G. Chartas

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

The nuclei of most galaxies are now believed to harbour supermassive black holes. The motions of stars in the central few light years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 × 106 solar masses (refs 2, 3). This object is spatially coincident with the compact radio source Sagittarius A* (Sgr A*) at the dynamical centre of the Galaxy, and the radio emission is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. Sgr A* is, however, much fainter than expected at all wavelengths, especially in X-rays, which has cast some doubt on this model. The first strong evidence for X-ray emission was found only recently. Here we report the discovery of rapid X-ray flaring from the direction of Sgr A*, which, together with the previously reported steady X-ray emission, provides compelling evidence that the emission is coming from the accretion of gas onto a supermassive black hole at the Galactic Centre.

The Chandra Deep Survey of the Hubble Deep Field-North Area. II. Results from the Caltech Faint Field Galaxy Redshift Survey Area*

A deep X-ray survey of the Hubble Deep Field-North (HDF-N) and its environs is performed using data collected by the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Currently a 221.9 ks exposure is available, the deepest ever presented, and here we give results on X-ray sources located in the 86 × 87 area covered by the Caltech Faint Field Galaxy Redshift Survey (the Caltech area). This area has (1) deep photometric coverage in several optical and near-infrared bands; (2) extensive coverage at radio, submillimeter, and mid-infrared wavelengths; and (3) some of the deepest and most complete spectroscopic coverage ever obtained. It is also where the X-ray data have the greatest sensitivity; the minimum detectable fluxes in the 0.5-2 keV (soft) and 2-8 keV (hard) bands are ≈1.3 × 10-16 and ≈6.5 × 10-16 ergs cm-2 s-1, respectively. More than ≈80% of the extragalactic X-ray background in the hard band is resolved. The 82 Chandra sources detected in the Caltech area are correlated with more than 25 multiwavelength source catalogs, and the results of these correlations as well as spectroscopic follow-up results obtained with the Keck and Hobby-Eberly Telescopes are presented. All but nine of the Chandra sources are detected optically with R 26.5. Redshifts are available for 39% of the Chandra sources, including 96% of the sources with R 5.0) objects. A total of 16 of the 67 1.4 GHz μJy sources in the Caltech area are detected in the X-ray band, and the detection rates for starburst-type and AGN-candidate μJy sources are comparable. Only two of the 17 red, optically faint (I > 25) μJy sources are detected in X-rays. While many of the starburst-type μJy sources appear to contain obscured active galactic nuclei (AGNs), the Chandra data are consistent with the majority of the μJy radio sources being powered by star formation. A total of 11 of the ≈100 ISO mid-infrared sources found in and near the HDF-N are detected in X-rays. In the HDF-N itself, where both the infrared coverage and the X-ray coverage are deepest, it is notable that six of the eight Chandra sources are detected by ISO; most of these are known to be AGNs where the X-ray and infrared detections reveal both the direct and indirect accretion power being generated. The high X-ray-to-infrared matching rate bodes well for future sensitive infrared observations of faint X-ray sources. Four of the 33 very red objects that have been identified in the Caltech area are detected in X-rays; these four are among our hardest Chandra sources, and we argue that they contain moderately luminous obscured AGNs. Overall, however, the small Chandra detection fraction suggests a relatively small AGN content in the optically selected very red object population. A stacking analysis of the very red objects not detected individually by Chandra yields a soft-band detection with an average soft-band X-ray flux of ≈1.9 × 10-17 ergs cm-2 s-1; the observed emission may be associated with the hot interstellar media of moderate-redshift elliptical galaxies. Constraints on AGN candidates, extended X-ray sources, and Galactic objects in the Caltech area are also presented.

The Chandra Deep Field North Survey. V. 1 Ms Source Catalogs

An extremely deep X-ray survey (≈1 Ms) of the Hubble Deep Field North (HDF-N) and its environs (≈450 arcmin2) has been performed with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. This is one of the two deepest X-ray surveys ever performed; for point sources near the aim point, it reaches 0.5–2.0 and 2–8 keV flux limits of ≈3 × 10-17 and ≈2 × 10-16 ergs cm-2 s-1, respectively. Here we provide source catalogs, along with details of the observations, data reduction, and technical analysis. Observing conditions, such as background, were excellent for almost all of the exposure. We have detected 370 distinct point sources: 360 in the 0.5–8.0 keV band, 325 in the 0.5–2.0 keV band, 265 in the 2–8 keV band, and 145 in the 4–8 keV band. Two new Chandra sources in the HDF-N itself are reported and discussed. Source positions are accurate to within 06–17 (at ≈90% confidence), depending mainly on the off-axis angle. We also detect two highly significant extended X-ray sources and several other likely extended X-ray sources. We present basic number count results for sources located near the center of the field. Source densities of 7100 deg-2 (at 4.2 × 10-17 ergs cm-2 s-1) and 4200 deg-2 (at 3.8 × 10-16 ergs cm-2 s-1) are observed in the soft and hard bands, respectively.

CHANDRA Detects Relativistic Broad Absorption Lines from APM 08279+5255

We report the discovery of X-ray broad absorption lines (BALs) from the BAL quasar APM 08279+5255 originating from material moving at relativistic velocities with respect to the central source. The large flux magnification by a factor of ~100 provided by the gravitational lens effect combined with the large redshift (z = 3.91) of the quasar have facilitated the acquisition of the first high signal-to-noise X-ray spectrum of a quasar containing X-ray BALs. Our analysis of the X-ray spectrum of APM 08279+5255 places the rest-frame energies of the two observed absorption lines at 8.1 and 9.8 keV. The detection of each of these lines is significant at a greater than 99.9% confidence level based on the F-test. Assuming that the absorption lines are from Fe XXV Kα, the implied bulk velocities of the X-ray BALs are ~0.2c and ~0.4c, respectively. The observed high bulk velocities of the X-ray BALs combined with the relatively short recombination timescales of the X-ray-absorbing gas imply that the absorbers responsible for the X-ray BALs are located at radii of 2 × 1017 cm, within the expected location of the UV absorber. With this implied geometry, the X-ray gas could provide the necessary shielding to prevent the UV absorber from being completely ionized by the central X-ray source, consistent with hydrodynamical simulations of line-driven disk winds. Estimated mass-outflow rates for the gas creating the X-ray BALs are typically less than a solar mass per year. Our spectral analysis also indicates that the continuum X-ray emission of APM 08279+5255 is consistent with that of a typical radio-quiet quasar with a spectral slope of Γ = 1.72.

The X-Ray Properties of the Most Luminous Quasars from the Sloan Digital Sky Survey

This article studies the X-ray properties of a representative sample of 59 of the most optically luminous quasars in the universe spanning a redshift range of z ≈ 1.5-4.5.

XMM-Newton Reveals the Quasar Outflow in PG 1115+080

We report on an observation of the broad absorption line (BAL) quasar PG 1115+080 performed with the XMM-Newton observatory. Spectral analysis reveals the second case of a relativistic X-ray-absorbing outflow in a BAL quasar. The first case was revealed in a recent observation of APM 08279+5255 with the Chandra X-Ray Observatory. As in the case of APM 08279+5255, the observed flux of PG 1115+080 is greatly magnified by gravitational lensing. The relatively high redshift (z = 1.72) of the quasar places the redshifted energies of resonant absorption features in a sensitive portion of the XMM-Newton spectral response. The spectrum indicates the presence of complex low-energy absorption in the 0.2-0.6 keV observed energy band and high-energy absorption in the 2-5 keV observed energy band. The high-energy absorption is best modeled by two Gaussian absorption lines with rest-frame energies of 7.4 and 9.5 keV. Assuming that these two lines are produced by resonant absorption due to Fe XXV Kα, we infer that the X-ray absorbers are outflowing with velocities of ~ 0.10c and ~ 0.34c, respectively. We have detected significant variability of the energies and widths of the X-ray BALs in PG 1115+080 and APM 08279+5255 over timescales of 19 and 1.8 weeks (proper time), respectively. The BAL variability observed from APM 08279+5255 supports our earlier conclusion that these absorbers are most likely launched at relatively small radii of 1016(Mbh/M8)1/2 cm. A comparison of the ionization properties and column densities of the low-energy and high-energy absorbers indicates that these absorbers are likely distinct; however, higher spectral resolution is needed to confirm this result. Finally, we comment on prospects for constraining the kinematic and ionization properties of these X-ray BALs with the next generation of X-ray observatories.

X-Ray Spectroscopy of Quasi-Stellar Objects with Broad Ultraviolet Absorption Lines

For the population of quasi-stellar objects (QSOs) with broad ultraviolet absorption lines, we are just beginning to accumulate X-ray observations with enough counts for spectral analysis at CCD resolution. From a sample of eight QSOs (including four broad absorption line [BAL] QSOs and three mini-BAL QSOs) with ASCA or Chandra spectra with more than 200 counts, general patterns are emerging. Their power-law X-ray continua are typical of normal QSOs with Γ ≈ 2.0, and the signatures of a significant column density [NH ≈ (0.1-4) × 1023 cm-2] of intrinsic, absorbing gas are clear. Correcting the X-ray spectra for intrinsic absorption recovers a normal ultraviolet-to-X-ray flux ratio, indicating that the spectral energy distributions of this population are not inherently anomalous. In addition, a large fraction of our sample shows significant evidence for complexity in the absorption. The subset of BAL QSOs with broad Mg II absorption apparently suffers from Compton-thick absorption completely obscuring the direct continuum in the 2-10 keV X-ray band, complicating any measurement of their intrinsic X-ray spectral shapes.

High-Resolution X-Ray Spectroscopy and Modeling of the Absorbing and Emitting Outflow in NGC 3783

The high-resolution X-ray spectrum of NGC 3783 shows several dozen absorption lines and a few emission lines from the H-like and He-like ions of O, Ne, Mg, Si, and S, as well as from Fe XVII-Fe XXIII L-shell transitions. We have reanalyzed the Chandra HETGS spectrum using better flux and wavelength calibrations, along with more robust methods. Combining several lines from each element, we clearly demonstrate the existence of the absorption lines and determine that they are blueshifted relative to the systemic velocity by -610 ? 130 km s-1. We find the Ne absorption lines in the High-Energy Grating spectrum to be resolved with FWHM = 840 km s-1; no other lines are resolved. The emission lines are consistent with being at the systemic velocity. We have used regions in the spectrum where no lines are expected to determine the X-ray continuum, and we model the absorption and emission lines using photoionized-plasma calculations. The model consists of two absorption components, with different covering factors, which have an order-of-magnitude difference in their ionization parameters. The two components are spherically outflowing from the active galactic nucleus, and thus contribute to both the absorption and the emission via P Cygni profiles. The model also clearly requires O VII and O VIII absorption edges. The low-ionization component of our model can plausibly produce UV absorption lines with equivalent widths consistent with those observed from NGC 3783. However, we note that this result is highly sensitive to the unobservable UV to X-ray continuum, and the available UV and X-ray observations cannot firmly establish the relationship between the UV and X-ray absorbers. We find good agreement between the Chandra spectrum and simultaneous ASCA and RXTE observations. The 1 keV deficit previously found when modeling ASCA data probably arises from iron L-shell absorption lines not included in previous models. We also set an upper limit on the FWHM of the narrow Fe K? emission line of 3250 km s-1. This is consistent with this line originating outside the broad-line region, possibly from a torus.

A Survey of Extended Radio Jets with Chandra and the Hubble Space Telescope

We present the results from an X-ray and optical survey of a sample of 17 radio jets in AGN performed with Chandra and HST. The sample was selected from the radio and is unbiased toward detection at shorter wavelengths, but preferentially it includes beamed sources. We find that X-ray emission is common on kpc-scales, with over half radio jets exhibiting at least one X-ray knot on the Chandra images. The distributions of the radio-to-X-ray and radio-to-optical spectral indices for the detected jets are similar to the limits for the non-detections,suggesting all bright radio jets have X-ray counterparts which will be visible in longer observations. Comparing the radio and X-ray morphologies shows that the majority of the X-ray jets have structures that closely map the radio. Analysis of the SED of the jet knots suggest the knots in which the X-ray and radio morphologies track each other produce X-rays by IC scattering of the Cosmic Microwave Background. The remaining knots produce X-rays by the synchrotron process. Spectral changes are detected along the jets, with the ratio of the X-ray-to-radio and optical-to-radio flux densities decreasing from the inner to the outer regions. This suggests the presence of an additional contribution to the X-ray flux in the jets inner part, either from synchrotron or IC of the stellar light. Alternatively, in a pure IC/CMB scenario, the plasma decelerates as it flows from the inner to the outer regions. Finally, the X-ray spectral indices for the brightest knots are flat, indicating that the bulk of the luminosity of the jets is emitted at GeV energies, and raising the interesting possibility of future detections with GLAST.

An Exploratory Chandra Survey of a Well-defined Sample of 35 Large Bright Quasar Survey Broad Absorption Line Quasars

We present 4–7 ks Chandra observations of 35 broad absorption line (BAL) quasars from the Large Bright Quasar Survey, the largest sample of sensitive, 0.5–8.0 keV X-ray observations of this class of quasars to date. The limitedrangesinbothredshift(z ¼ 1:42 2:90)andultravioletluminosity(afactorof � 12)ofthesamplealsomake itrelativelyuniform.Of 35 targets,27 aredetected fora detectionfractionof 77%, and we confirmprevious studies that find BAL quasars to be generally X-ray-weak. Five of the eight nondetections are known low-ionization BAL quasars, confirming reports of extreme X-ray weakness in this subset (� 10% of optically selected BAL quasars). ThoseBALquasarswiththehardestX-rayspectraarealsotheX-rayweakest,consistentwiththeinterpretationthat intrinsic absorption is the primary cause of X-ray weakness in this class of quasars as a whole. Furthermore, the observed trend is not consistent with simple neutral absorption, supporting findings from spectroscopic observations of individual targets that BAL quasars typically exhibit complex X-ray absorption (e.g., partially covering orionizedabsorbers).AssumingnormalquasarX-raycontinuaandusingthehard-band(observed-frame2–8keV) X-ray flux to ‘‘correct’’ for the effects of intrinsic absorption at softer energies increases the relative X-ray to optical flux ratios to much closer to the range for normal quasars, further indicating that typically neither are highionizationBALquasarsintrinsicallyX-ray-weak,nordotheysufferfromCompton-thickabsorption.Ingeneral,we find no evidence for correlations between X-ray weakness and ultraviolet absorption-line properties, with the exception of a likely correlation between the maximum outflow velocity of C iv absorption and the magnitude of X-ray weakness. We discuss the implications of our results for disk-wind models of BAL outflows in quasars. Subject headingg galaxies: active — quasars: absorption lines — quasars: general — X-rays: galaxies