arXiv subjects

W. N. Brandt

Publications and source records attributed to W. N. Brandt.

At least 487 records · Page 27Linked to original sources

arXiv2005

The X-ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei

We perform a detailed investigation of moderate-to-high quality X-ray spectra of ten of the most luminous active galactic nuclei (AGNs) known at z>4 (up to z~6.28). This study includes five new XMM observations and five archived X-ray observations (four by XMM and one by Chandra). We find that the X-ray power-law photon indices of our sample, composed of eight radio-quiet sources and two that are moderately radio loud, are not significantly different from those of lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen column densities, N_H<~10^{22}-10^{23} cm^{-2}, indicate that these AGNs are not significantly absorbed. A joint fit performed on our eight radio-quiet sources, with a total of ~7000 photons, constrains the mean photon index of z>4 radio-quiet AGNs to Gamma=1.97^{+0.06}_{-0.04}, with no detectable intrinsic dispersion from source to source. We also obtain a strong constraint on the mean intrinsic column density, N_H<~3x10^{21} cm^{-2}, showing that optically selected radio-quiet AGNs at z>4 are, on average, not more absorbed than their lower-redshift counterparts. All this suggests that the X-ray production mechanism and the central environment in radio-quiet AGNs have not significantly evolved over cosmic time. The mean equivalent width of a putative neutral narrow Fe Ka line is constrained to be <~190 eV, and similarly we place constraints on the mean Compton reflection component (R<~1.2). None of the AGNs varied on short (~1 hr) timescales, but on longer timescales (months-to-years) strong variability is observed in four of the sources. In particular, the X-ray flux of the z=5.41 radio-quiet AGN SDSS 0231-0728 dropped by a factor of ~4 over a rest-frame period of 73 d. This is the most extreme X-ray variation observed in a luminous z>4 radio-quiet AGN.

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arXiv2005

Deep Extragalactic X-ray Surveys

Deep surveys of the cosmic X-ray background are reviewed in the context of observational progress enabled by the Chandra X-ray Observatory and the X-ray Multi-Mirror Mission-Newton. The sources found by deep surveys are described along with their redshift and luminosity distributions, and the effectiveness of such surveys at selecting active galactic nuclei (AGN) is assessed. Some key results from deep surveys are highlighted including (1) measurements of AGN evolution and the growth of supermassive black holes, (2) constraints on the demography and physics of high-redshift AGN, (3) the X-ray AGN content of infrared and submillimeter galaxies, and (4) X-ray emission from distant starburst and normal galaxies. We also describe some outstanding problems and future prospects for deep extragalactic X-ray surveys.

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arXiv2005

A Remarkable Low-Mass X-ray Binary within 0.1 pc of the Galactic Center

Recent X-ray and radio observations have identified a transient low-mass X-ray binary (LMXB) located only 0.1 pc in projection from the Galactic center, CXOGC J174540.0-290031. In this paper, we report the detailed analysis of X-ray and infrared observations of the transient and its surroundings. Chandra bservations detect the source at a flux of F_X = 2e-12 erg cm^-2 s^-1 (2-8 keV). After accounting for absorption both in the interstellar medium and in material local to the source, the implied luminosity of the source is only L_X = 4e34 erg/s (2-8 keV; D=8 kpc). However, the diffuse X-ray emission near the source also brightened by a factor of 2. The enhanced diffuse X-ray emission lies on top of a known ridge of dust and ionized gas that is visible infrared images. We interpret the X-ray emission as scattered flux from the outburst, and determine that the peak luminosity of CXOGC J174540.0-290031 was >2e36 erg/s. We suggest that the relatively small observed flux results from the fact that the system is observed nearly edge-on, so that the accretion disk intercepts most of the flux emitted along our line of sight. We compare the inferred peak X-ray luminosity to that of the radio jet. The ratio of the X-ray to radio luminosities, L_X/L_R<1e4, is considerably smaller than in other known LMXBs (> 1e5). This is probably because the jets are radiating with unusually high efficiency at the point where they impact the surrounding interstellar medium. This hypothesis is supported by a comparison with mid-infrared images of the surrounding dust. Finally, we find that the minimum power required to produce the jet, L_jet~1e37 erg/s, is comparable to the inferred peak X-ray luminosity. This is the most direct evidence yet obtained that LMXBs accreting at low rates release about half of their energy as jets.

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arXiv2005

Rapid Growth of Black Holes in Massive Star-Forming Galaxies

The tight relationship between the masses of black holes and galaxy spheroids in nearby galaxies implies a causal connection between the growth of these two components. Optically luminous quasars host the most prodigious accreting black holes in the Universe and can account for >30% of the total cosmological black-hole growth. As typical quasars are not, however, undergoing intense star formation and already host massive black holes [>10^(8) M(Sun)], there must have been an earlier pre-quasar phase when these black holes grew [mass range ~10^(6)-10^(8) M(Sun)]. The likely signature of this earlier stage is simultaneous black-hole growth and star formation in distant (i.e., z>1; >8 billion light years away) luminous galaxies. Here we report ultra-deep X-ray observations of distant star-forming galaxies that are bright at submillimetre wavelengths. We find that the black holes in these galaxies are growing almost continuously throughout periods of intense star formation. This activity appears to be more tightly associated with these galaxies than any other coeval galaxy populations. We show that the black-hole growth from these galaxies is consistent with that expected for the pre-quasar phase.

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arXiv2005

X-ray Lighthouses of the High-Redshift Universe. II. Further Snapshot Observations of the Most Luminous z>4 Quasars with Chandra

We report on Chandra observations of a sample of 11 optically luminous (Mb<-28.5) quasars at z=3.96-4.55 selected from the Palomar Digital Sky Survey and the Automatic Plate Measuring Facility Survey. These are among the most luminous z>4 quasars known and hence represent ideal witnesses of the end of the "dark age ''. Nine quasars are detected by Chandra, with ~2-57 counts in the observed 0.5-8 keV band. These detections increase the number of X-ray detected AGN at z>4 to ~90; overall, Chandra has detected ~85% of the high-redshift quasars observed with snapshot (few kilosecond) observations. PSS 1506+5220, one of the two X-ray undetected quasars, displays a number of notable features in its rest-frame ultraviolet spectrum, the most prominent being broad, deep SiIV and CIV absorption lines. The average optical-to-X-ray spectral index for the present sample ( =-1.88+/-0.05) is steeper than that typically found for z>4 quasars but consistent with the expected value from the known dependence of this spectral index on quasar luminosity. We present joint X-ray spectral fitting for a sample of 48 radio-quiet quasars in the redshift range 3.99-6.28 for which Chandra observations are available. The X-ray spectrum (~870 counts) is well parameterized by a power law with Gamma=1.93+0.10/-0.09 in the rest-frame ~2-40 keV band, and a tight upper limit of N_H~5x10^21 cm^-2 is obtained on any average intrinsic X-ray absorption. There is no indication of any significant evolution in the X-ray properties of quasars between redshifts zero and six, suggesting that the physical processes of accretion onto massive black holes have not changed over the bulk of cosmic time.

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arXiv2004

An Overabundance of Transient X-ray Binaries within 1 pc of the Galactic Center

During five years of Chandra observations, we have identified seven X-ray transients located within 23 pc of Sgr A*. These sources each vary in luminosity by more than a factor of 10, and have peak X-ray luminosities greater than 5e33 erg/s, which strongly suggests that they are accreting black holes or neutron stars. The peak luminosities of the transients are intermediate between those typically considered outburst and quiescence for X-ray binaries. Remarkably four of these transients lie within only 1 pc of Sgr A*. This implies that, compared to the numbers of similar systems located between 1 and 23 pc, transients are over-abundant by a factor of 20 per unit stellar mass within 1 pc of Sgr A*. It is likely that the excess transient X-ray sources are low-mass X-ray binaries that were produced, as in the cores of globular clusters, by three-body interactions between binary star systems and either black holes or neutron stars that have been concentrated in the central parsec through dynamical friction. Alternatively, they could be high-mass X-ray binaries that formed among the young stars that are present in the central parsec.

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arXiv2004

The unresolved hard X-ray background: the missing source population implied by the Chandra and XMM-Newton deep fields

We extend our earlier work on X-ray source stacking in the deep XMM-Newton observation of the Lockman Hole, to the 2 Ms Chandra Deep Field North and the 1 Ms Chandra Deep Field South. The XMM-Newton work showed the resolved fraction of the X-ray background to be ~80-100 per cent at <2 keV but this decreased to only ~50 per cent above ~8 keV. The CDF-N and CDF-S probe deeper, and are able to fill-in some of the missing fraction in the 4-6 keV range, but the resolved fraction in the 6-8 keV band remains only ~60 per cent, confirming the trend seen with XMM-Newton. The missing X-ray background component has a spectral shape that is consistent with a population of highly obscured AGN at redshifts \~0.5-1.5 and with absorption column densities of ~10^23 - 10^24 cm^-2.

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arXiv2004

X-rays from the First Massive Black Holes

We briefly review some recent results from Chandra and XMM-Newton studies of the highest redshift (z > 4) active galactic nuclei (AGNs). Specific topics covered include radio-quiet quasars, radio-loud quasars, moderate-luminosity AGNs in X-ray surveys, and future prospects. No significant changes in AGN X-ray emission properties have yet been found at high redshift, indicating that the small-scale X-ray emission regions of AGNs are insensitive to the dramatic changes on larger scales that occur from z = 0-6. X-ray observations are also constraining the environments of high-redshift AGNs, relevant emission processes, and high-redshift AGN demography.

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arXiv2004

A Chandra observation of the z=2.285 galaxy FSC10214+4724: Evidence for a Compton-thick quasar?

We present a ~20 ks Chandra ACIS-S observation of the strongly lensed z=2.285 ultra-luminous infrared galaxy FSC10214+4724. Although this observation achieves the equivalent sensitivity of an up-to ~4 Ms Chandra exposure (when corrected for gravitational lensing), the rest-frame 1.6-26.3 keV emission from FSC10214+4724 is weak (L_X~2E42 erg/s for a lensing boost of ~100); a significant fraction of this X-ray emission appears to be due to vigorous star-formation activity. If FSC10214+4724 hosts a quasar, as previously suggested, then it must be obscured by Compton-thick material. We compare FSC10214+4724 to high-redshift SCUBA galaxies and discuss the X-ray identification of Compton-thick AGNs at high redshift.

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arXiv2004

X-ray Insights Into Interpreting CIV Blueshifts and Optical/UV Continua

We present 0.5-8.0 keV Chandra observations of six bright quasars that represent extrema in quasar emission-line properties -- three quasars each with small and large blueshifts of the CIV emission line with respect to the systemic redshift of the quasars. Supplemented with seven archival Chandra observations of quasars that met our selection criteria, we investigate the origin of this emission-line phenomenon in the general context of the structure of quasars. We find that the quasars with the largest CIV blueshifts show evidence, from joint-spectral fitting, for intrinsic X-ray absorption (N_H ~ 10^22 cm^-2). Given the lack of accompanying CIV absorption, this gas is likely to be highly ionized, and may be identified with the shielding gas in the disk-wind paradigm. Furthermore, we find evidence for a correlation of alpha_uv, the ultraviolet spectral index, with the hardness of the X-ray continuum; an analysis of independent Bright Quasar Survey data from the literature supports this conclusion. This result points to intrinsically red quasars having systematically flatter hard X-ray continua without evidence for X-ray absorption. We speculate on the origins of these correlations of X-ray properties with both CIV blueshift and alpha_uv and discuss the implications for models of quasar structure.

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arXiv2004

X-ray Properties of Lyman Break Galaxies in the Great Observatories Origins Deep Survey

We constrain the X-ray emission properties of Lyman break galaxies (LBGs) at z ~ 3-6 using the 2 Ms Chandra Deep Field-North and 1 Ms Chandra Deep Field-South. Large samples of LBGs were discovered using HST as part of the Great Observatories Origins Deep Survey (GOODS). Deep optical and X-ray imaging over the GOODS fields have allowed us to place the most significant constraints on the X-ray properties of LBGs to date. Mean X-ray properties of 449, 1734, 629, and 247 LBGs with z ~ 3, 4, 5, and 6, respectively, were determined using stacking techniques. When stacked, we detect X-ray emission from LBGs at z ~ 3 (7 sigma) and from an optically bright subset (brightest 25%) of LBGs at z ~ 4 (3 sigma); the latter is the highest redshift detection yet for "normal" galaxies in the X-ray band. The effective exposure times for these stacked observations are 0.7 and 0.5 Gs, respectively. The derived average rest-frame 2.0-8.0 keV luminosities are 1.5 and 1.4 x 10^41 erg s^-1, respectively. X-ray emission from these LBGs is likely due to high mass X-ray binaries (HMXBs) and Type II supernovae; the corresponding star formation rates are 85-240 M_sol yr^-1. The X-ray to B-band mean luminosity ratio (Lx/LB) at z~3 is somewhat elevated with respect to that measured for starburst galaxies in the local Universe. When stacking full samples of LBGs at z ~ 4, 5, and 6 we do not obtain significant detections and derive rest-frame 2.0-8.0 keV luminosity upper limits of 0.9, 2.8, and 7.1 x 10^41 erg s^-1, respectively. These upper limits constrain any widespread AGN activity in these objects to be modest at best. Furthermore, we find that ~ 0.5% of our LBGs from z ~ 3-6 are detected individually in the X-ray band. Continued ...

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arXiv2004

The narrow-line quasar NAB 0205+024 observed with XMM-Newton

The XMM-Newton observation of the narrow-line quasar NAB 0205+024 reveals three striking differences since it was last observed in the X-rays with ASCA. Firstly, the 2-10 keV power-law is notably steeper. Secondly, a hard X-ray flare is detected, very similar to that seen in I Zw 1. Thirdly, a strong and broad emission feature is detected with the bulk of its emission redward of 6.4 keV, and extending down to ~5 keV in the rest frame. The most likely explanation for the broad feature is neutral iron emission emitted from a narrow annulus of an accretion disc close to the black hole. The hard X-ray flare could be the mechanism which illuminates this region of the disc, allowing for the emission line to be detected. The combination of effects can be understood in terms of the `thundercloud' model proposed by Merloni & Fabian.

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arXiv2004

Long-term spectral changes in the partial-covering candidate NLS1 1H 0707-495

We compare two XMM-Newton observations of the Narrow-Line Seyfert 1 galaxy 1H 0707-495 separated by two years, and discuss the results in terms of the partial-covering phenomenon. The second longer observation once again displays a sharp (< 200 eV) spectral drop above 7 keV; however, in contrast to the first observation, the edge depth and energy have changed significantly. In addition to changes in the edge parameters, the high-energy spectrum appears steeper. The changes in the high-energy continuum can be adequately explained in terms of a partial-covering absorber outflowing from the central region. The low-energy spectrum also shows significant long-term spectral variability, including: a substantial increase in the disc temperature; detection of a ~0.9 keV emission feature; and the presence of warm absorption that was also detected during the ASCA mission, but not seen during the first XMM-Newton observation. The large increase in disc temperature, and more modest rise in luminosity, can be understood if we consider the slim-disc model for 1H 0707-495. In addition, the higher disc luminosity could be the driving force behind the outflow scenario and the re-appearance of a warm medium during the second XMM-Newton observation.

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arXiv2004

Obscured AGN and the X-ray, Optical and Far-Infrared Number Counts of AGN in the GOODS Fields

The deep X-ray, optical, and far-infrared fields that constitute GOODS are sensitive to obscured AGN (N_H>10^{22} cm^{-2}) at the quasar epoch (z~2-3), as well as to unobscured AGN as distant as z~7. Luminous X-ray emission is a sign of accretion onto a supermassive black hole and thus reveals all but the most heavily obscured AGN. We combine X-ray luminosity functions with appropriate spectral energy distributions for AGN to model the X-ray, optical and far-infrared flux distributions of the X-ray sources in the GOODS fields. A simple model based on the unified paradigm for AGN, with ~3 times as many obscured AGN as unobscured, successfully reproduces the z-band flux distributions measured in the deep HST ACS observations on the GOODS North and South fields. This model is also consistent with the observed spectroscopic and photometric redshift distributions once selection effects are considered. The previously reported discrepancy between observed spectroscopic redshift distributions and the predictions of population synthesis models for the X-ray background can be explained by bias against the most heavily obscured AGN generated both by X-ray observations and the identification of sources via optical spectroscopy. We predict the AGN number counts for Spitzer MIPS 24 um and IRAC 3.6-8 um observations in the GOODS fields, which will verify whether most AGN in the early Universe are obscured in the optical. Such AGN should be very bright far-infrared sources and include some obscured AGN missed even by X-ray observations.

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arXiv2004

The Fall of AGN and the Rise of Star-Forming Galaxies: A Close Look at the Chandra Deep Field X-ray Number Counts

We investigate the X-ray number counts in the 1-2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into Active Galactic Nuclei (AGN), star-forming galaxies, and Galactic stars based on several criteria. We find that AGN continue to dominate the number counts in the 0.5-2.0 keV and 2-8 keV bands. At flux limits of ~2.5e-17 erg cm-2 s-1 (0.5-2.0 keV) and ~1.4e-16 erg cm-2 s-1 (2-8 keV), the overall AGN source densities are 7166 (+304/-292) and 4558 (+216/-207) sources deg-2, respectively; these are factors of ~10-20 higher than found in the deepest optical spectroscopic surveys. While still a minority, the number counts of star-forming galaxies climb steeply such that they eventually achieve source densities of 1727 (+187/-169) and 711 (+270/-202) sources deg-2 at the CDF 0.5-2.0 keV and 2-8 keV flux limits, respectively. Adopting recent XRB flux densities measurements, the CDFs resolve a total of 89.5% (+5.9%/-5.7%) and 86.9% (+6.6%/-6.3%) of the extragalactic 0.5-2.0 keV and 2-8 keV XRBs, respectively. Extrapolation of the number-count slopes can easily account for the entire 0.5-2.0 keV and 2-8 keV XRBs to within statistical errors. We also revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent except for sources in the 2-8 keV band below F(2-8 keV)~1e-15 erg cm-2 s-1, where deviations gradually increase to ~3.9 sigma.

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arXiv2004

X-ray Surveys and Wide-Field Optical/Near-Infrared Imaging with the Joint Dark Energy Mission

I briefly describe a few important scientific issues that could be addressed effectively via the combination of data from JDEM and X-ray missions. The topics covered are largely focused on active galactic nuclei (AGN) and include (1) the selection of AGN via X-ray emission and optical variability, (2) nuclear outbursts in galaxies due to transient fueling of their supermassive black holes, (3) moderate-luminosity AGN at high redshift (z > 4) found via application of "dropout" techniques to X-ray sources, and (4) the host-galaxy morphologies of X-ray selected AGN. I also describe the substantial challenges to obtaining wide-field X-ray data with sufficient sensitivity to complement JDEM properly.

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arXiv2004

The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center

We examine the X-ray spectra and variability of the sample of X-ray sources with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We find that the spectra of the point sources near the Galactic center are very hard between 2--8 keV, even after accounting for absorption. When modeled as power laws the median photon index is Gamma=0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT>8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly-ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source, or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact to due local absorption that partially-covers the X-ray emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short time scales or changes in their mean X-ray flux on long time scales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.

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arXiv2004

First Simultaneous NIR/X-ray Detection of a Flare from SgrA*

We report on the first simultaneous near-infrared/X-ray detection of the Sgr A* counterpart which is associated with the massive black hole at the center of the Milky Way. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory. We also report on quasi-simultaneous observations at a wavelength of 3.4 mm using the Berkeley-Illinois-Maryland Association (BIMA) array. A flare was detected in the X-domain with an excess 2-8 keV luminosity of about 6$\times10^{33}$ erg/s. A fading flare of Sgr A* with $>$2 times the interim-quiescent flux was also detected at the beginning of the NIR observations, that overlapped with the fading part of the X-ray flare. Compared to 8-9 hours before the NIR/X-ray flare we detected a marginally significant increase in the millimeter flux density of Sgr A* during measurements about 7-9 hours afterwards. We find that the flaring state can be conveniently explained with a synchrotron self-Compton model involving up-scattered sub-millimeter photons from a compact source component, possibly with modest bulk relativistic motion. The size of that component is assumed to be of the order of a few times the Schwarzschild radius. The overall spectral indices $α_{NIR/X-ray}$ ($S_ν$$\propto$$ν^{-α}$) of both states are quite comparable with a value of $\sim$1.3. Since the interim-quiescent X-ray emission is spatially extended, the spectral index for the interim-quiescent state is probably only a lower limit for the compact source Sgr A*. A conservative estimate of the upper limit of the time lag between the ends of the NIR and X-ray flare is of the order of 15 minutes.

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