* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The new X-ray universe
Corvus (constellation) wikipedia , lookup
Leibniz Institute for Astrophysics Potsdam wikipedia , lookup
Spitzer Space Telescope wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Modified Newtonian dynamics wikipedia , lookup
Astronomy in the medieval Islamic world wikipedia , lookup
International Year of Astronomy wikipedia , lookup
Gamma-ray burst wikipedia , lookup
Theoretical astronomy wikipedia , lookup
History of astronomy wikipedia , lookup
History of gamma-ray burst research wikipedia , lookup
H II region wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Hubble Deep Field wikipedia , lookup
High-velocity cloud wikipedia , lookup
Star formation wikipedia , lookup
Observational astronomy wikipedia , lookup
X-ray astronomy wikipedia , lookup
History of X-ray astronomy wikipedia , lookup
X-ray astronomy satellite wikipedia , lookup
X-ray astronomy The new X-ray universe Phil Charles and Andy Fabian review the changes brought to X-ray astronomy by the two orbiting observatories, XMM-Newton and Chandra. e describe here the changes to X-ray astronomy that are being brought about by the two new orbiting X-ray observatories, NASA’s Chandra and ESA’s XMM-Newton. Between them they bring a dramatic gain in sensitivity, spatial resolution (now comparable to ground-based optical and infrared telescopes) and spectral resolution. The superb Chandra mirrors rapidly resolved the diffuse X-ray background and have produced stunning detailed images of a wide variety of cosmic objects. Large gains have been made by both missions in spectroscopic performance, with individual X-ray emission lines now detectable in hot stellar coronae, supernova remnants, X-ray binaries, active galactic nuclei and galaxy clusters. W X -ray astronomy took a major step in the 1970s with the first scanning satellites, such as UHURU and Ariel V, but it required the imaging capability (using grazing incidence mirrors) of the Einstein Observatory (launched in 1978) in order to reach the sensitivity level necessary to study significant samples of normal stars and galaxies. This enabled the Einstein observatory to be used to tackle one of the longest standing problems in X-ray astronomy, namely the nature of the diffuse hard X-ray background, giving the first direct indication that, at least at low energies, it might be due to large numbers of very faint, distant sources. X-ray astronomy is now well established as a tool for tracing the hot universe, in the aftermath of supernova explosions, binary star systems involving white dwarfs, neutron stars and black holes, active galaxies and quasars, and hot intergalactic gas. The missions that followed Einstein in the next two decades (EXOSAT, GINGA, ROSAT, ASCA and BeppoSAX) barely improved on spatial resolution, which remained substantially worse (factors of 5–50) than most other December 2001 Vol 42 1: This NASA X-ray image of the central arcminute of M31 demonstrates Chandra’s superb imaging qualities. The two sources at the centre (blown up in the inset) are barely an arcsecond apart. The red contours from HST show the location of the supermassive black hole. The X-ray flux is weak because accretion onto the galaxy nucleus is very slow. The blue source is much cooler than the others, and is probably a white dwarf accreting so fast from its companion that there is continuous nuclear burning on the surface of the star (a “supersoft source”). (X-ray: NASA/SAO/CXC/M Garcia et al.; Optical: NASA/GSFC/T Brown et al.) wavelengths, particularly from the ground. Several of the missions opened up the field of X-ray imaging spectroscopy and all of them enabled significant scientific advances to be made. The area of high spectral resolution did not, however, improve. XMM-Newton and Chandra have changed all this, and in highly complementary ways. Chandra provides the sub-arcsecond resolution equivalent to the spatial resolution of groundbased optical and infrared telescopes, but at X-ray wavelengths (figure 1). Its mirrors are the finest yet produced in X-ray astronomy, having a surface figure of ~0.5 arcsec. Such high resolution enables it to overcome the problems of source confusion that limited earlier missions in, for example, studying individual sources in other galaxies. XMM-Newton has a large collecting area from its multiply nested mirrors which makes it particularly sensitive, enabling the detection of weaker and variable sources. Grating spectrometers have been developed for both observatories that improve dramatically on the technology used in the 1970s and 80s. Chandra uses deployable transmission gratings that can be introduced into the field of view (figure 2), whereas XMM-Newton has permanently mounted grazing-incidence reflection gratings that intercept approximately half of the light 6.11 X-ray astronomy ISIM optical bench solar array aspect camera sun shield LETG 2: A schematic of the light path through Chandra. X-rays are absorbed at normal incidence; X-ray telescopes depend on glancing incidence, and so consist of nested barrel-shaped mirrors (NASA). HRMA ACIS/HRC HETG electrical boxes thermal shroud 3: The centre of the starburst galaxy M82, in X-rays. The bright source near the centre of the image, about 600 light years from the dynamical centre (small green +) of the galaxy, increased dramatically in intensity over three months (compare left and right panels) after which it decreased – a pattern of variability characteristic of a black hole. (NASA/SAO/CXC.) oxygen VII triplet oxygen VIII Lyα He-like H-like neon IX neon X triplet Lyα He-like H-like magnesium XI triplet direct image, all energies He-like 4: Spectral resolution illustrated by this supernova remnant E0102-723. 5: Chandra’s view of the Centaurus galaxy cluster, revealing hitherto-unseen details of this cooling-flow cluster, especially in the central region. 6.12 from two of its three mirrors (hence producing high-quality X-ray spectra as part of all observations). These instruments have brought major improvements in spectral resolution but with a staggering gain in throughput, which means they can be used to study a much wider range of objects than was possible with, for example, the Einstein grating spectrometer. This enables a much clearer picture of the temperature, structure and behaviour of sources, whether galaxy clusters, active galactic nuclei or binary stars. Both instruments – described in more detail in the boxes on pages 6.14 and 6.16 – have a high throughput, providing a much wider range of accessible science in the available observing time than hitherto possible. The spatial resolution of Chandra is especially useful in crowded source regions, such as the huge numbers of individual sources that are being resolved in M31, the nucleus of which has finally been resolved in X-rays. It is now possible to monitor variability in these faint sources, giving yet more information about the processes responsible for their emission. For example, figure 3 shows a source identified in M82 December 2001 Vol 42 X-ray astronomy Starburst galaxies Extragalactic astronomy has benefited from the sensitivity and the volume of observations now possible. Comparisons of the number of X-ray sources in galaxies shows a fourfold variation from one to the next, implying different evolutionary histories. Several starburst galaxies – galaxies undergoing extensive star formation – have been found to contain ultraluminous X-ray sources, probably massive black-hole systems. The first Chandra observations of NGC253 suggest that the observations come from just a fraction of the mass in the starburst winds, the densest parts where it meets the galaxy’s interstellar medium. In future, spectroscopic observations should resolve enough of the starburst winds in order to constrain the gas masses involved. Galaxy clusters The high spectral resolution and sensitivity offered by XMM-Newton has broadened our knowledge of clusters of galaxies, especially those associated with cooling flows. The RGS spectrometers have provided the first high spectral resolution spectra of the central regions (figure 6 shows data for Ser 159-03, as an example). The spectra show the presence of gas at about one third of the cluster virial temDecember 2001 Vol 42 6a: The radial distribution of Fe, T and density in Ser 159-03. Key items are: the temperature drop in the centre (ascribed to cooling gas), the temperature drop in the outer parts, and the clear abundance gradient. b: RGS spectrum of this cluster, for which the following applies: RGS data show O VIII, Ne X and Fe XXIV/XXIII lines. Fe XVII/XVIII lines are weak or absent. Typically there is five times less cool gas compared to the isobaric model. (a) 3.0 2.5 T (keV) 2.0 1.5 Fe abundance 1.0 0.4 0.2 nH (m– 3) 0 104 1000 100 0.1 10 1 radius (arcmin) (b) 0.06 counts/s/Å 0.04 0.02 0 (observed – model) /model which turned out to be highly variable (indicating that it was a single object, not a close group of sources) and at the known distance of M82 this implied that it was “ultraluminous” (Lx >1039 erg s–1; the source in M82 varies up 1041 erg s–1). While the existence of such objects was hinted at by earlier missions, a substantial number have now been found in Chandra and XMM observations of nearby galaxies. If scaled simply relative to accreting X-ray binaries in our own galaxy, then this suggests that they may be accreting black holes with a compact object mass of around 100 Suns. If confirmed this has important consequences for the late stages in the evolution of the supermassive stars that are needed to have formed these objects. However, there are alternative interpretations where the high luminosity is a result of “beaming” in our direction which makes the objects look intrinsically brighter than they really are. In another case, the active RS CVn II Peg was seen to flare during observation with Chandra’s high-energy transmission grating, revealing the dramatic change in temperature structure of the plasma as it moves from quiescent to flaring conditions, a process that has links with the much smaller-scale flares observed from the Sun. Spectra also showed a significant Fe deficiency. We describe below some of the highlights of observing with XMM-Newton and Chandra, in fields as varied as galaxy clusters, quasars, X-ray binaries and the Galactic Centre. 10 15 20 25 30 35 10 15 20 25 wavelength (Å) 30 35 1 0 –1 perature there, but register the absence or only a low level of emission from Fe XVII and Fe XVIII ions expected if gas continues to cool below 1 keV or so. There is much less cool gas than would be expected for most simple isobaric cooling-flow models. No unique explanation exists for this characteristic. Chandra observations of nearby cooling-flow clusters, the Perseus cluster, Abell 1795, and the Centaurus cluster (figure 5), show considerable detail of the structure in the central cooler region. Holes in the Perseus cluster X-ray emission are formed by radio lobes, but expanding only slowly since no shocks are evident. Two outer holes correspond to low- frequency radio spurs, and the hot X-ray gas shows a spiral structure that may be due to the angular momentum of infalling material. For A1795, a linear X-ray structure correlates with a filament in Hα emission, and is cooler than the surrounding cluster and may have been formed by cooling gas. Maps of temperature, absorption and abundance in the Centaurus cluster indicate decreasing temperature and decreasing absorption towards the centre of the cluster following a peak at about 20 kpc. Measuring mass and dark matter Chandra’s spatial resolution makes it a robust tool for measuring mass. So far, observations 6.13 X-ray astronomy Chandra have confirmed that hot gas exists within groups of galaxies that have low X-ray luminosity, and in some with very low velocity dispersion. These groups are in the majority; if otherwise invisible hot gas is a general feature of them, then this may account for the bulk of the baryonic mass at this scale. Chandra’s resolution makes it a robust tool for mass determination in galaxy clusters. In Abell 2390, Abell 1835 and RXJ1347.5-1145, all X-ray luminous, relatively relaxed clusters of galaxies, the mass profiles determined from the Chandra data are in good agreement with the predictions from numerical simulations. The best-fit X-ray mass models matched independent results from gravitational lensing studies and, where available, optical measurements of the galaxy velocity dispersions in the clusters (figure 8). Using the Chandra measurements, the X-ray gas mass fraction as a function of radius in the clusters was determined and, assuming that massive clusters give a fair sample of the properties of the universe as a whole, the results suggest the relation Ωmatter = 0.33 – 0.40 h50–0.5. Observations have also confirmed that hot gas exists within groups of galaxies that have low X-ray luminosity, and in some with very low velocity dispersion. These groups are in the majority; if 6.14 7: Final polishing and checking of one of Chandra’s superb mirrors, which allow the spectacular results. other, ACIS-S, is a spectroscopic array with six chips, used in the main with the grating. There is also a High Resolution Camera (HRC; PI Steve Murray, SAO) comprising two microchannel plate detectors. HRC-I uses a 90 mm2 detector optimized for imaging, with a 30 arcmin field of view and ~0.5 arcsec spatial resolution. HRC-S has a 20 × 300 mm rectangular detector optimized for use with the Low Energy Transmission Grating experiment (PI Bert Brinkman, Utrecht). The High Energy Transmission Grating is used with ACIS-S (PI Claud Canizares MIT). 8: A comparison of the projected total mass determined from the Chandra X-ray data for Abell 2390 (68% confidence limits shown in purple) with the strong lensing result of Pierre et al. (1996, red circle) and the weak lensing results of Squires et al. (1996, orange). The effective velocity dispersion associated with the best-fit X-ray mass model is in good agreement with the optically determined value. 1015 mass (M) The Chandra X-ray Observatory started life as NASA’s Advanced X-ray Astrophysics Facility (AXAF). It was renamed in honour of Subrahmanyan Chandrasekhar. It was launched and deployed by the Space Shuttle Columbia on 23 July 1999, into an elliptical high-Earth orbit that allows long-duration uninterrupted exposures of celestial objects. The combination of high resolution, large collecting area and sensitivity to higher energy X-rays make it possible for Chandra to study extremely faint sources, sometimes strongly absorbed, in crowded fields. The observatory is expected to have a ten-year mission. Chandra has a spatial resolution of < 1 arcsec over an energy range from 0.1 to 10 keV. It follows a 64-hour, highly eccentric Earth orbit. The observatory carries a Wolter Type 1 grazing incidence iridiumcoated imaging telescope with a field of view ~30 arcmin across and an effective area of 800 and 400 cm2 at 0.25 and 5 keV respectively. The detectors are as follows. The AXAF Charge Coupled Imaging Spectrometer (ACIS; PI Gordon Garmire, Penn State University) has two CCD arrays for a total of 10 chips. One array (ACIS-I) is primarily for imaging and uses four chips; the 1014 100 200 500 1000 radius (kpc) 2000 otherwise invisible hot gas is a general feature among them, then this may account for the bulk of the baryonic mass at this scale. Quasars High-resolution spectroscopy has changed researchers’ perceptions of quasars, providing some localization of the sources of particular lines. This is another field in which combining observations from both Chandra and XMMNewton brings benefits in terms of details and in the number of objects examined. Combining data from 10 AGN shows a correlation between the ionization state of the disk and the AGN luminosity. Largely neutral iron gives a strong line at 6.4 keV; ionized iron produces lines at 6.7 keV and 6.97 keV. Iron appears largely as a neutral line in many Seyfert 1 type galactic nuclei; it becomes more ionized as the luminosity of the central source increases. The high sensitivity and spectral resolution is enabling weak narrow line components to be readily found: there is a narrow, neutral iron line of similar width and relative strength in all but the most luminous sources, implying some common origin in a molecular torus or possibly the optical broad-line-region (BLR). The EPIC consortium has examined several December 2001 Vol 42 X-ray astronomy December 2001 Vol 42 Markarian 766 counts s–1 Å–1 0.08 0.06 C VI Lyα 0.04 0.02 0 VIII Lyα N VII Lyα residuals 0 0.02 0 –0.02 5 10 15 20 25 wavelength (Å) 35 30 MCG – 6 – 30 –15 counts s–1 Å–1 0.08 0.06 C VI Lyα 0.04 0.02 0 VIII Lyα N VII Lyα 0 residuals AGNs including the luminous quasar PKS 0558-504, which shows a featureless X-ray continuum in which a large, soft X-ray excess dominates the high-energy features. The data are consistent with a multitemperature accretion-disk corona. Variations in the X-ray continuum, such as an increase of 35% in less than an hour, suggest that there is a high accretion rate. In this source there is no sign yet of a neutral iron line at 6.4 keV, although there is a highly ionized iron line at 6.97 keV which may be variable. Mrk 205, an intermediate-luminosity QSO, has a two-component iron line, which can be fitted by a narrow 6.4 keV (neutral) component, possibly arising from the molecular torus and/or BLR, and a broad 6.7 keV (Helike) component, possibly arising from a partially ionized accretion disk. A similar two-component profile is seen in the lower luminosity Narrow-Line Seyfert 1 galaxy, Mrk 359, although here the narrow line appears stronger and the broad component is more neutral. The HETG is also probing circumsource regions in AGN. Extended emission from the Seyfert 1.5 galaxy NGC 4151 shows evidence for both photoionized and collisionally ionized material in a multicomponent narrow line region. The emission line spectrum of the Seyfert 2 galaxy NGC 1068 looks remarkably similar to that of NGC 4151, but with a relatively weaker continuum. The spectra of two high-redshift quasars were searched for evidence of resonance absorption lines from an enriched warm intergalactic medium, but no such lines have been seen to date. This nondetection does not yet set significant constraints on cosmological models of the intergalactic medium. Results from the grating spectrometers on board Chandra and XMMNewton vividly demonstrate the power of high-resolution spectroscopy in the soft X-ray band. Never before have AGN spectra been measured in such detail: chemical composition, temperatures, densities, ionization states and velocities of the emitting gas can be derived directly from the spectra, and the location of the gas in the AGN can be pinpointed. A variety of physical scenarios have already been found and studied quantitatively for the different AGN classes. An exciting but still-controversial RGS result is the identification of emission lines from a relativistic disk around a massive rotating black hole in the Narrow Line Seyfert 1 galaxies MCG-6-30-15 and Mrk 766 (figure 9a). A model for these sources has been constructed involving a single power-law absorbed by ionized gas with very strong, He-like, oxygen, nitrogen and carbon emission lines, gravitationally redshifted and broadened by relativistic effects in the vicinity of a Kerr black hole (fig 9b). An alternative interpretation of the spectrum obtained with the HETG from the 0.02 0 –0.02 5 10 15 20 30 25 35 wavelength (Å) 9 (a and b): XMM-Newton RGS spectra of the Narrow Line Seyfert 1 galaxies MCG-6-30-15 and Mrk766, fitted with a model composed of a power-law, warm absorption and emission lines of H-like oxygen, nitrogen and carbon, gravitationally redshifted and broadened by relativistic effects in the vicinity of a Kerr black hole. 10: A simulation of an X-ray binary. (R Hynes, University of Southampton.) 6.15 X-ray astronomy XMM-Newton The European Space Agency’s X-ray Multi-Mirror satellite, known as XMM-Newton, was launched from Kourou, French Guiana, on 10 December 1999, into a 48-hour elliptical orbit around the Earth, inclined at 40° with a southern apogee at 114 000 km and the perigee altitude of 7000 km. The eccentric orbit means that very long, uninterrupted observations can be made. It has an operational lifetime of up to 10 years. XMM-Newton is a three-axis stabilized spacecraft with a pointing accuracy of one arcsec. It carries three X-ray telescope modules, each shaped like a barrel and containing 58 high-precision concentric mirrors, delicately nested to offer the largest collecting area possible. Each mirror is made of nickel, coated with gold, and sits just millimetres from its neighbour. Together they have an area greater than that of a tennis court. The observatory covers a range of 0.1–12 keV (0.1–10 nm spectral range), first of these objects is that this feature can be wholly explained by a dusty warm absorber alone. The apparent redshifted OVII edge can be fully explained by a combination of the overlapping 1s2 – 1s2p (n > 5) OVII resonance line series from a warm absorber, and the Fe L3 and L2 edges from embedded dust. X-ray binaries Chandra and XMM-Newton are ideally suited to observations of X-ray binaries, particularly able to exploit the high orbits of both spacecraft which offer long, uninterrupted viewing times (conventional near-Earth orbit missions can only view any given point on the sky for typically 40 min out of every 100 min orbit, as a result of Earth-occultation and other constraints). This is because most powerful X-ray binaries have orbital periods of a few hours to a day or so. The improved sensitivity has enabled observations to be made of transient X-ray sources in quiescence, for which spectral and variability studies can be performed that were not previously possible. These are providing valuable insights into the differences between black holes and neutron stars where the latter systems appear to be brighter than equivalent objects thought to contain black holes. This is explained as possible direct evidence for the existence of an event horizon around the black hole, in that hot gas that crosses this region is lost from view for ever, whereas in a neutron star binary this hot material will come to rest on the surface of the neutron star and radiate its energy back out. 6.16 11: The nested mirrors of XMM-Newton. giving a collecting area of 4300 cm2 at 1.5 keV and 1800 cm2 at 8 keV. The resolution is 5 arcsec (full width half-maximum), 14 arcsec (half energy width) at all wavelengths. There are three main scientific instruments on board. The three European Photon Imaging Cameras (EPIC) were produced by a consortium of 10 institutes in four nations: the UK, Italy, France and Germany. Principal Investigator is Martin Turner of the X-ray Astronomy Group at Leicester University. One of the cameras uses a new type of CCD (PN) developed by the Max An especially exciting observation of an X-ray binary was made by Chandra which found the first X-ray “P Cygni” profiles from the extreme object Circinus X-1. Such profiles indicate that there is dense, outflowing material, but to see this in such highly ionized lines as silicon XIV means that we are watching the interaction of matter very close to the compact object (probably a neutron star). This is because the line profiles (seen in X-rays for the very first time here) require velocities of around 2000 km s–1 in material that must be at temperatures of typically tens of millions of degrees (to produce the level of ionization). This is the analogy in X-ray binaries, according to the observers of this event, to Broad Absorption Line quasars, where similar (but much larger scale) outflows are believed to be driven by a powerful central source. One example is afforded by the Chandra coverage of the recent outbursting transient XTE J1118+480. Combined with EUVE and RXTE data, it was possible to construct an unbroken spectral energy distribution from extreme-UV to hard X-rays. High spectral resolution observations have revealed richly structured spectra containing both emission lines and absorption edges. These contain valuable clues about photoionized winds, disk atmospheres and other structures within the binary; recent XMM results on EXO 0748-676 show the power of such observations. Even when no lines are seen, it is of interest to rule out geometries that predict significant line emission. For example, the lack of emission lines in LMC Planck Institute of Extraterrestrial Physics in Garching, Germany. There are two Reflection Grating Spectrometers (RGS). The Principal Investigator is Prof. Bert Brinkman of the High-Energy Astronomy division, SRON, Utrecht, Netherlands, with co-Investigator Steven Kahn from Columbia University, NY, USA. The Optical Monitor (OM), co-aligned with the main X-ray telescope, will give the XMM-Newton mission a multiwavelength capacity. The Mullard Space Science Laboratory (MSSL) has supplied this 30 cm aperture Ritchey-Chretien telescope (with a 170–600 nm spectral range). OM Principal Investigator is Keith Mason of MSSL. In addition, XMM-Newton has a particle detector, the EPIC Radiation Monitor System (ERMS), developed by the Centre d’Etude Spatiale des Rayonnements (CESR) in Toulouse, France. It will measure radiation levels in the Earth’s radiation belts and from solar flares, radiation that can perturb the sensitive CCD detectors of the main science instruments. X-3 has been used to argue that accretion in this high-mass X-ray binary and black hole candidate is by Roche lobe overflow rather than by wind accretion. The future We can only hope here to have whetted the appetite for the feast of new results and insight that will follow the opening of the frontier of high-resolution X-ray spectroscopy. Already there are indications of exciting new results such as X-ray spectroscopy of the twin-jet source SS433 revealing a forest of X-ray emission lines, including components associated with the relativistic jets. And deep X-ray imaging of globular clusters is finally helping to resolve the variety of populations that are present. We already know of a dozen or so luminous X-ray binaries associated with globular clusters (in itself an indication of their enhanced formation rates in the dense stellar environments of the core), and one of these (M15) has now been shown to contain two bright X-ray sources, separated by only 2.7 arcsec. But it is the study of the faint X-ray source population that Chandra and XMM-Newton are contributing to most, as their sensitivity allows them to reach almost the level of normal stellar coronal activity as displayed by the Sun! ● Prof. Phil Charles, University of Southampton; Prof. Andy Fabian, Institute of Astronomy, University of Cambridge. This article was based on the RAS Discussion Meeting on Advances in X-ray astronomy, reported in The Observatory. December 2001 Vol 42