arXiv subjects
Publications and source records attributed to W. N. Brandt.
The study of the X-ray properties of z>4 quasars has grown substantially over the last few years, mostly thanks to Chandra. Here we focus on (1) the average X-ray properties of an optically selected, luminous sample of nine z>4 Palomar Digital Sky Survey quasars recently observed by Chandra, and (2) the X-ray spectral properties of three z>4 AGNs in the Chandra Deep Field-North (CDF-N) using the 2 Ms exposure.
We briefly report here on our ongoing X-ray spectroscopy and variability analyses of 136 faint X-ray sources discovered in the 2 Ms Chandra Deep Field North observations spanning 1.4x10-15 to 2x10-13 ergs s-1 cm-2. Most sources are adequatley fitted with a simple intrinsic absorption plus power-law model, although the resulting photon index is much lower than the canonical average unabsorbed spectral slope found locally (suggesting additional spectral complexity). Among the 136 sources, 7% show significant Fe (Ka) emission features, with equivalent widths in the range 0.1-1.3 keV. Two of these emission-line sources appear to be Compton thick. For the subset of sources with redshifts, we also find that fewer than 10% are likely to be Compton thick. Finally, ~60% of the sample show signs of long-term variability. This fraction climbs to ~90% for the bright sources (>500 cnts) or BLAGN.
We present the results obtained from exploratory Chandra observations of nine high-redshift (z=4.09-4.51) quasars, selected from among the optically brightest Palomar Digital Sky Survey quasars known. Their broad-band spectral energy distributions are characterized, on average, by steeper alpha_ox values ( =-1.81+/-0.03) than those of lower-redshift, lower-luminosity samples of quasars. We find a significant correlation between AB1450(1+z) magnitude and soft X-ray flux, suggesting that the engine powering the UV and X-ray emission is the same. The joint 2-30 keV rest-frame X-ray spectrum is well parameterized by a Gamma=2.0+/-0.2 power-law model with no evidence for intrinsic absorption (N_H<8.8 10^{21} cm^-2).
We present observations of the intrinsic absorption in the Seyfert 1 galaxy NGC 3783 obtained with the STIS/HST and FUSE. We have coadded multiple STIS and FUSE observations to obtain a high S/N averaged spectrum spanning 905-1730 A. The averaged spectrum reveals absorption in O VI, N V, C IV, N III, C III and the Lyman lines up to LyE in the three blueshifted kinematic components previously detected in the STIS spectrum (at radial velocities of -1320, -724, and -548 km/s). The highest velocity component exhibits absorption in Si IV. We also detect metastable C III* in this component, indicating a high density in this absorber. We separate the individual covering factors of the continuum and emission-line sources as a function of velocity in each kinematic component using the LyA and LyB lines. Additionally, we find that the continuum covering factor varies with velocity within the individual kinematic components, decreasing smoothly in the wings of the absorption by at least 60%. The covering factor of Si IV is found to be less than half that of H I and N V in the high velocity component. Additionally, the FWHM of N III and Si IV are narrower than the higher ionization lines in this component. These results indicate there is substructure within this absorber. We derive a lower limit on the total column (N_H>=10^{19}cm^{-2}) and ionization parameter (U>=0.005) in the low ionization subcomponent of this absorber. The metastable-to-total C III column density ratio implies n_e~10^9 cm^{-3} and an upper limit on the distance of the absorber from the ionizing continuum of R<=8x10^{17} cm.
Understanding the nature of Extremely Red Objects [EROs; (I-K)>=4] is one of the most challenging issues in observational cosmology. Here we report on the X-ray constraints provided by the 2 Ms Chandra Deep Field-North Survey (CDF-N). X-ray emission has been detected from 11 out of 36 EROs (approx 30%). Five of these have hard X-ray emission and appear to be obscured AGNs, while non-AGN emission (star formation or normal elliptical galaxy emission) is likely to be the dominant source of X-rays from the soft X-ray sources detected at the faintest X-ray flux levels.
X-ray absorption and emission lines now serve as powerful diagnostics of the outflows from active galaxies. Detailed X-ray line studies of outflows have recently been enabled for a significant number of active galaxies via the grating spectrometers on Chandra and XMM-Newton. We will review some of the recent X-ray findings on active galaxy outflows from an observational perspective. We also describe some future prospects. X-ray absorption lines from H-like and He-like ions of C, N, O, Ne, Mg, Al, Si, and S are often seen. A wide range of ionization parameter appears to be present in the absorbing material, and inner-shell absorption lines from lower ionization ions, Fe L-shell lines, and Fe M-shell lines have also been seen. The X-ray absorption lines are typically blueshifted relative to the systemic velocity by a few hundred km s^{-1}, and they often appear kinematically consistent with UV absorption lines of C IV, N V, and H I. The X-ray absorption lines can have complex profiles with multiple kinematic components present as well as filling of the absorption lines by emission-line photons. A key remaining uncertainty is the characteristic radial location of the outflowing gas; only after this quantity is determined will it be possible to calculate reliably the amount of outflowing gas and the kinetic luminosity of the outflow.
We present an XMM-Newton observation of the bright, narrow-line, ultrasoft Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parameterised using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionised accretion disc, inverse-Compton scattering of soft disc photons by thermal electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal distribution. We emphasise the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc.
We investigate the relationship between faint X-ray and 1.4 GHz radio source populations detected within 3' of the Hubble Deep Field North using the 1 Ms Chandra and 40 uJy VLA surveys. Within this region, we find that ~42% of the 62 X-ray sources have radio counterparts and ~71% of the 28 radio sources have X-ray counterparts; thus a 40 uJy VLA survey at 1.4 GHz appears to be well-matched to a 1 Ms Chandra observation. Among the different source populations sampled, we find that the majority of the 18 X-ray detected emission-line galaxies (ELGs) have radio and mid-infrared ISOCAM counterparts and appear to be luminous star-forming galaxies at z=0.3-1.3. Importantly, the radio-detected ELGs make up ~35% of the X-ray source population at 0.5-8.0 keV X-ray fluxes between ~(1-5)x10e-16 erg/cm2/s and signal the emergence of the luminous, high-z starburst galaxy population in the X-ray band. We find that the locally-determined correlation between X-ray luminosities and 1.4 GHz radio luminosity densities of the late-type galaxies can easily be extended to include the luminous intermediate-redshift ELGs, suggesting that the X-ray and radio emission processes are generally associated in star-forming galaxies. This result implies that the X-ray emission can be used as an indicator of star formation rate for star-forming galaxies. Finally, we show that there appear to be two statistically distinct types of ISOCAM-detected star-forming galaxies: those with detectable radio and X-ray emission and those without. The latter type may have stronger mid-infrared emission-line features that increase their detectability at mid-infrared wavelengths.
We report the discovery of X-ray broad absorption lines (BALs) from the BALQSO 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 the > 99.9% confidence level based on the F-test. Assuming that the absorption lines are from Fe xxv Kalpha, 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 time-scales of the X-ray absorbing gas imply that the absorbers responsible for the X-ray BALs are located at radii of < 2 x 10^(17) 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 Gamma = 1.72(-0.05,+0.06).
We present the optical, near-infrared, submillimeter, and radio follow-up catalog of the X-ray selected sources from the 1 Ms Chandra observation of the Hubble Deep Field North region. We have B, V, R, I, and z' magnitudes for the 370 X-ray point sources, HK' magnitudes for 276, and spectroscopic redshifts for 182. We present high-quality spectra for 175 of these. The redshift distribution shows indications of structures at z=0.843 and z=1.0175 (also detected in optical surveys) which could account for a part of the field-to-field variation seen in the X-ray number counts. The flux contributions separated into unit bins of redshift show that the z<1 spectroscopically identified sources already contribute about one-third of the total flux in both the hard and soft bands. We find from ratios of the X-ray counts that the X-ray spectra are well-described by absorption of an intrinsic Gamma=1.8 power-law, with log NH values ranging from 21 to 23.7. We estimate that the Chandra sources that produce 87% of the HEAO-A X-ray background (XRB) at 3 keV produce 57% at 20 keV, provided that at high energies the spectral shape of the sources continues to be well-described by a Gamma=1.8 power-law. However, when the Chandra contributions are renormalized to the BeppoSAX XRB at 3 keV, the shape matches fairly well the observed XRB at both energies. Thus, whether a substantial population of as-yet undetected Compton-thick sources is required to completely resolve the XRB above 10 keV depends critically on how the currently discrepant XRB measurements in the 1-10 keV energy range tie together with the higher energy XRB. (Abridged)
In recent years, luminous X-ray outbursts with variability amplitudes as high as ~ 400 have been serendipitously detected from a small number of active and inactive galaxies. These outbursts may result from the tidal disruptions of stars by supermassive black holes as well as accretion-disk instabilities. In order to place the first reliable constraints on the rate of such outbursts in the Universe and test the stellar tidal disruption hypothesis, we have performed a systematic and complete survey for them by cross-correlating ROSAT All-Sky Survey (RASS) and pointed PSPC data. We have detected five galaxies that were in outburst during the RASS, three of which show no signs of nuclear activity; these objects had been reported on individually in previous studies. After making reasonable corrections for the complicated selection effects, we conclude that the rate of large-amplitude X-ray outbursts from inactive galaxies in the local Universe is ~ 9.1 x 10^{-6} galaxy^{-1} yr^{-1}. This rate is consistent with the predicted rate of stellar tidal disruption events in such galaxies. When only the two active galaxies are considered, we find a rate for active galaxies of ~ 8.5 x 10^{-4} galaxy^{-1} yr^{-1}. In order to place tighter constraints on these rates, additional outbursts must be detected.
We are in the process of obtaining a complete sample of exploratory X-ray observations of broad absorption line (BAL) QSOs from the Large Bright Quasar Survey (LBQS). This survey is composed of short (5-7 ks) Chandra ACIS-S3 observations designed to significantly constrain X-ray fluxes and hardness ratios. Our sample avoids the selection biases of some previous surveys while providing X-ray data of similar quality. Of 17 BAL QSOs observed to date, 13 are detected. In general, our results are consistent with absorption as the primary cause of X-ray weakness in BAL QSOs. As found by Green et al. (2001), those BAL QSOs with low-ionization absorption lines are found to be notably X-ray weaker than BAL QSOs with only high ionization lines.
We review X-ray studies of the highest redshift (z > 4) quasars, focusing on recent advances enabled largely by the capabilities of Chandra and XMM-Newton. Included are discussions of basic X-ray population studies, X-ray spectroscopy, high-redshift AGN in X-ray surveys, and future prospects.
We present results from a 900 ks exposure of NGC 3783 with the High-Energy Transmission Grating Spectrometer on board the Chandra X-ray Observatory. The resulting X-ray spectrum has the best combination of signal-to-noise and resolution ever obtained for an AGN. This spectrum reveals absorption lines from H-like and He-like ions of N, O, Ne, Mg, Al, Si, and S. There are also possible absorption lines from H-like and He-like Ar and Ca. We also identify inner-shell absorption from lower-ionization ions such as Si_VII-Si_XII and S_XII-S_XIV. The iron absorption spectrum is very rich; L-shell lines of Fe_XVII-Fe_XXIV are detected, strong complex of M-shell lines, and probable resonance lines from Fe_XXV. The absorption lines are blueshifted relative to the systemic velocity by a mean velocity of -590+-150 km/s. We resolve many of the absorption lines, and their mean FWHM is 820+-280 km/s. We do not find correlations between the velocity shifts or the FWHMs with the ionization potentials of the ions. Most absorption lines show asymmetry, having more extended blue wings than red wings. In O_VII we have resolved this asymmetry to be from an additional absorption system at ~ -1300 km/s. The two X-ray absorption systems are consistent in velocity shift and FWHM with the ones identified in the UV lines of C IV, N V, and H I. Equivalent width measurements for all lines are given and column densities are calculated for several ions. We resolve the narrow Fe_Kαline at 6398.2+-3.3 eV to have a FWHM of 1720+-360 km/s, which suggests that this narrow line may be emitted from the outer part of the broad line region or the inner part of the torus. We also detect a `Compton shoulder' redward of the narrow Fe_Kαline which indicates that it arises in cold, Compton-thick gas.
We report on exploratory Chandra observations of the three highest redshift quasars known (z = 5.82, 5.99, and 6.28), all found in the Sloan Digital Sky Survey. These data, combined with a previous XMM-Newton observation of a z = 5.74 quasar, form a complete set of color-selected, z > 5.7 quasars. X-ray emission is detected from all of the quasars at levels that indicate that the X-ray to optical flux ratios of z ~ 6 optically selected quasars are similar to those of lower redshift quasars. The observations demonstrate that it will be feasible to obtain quality X-ray spectra of z ~ 6 quasars with current and future X-ray missions.
Using the 1 Ms Chandra Deep Field North and 15 micron ISOCAM HDF-N surveys we find a tight correlation between the population of strongly evolving starbursts discovered in faint 15 micron ISOCAM surveys and the apparently normal galaxy population detected in deep X-ray surveys. Up to 100% of the X-ray detected emission-line galaxies (ELGs) have 15 micron counterparts, in contrast to 10-20% of the X-ray detected absorption-line galaxies and AGN-dominated sources. None of the X-ray detected ELGs is detected in the hard band (2-8 keV), and their stacked-average X-ray spectral slope of Gamma=2.0 suggests a low fraction of obscured AGN activity within the X-ray detected ELG population. The characteristics of the z=0.4-1.3 X-ray detected ELGs are consistent with those expected for M82 and NGC 3256-type starbursts; these X-ray detected ELGs contribute ~2% of the 0.5-8.0 keV X-ray background. The only statistical difference between the X-ray detected and X-ray undetected 15 micron selected ELGs is that a much larger fraction of the former have radio emission.
The luminous X-ray binary Circinus X-1 has been observed twice near zero orbital phase using the High-Energy Transmission Grating Spectrometer (HETGS) onboard Chandra. The source was in a high-flux state during a flare for the first observation, and it was in a low-flux state during a dip for the second. Spectra from both flux states show clear P Cygni lines, predominantly from H-like and He-like ion species. These indicate the presence of a high-velocity outflow from the Cir X-1 system which we interpret as an equatorial accretion-disk wind, and from the blueshifted resonance absorption lines we determine outflow velocities of 200 - 1900 km/s with no clear velocity differences between the two flux states. The line strengths and profiles, however, are strongly variable both between the two observations as well as within the individual observations. We characterize this variability and suggest that it is due to both changes in the amount of absorbing material along the line of sight as well as changes in the ionization level of the wind. We also refine constraints on the accretion-disk wind model using improved plasma diagnostics such as the He-like Mg XI triplet, and we consider the possibility that the X-ray absorption features seen from superluminal jet sources can generally be explained via high-velocity outflows.
Chandra has performed a 1.4 Ms survey centred on the Hubble Deep Field-North (HDF-N), probing the X-ray Universe 55-550 times deeper than was possible with pre-Chandra missions. We describe the detected point and extended X-ray sources and discuss their overall multiwavelength (optical, infrared, submillimeter, and radio) properties. Special attention is paid to the HDF-N X-ray sources, luminous infrared starburst galaxies, optically faint X-ray sources, and high-to-extreme redshift AGN. We also describe how stacking analyses have been used to probe the average X-ray emission properties of normal and starburst galaxies at cosmologically interesting distances. Finally, we discuss plans to extend the survey and argue that a 5-10 Ms Chandra survey would lay key groundwork for future missions such as XEUS and Generation-X.