelength image of the merging galaxies NGC 6240 showing the binary nucleus and hot X-ray gas in red, yellow and white and the optical emission from HST in blue. (NASA/CXC) At much smaller masses, the recent detection of the merging neutron stars, GW 170817, via a GW signal detected by LIGO/Virgo, followed ∼2 s later by a γ-ray detection with both NASA's Fermi Gamma-ray Space Telescope and ESA's INTErnational Gamma-Ray Astrophysics Laboratory (Integral), opened up a new window on celestial sources through “multimessenger astronomy”. Chandra played an important role in tracking and constraining models for GW 170817. The initial kilonova observed in the optical reddened and faded within two weeks (Drout et al. 2017). The radio and X-ray jet continued to increase in flux over ∼160 days, demonstrating the presence of a ∼30° off-axis, structured jet which, as it expanded and broadened, gradually intercepted our line of sight. The X-ray and radio fluxes increased together, as expected for synchrotron radiation, peaked and then decayed as our view transitioned to seeing directly into the broadened, relativistic jet core. Figure 11 (Margutti et al. 2019) shows a schematic of the model along with the X-ray light curve of the source. 11 Open in new tabDownload slide (Left) Central kilonova and jet of GW 170817. The jet expanded and X-ray emission increased until Chandra's line-of-sight was into the jet core. (Right) X-ray light curve of GW 170817, colours indicating line-of-sight. The best-fitting off-axis relativistic jet model is in blue (Wu & MacFadyen 2018). (Reproduced from Margutti et al. 2019) 11 Open in new tabDownload slide (Left) Central kilonova and jet of GW 170817. The jet expanded and X-ray emission increased until Chandra's line-of-sight was into the jet core. (Right) X-ray light curve of GW 170817, colours indicating line-of-sight. The best-fitting off-axis relativistic jet model is in blue (Wu & MacFadyen 2018). (Reproduced from Margutti et al. 2019) Cosmological topics Clusters of galaxies, the largest gravitationally bound structures, trace both the growth of structure and the location of baryons in the universe, properties that provide direct and independant cosmological constraints. Prior to Chandra, ROSAT X-ray data on the Coma cluster had suggested that the universe has low density (White et al. 1993a, b). With Chandra, rapid advances have been made in the use of clusters for cosmology, finding results consistent with those from Type Ia supernovae. The mean matter density of the universe, Ωm = 0.3 ± 0.04, and the dark energy equation of state, w = p/ρ = −1.26 ± 0.24, where (p, ρ) are the dark energy pressure and density respectively, are consistent with the value of w = −1, i.e. dark energy described a cosmological constant in general relativity (Allen et al. 2002, 2004). Clusters have also provided a test of the properties of dark matter, which accounts for ∼83% of their total mass. The first dramatic example was the observation that the location of the dark matter, inferred by gravitational lensing in visible light images, in the merging system known as the Bullet cluster (1E 0657-558) aligned with the galaxies, while that of the X-ray gas lagged behind as a result of self-interaction during the encounter (figure 12). This separation places constraints on the self-interaction cross-section per unit mass of dark matter, σ/m < 1.25 cm2 g−1, ruling out a number of candidates for dark matter particles (Clowe et al. 2006, Randall et al. 2008). This result has been supported by observations of several other merging systems (e.g. Harvey et al. 2015). 12 Open in new tabDownload slide A multiwavelength image of the merging clusters 1E 0657-558 (the Bullet cluster), demonstrating the differing locations of the galaxies (white), dark matter (blue, inferred from gravitational lensing), and hot X-ray-emitting gas (red) following the clusters' interaction. (NASA/CXC) 12 Open in new tabDownload slide A multiwavelength image of the merging clusters 1E 0657-558 (the Bullet cluster), demonstrating the differing locations of the galaxies (white), dark matter (blue, inferred from gravitational lensing), and hot X-ray-emitting gas (red) following the clusters' interaction. (NASA/CXC) Another cosmological problem that Chandra has attempted to address is that of the missing third of the baryons at low redshift compared with those at high redshift. The missing baryons are thought to reside in large-scale filaments of a warm–hot intergalactic medium (WHIM). Multiple observations by both Chandra and XMM-Newton have reported and disputed marginal detections against background sources (e.g. Nicastro et al. 2013, 2016) demonstrating that this experiment pushes the limits of both observatories. A recent positive result uses an updated technique of stacking Chandra High-Energy Transmission Grating data on the luminous AGN H1821+643 (z = 0.297) at redshifts corresponding to the 17 UV-observed absorption line systems along its line of sight. A 3.3σ detection of the prominent O Vii λ 21.6 absorption line has been obtained (Kovacs et al. 2019). The line equivalent width is ∼4.1 mÅ, with a derived column density NOV II ∼ 1.4 × 1015 cm−2 ⁠, consistent with WHIM expectations. A longer observation of this target has been approved in Chandra Cycle 21 to confirm – or not – and potentially improve the significance of this detection. Chandra's status and future Chandra continues to operate at high efficiency, with ∼70% wall-clock time on sky limited only by passage through the Earth's radiation belts, and the impact of Chandra's science remains high. There are a couple of aging-related factors that are being effectively managed at this time. The thermal insulation on Chandra is degrading and operations become more complex as the satellite warms, placing limits on the exposure times for individual observations at a given solar pitch angle. In addition, the build-up of contaminant on the ACIS window has reduced the response of that instrument at energies ∼1.5 keV. The performance degradations that result have made some science difficult (e.g. observations of very soft sources, long uninterrupted observations), and limits observing time allocated at high ecliptic latitude. However, Chandra has observed only ∼550 degrees2, ∼1.4% of the sky, to date, so there is plenty more sky to observe. The demand for time remains high and new science is continually enabled by the commissioning of new missions and telescopes. Recent and upcoming opportunities include: the birth of gravitational wave (multimessenger) astronomy (Abbott et al. 2017) with LIGO and Virgo; continued coordination with the Event Horizon Telescope (EHT; Event Horizon Telescope Collaboration et al. 2019) to observe X-rays originating close to the event horizon of the M87 and Sgr A∗ SMBHs; coordinated observations with the Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2014); follow-up of targets from the second (after ROSAT) all-sky X-ray survey with e-Rosita (Predehl 2017) on Spektr-RG/SRG; the upcoming launch from NASA of the X-ray Imaging and Spectroscopy Mission (XRISM), the Imaging X-ray Polarization Explorer (IXPE; Weisskopf et al. 2016), and of the James Webb Space Telescope (JWST); and the transient survey from the ground with the Large Synoptic Survey Telescope (LSST), to name a few. 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TI - Chandra's revolution in X-ray astronomyChandra
JF - Astronomy & Geophysics
DO - 10.1093/astrogeo/atz191
DA - 2019-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/chandra-s-revolution-in-x-ray-astronomychandra-3QamaoNDoT
SP - 6.19
VL - 60
IS - 6
DP - DeepDyve
ER -