arXiv subjects

W. N. Brandt

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

At least 577 records · Page 32Linked to original sources

arXiv2002

Exotic sources detected in the 1 Ms Chandra Deep Field North Survey

We provide constraints on optically faint (I>=24) X-ray sources, z>6 AGN, Very Red Objects (VROs; I-K>=4), and optically faint radio sources using the 1 Ms Chandra Deep Field North survey. We argue that the majority of the optically faint X-ray sources are obscured AGN at z=1-3. These sources comprise up to 50% of the X-ray detected AGN. Approximately 30% of the optically faint X-ray sources have properties consistent with those expected for z>6 AGN; however we argue that the majority of these sources lie at lower redshifts. The constraint we place on the source density of z>6 AGN (<~0.09 arcmin^{-2}) is below that predicted by some models. Approximately 30% of the optically faint X-ray sources are also VROs. Within the K<=20.1 VRO population, ~15% of the sources are AGN dominated. Conversely, we present evidence suggesting that non-AGN dominated VROs are detected at the flux limit of this 1 Ms Chandra survey. Finally, we show that ~50% of the optically faint radio source population are detected with X-ray emission. The majority of these sources appear to be obscured AGN.

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arXiv2002

2-8 keV X-ray number counts determined from Chandra blank field observations

We present the 2-8 keV number counts from the 1Ms Chandra observation of the Chandra Deep Field North (CDF-N). We combine these with the number counts from a 78ks exposure of the Hawaii Survey Field SSA22 and with the number counts obtained in independent analyses of the CDF-S and the Hawaii Survey Field SSA13 to determine the number counts from 2.10^(-16) to 10^(-13) ergs/cm2/s. Over this flux range the contribution to the X-ray background is 1.1 10^(-11) ergs/cm2/s/square degree. When the contributions above 10^(-13) ergs/cm2/s from BeppoSAX or ASCA observations are included, the total rises to 1.4 10^(-11) ergs/cm2/s/square degree. However, there appears to be substantial field-to-field variation in the counts in excess of the statistical uncertainties. When the statistical and flux calibration uncertainties (in both the background and source measurements) are taken into account, as much as 0.5 10^(-11) ergs/cm2/s/square degree could still be present in an unresolved component.

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arXiv2002

The Luminosity Dependence of UV Absorption in AGN

We describe the results of a survey of the UV absorption properties of the Boroson & Green sample of AGN, which extends from the Seyfert (M_V ~ -21) to the luminous quasar (M_V ~ -27) level. The survey is based mostly on HST archival data available for >~ 1/2 of the 87 sample objects. Our main result is that soft X-ray weak quasars (SXWQs, 10 AGN with alpha_ox<= -2) show the strongest UV absorption at a given luminosity, and their maximum outflow velocity, v_max, is strongly correlated with M_V (r_S=-0.95). This suggests that v_max is largely set by the luminosity, as expected for radiation-pressure driven outflows. Luminous SXWQs have preferentially low [O III] luminosity, which suggests they are physically distinct from unabsorbed AGN, while non-SXWQs with UV absorption are consistent with being drawn from the unabsorbed AGN population. We also find an indication that v_max/v_BLR increases with L/L_Edd, as expected for radiation-pressure driven outflows. This relation and the v_max vs. M_V relation may indicate that the radiation-pressure force multiplier increases with luminosity, and that the wind launching radius in non-SXWQs is ~10 times larger than in SXWQs.

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arXiv2001

X-raying the Ultraluminous Infrared Starburst Galaxy and Broad Absorption Line QSO, Markarian 231, with Chandra

With 40 ks of Chandra ACIS-S3 exposure, new information on both the starburst and QSO components of the X-ray emission of Markarian 231, an ultraluminous infrared galaxy and Broad Absorption Line QSO, has been obtained. The bulk of the X-ray luminosity is emitted from an unresolved nuclear point source, and the spectrum is remarkably hard with the majority of the flux emitted above 2 keV. Most notably, significant nuclear variability (a decrease of ~45% in approximately 6 hours) at energies above 2 keV indicates that Chandra has probed within light hours of the central black hole. Though we concur with Maloney & Reynolds that the direct continuum is not observed, this variability coupled with the 188 eV upper limit on the equivalent width of the FeKalpha emission line argues against the reflection-dominated model put forth by these authors based on their ASCA data. Instead, we favor a model in which a small, Compton-thick absorber blocks the direct X-rays, and only indirect, scattered X-rays from multiple lines of sight can reach the observer. Extended soft, thermal emission encompasses the optical extent of the galaxy and exhibits resolved structure. An off-nuclear X-ray source with a 0.35-8.0 keV luminosity of L_X=7x10^{39} erg s^{-1}, consistent with the ultraluminous X-ray sources in other nearby starbursts, is detected. We also present an unpublished FOS spectrum from the HST archive showing the broad C IV absorption.

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arXiv2001

The Chandra Deep Field North Survey. IX. Extended X-ray Sources

The ~1 Ms Chandra Deep Field North observation is used to study the extended X-ray sources in the region surrounding the Hubble Deep Field North (HDF-N), yielding the most sensitive probe of extended X-ray emission at cosmological distances to date. A total of six such sources are detected, the majority of which align with small numbers of optically bright galaxies. Their angular sizes, band ratios, and X-ray luminosities -- assuming they lie at the same distances as the galaxies coincident with the X-ray emission -- are generally consistent with the properties found for nearby groups of galaxies. One source is notably different and is likely to be a poor-to-moderate X-ray cluster at high redshift (i.e., z > 0.7). We are also able to place strong constraints on the optically detected cluster of galaxies ClG 1236+6215 at z=0.85 and the wide-angle-tail radio galaxy VLA J123725.7+621128 at z~1-2. With rest-frame 0.5--2.0 keV X-ray luminosities of <(3-15)e42 ergs s^{-1}, the environments of both sources are either likely to have a significant deficit of hot intra-cluster gas compared to local clusters of galaxies, or they are X-ray groups. We find the surface density of extended X-ray sources in this observation to be 167 (+97,-67) deg^{-2} at a limiting soft-band flux of approximately 3e-16 ergs s^{-1} cm^{-2}. No evolution in the X-ray luminosity function of clusters is needed to explain this value. (Abridged)

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arXiv2001

The Chandra Deep Field North Survey. X. X-ray Emission from Very Red Objects

The multi-wavelength properties of Very Red Objects (VROs; I-K>=4) are largely unknown as many of these sources are optically faint (I>=24) and undetected at most wavelengths. Here we provide constraints on the X-ray properties of VROs using the 1 Ms Chandra exposure of an 8.4'x8.4' region within the Hawaii Flanking-Field area containing the HDF-N. We find that VROs detected in the hard band (2-8 keV) have flat X-ray spectral slopes (Gamma ~ 0.9) and X-ray properties consistent with those expected from luminous obscured AGN. The fraction of such sources in the K<=20.1 VRO population is ~14%. Conversely, the average X-ray spectral slope of VROs detected in the soft band (0.5-2 keV) but not in the hard band is comparatively steep (Gamma>1.4), and the X-ray emission from these sources is consistent with that expected from less energetic processes (i.e., star formation, low-luminosity AGN activity, normal elliptical galaxy emission); star-formation and low-luminosity AGN activity scenarios are favored in those sources with irregular optical morphologies. Stacking analyses of the X-ray emission from VROs not individually detected at X-ray energies yield significant detections (>=99% confidence) in the soft band and the full band (0.5-8 keV). We find this X-ray emission is produced predominantly by the optically brightest VROs. The simplest explanation of this result is that we have detected the average X-ray emission from non-active VROs with low X-ray-to-optical flux ratios [log(FX/FI) ~ -2]; this is consistent with that expected if the majority of these VROs are ~ M* elliptical galaxies. A number of VROs are also detected with mid-IR (15um) and radio emission, and we provide constraints on the nature of this emission.

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arXiv2001

The Chandra Deep Field North Survey. VIII. X-ray Constraints on Spiral Galaxies from 0.4 < z < 1.5

We present a statistical X-ray study of spiral galaxies in the Hubble Deep Field-North and its Flanking Fields using the Chandra Deep Field North 1 Ms dataset. We find that approximately L_star galaxies with 0.4 < z < 1.5 have ratios of X-ray to B-band luminosity similar to those in the local Universe, although the data indicate a likely increase in this ratio by a factor of approximately 2-3. We have also determined that typical spiral galaxies at 0.4 < z < 1.5 should be detected in the 0.5--2 keV band in the flux range 3-6 X 10^-18 erg cm^-2 s^-1.

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arXiv2001

X-ray Spectroscopy of QSOs with Broad Ultraviolet Absorption Lines

For the population of 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 Gamma~2.0, and the signatures of a significant column density [N_H~(0.1-4)x10^{23} 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 MgII 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.

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arXiv2001

XMM-Newton discovery of a sharp spectral feature at ~7 keV in the Narrow-Line Seyfert 1 galaxy 1H 0707-495

We report the first detection of a sharp spectral feature in a Narrow-Line Seyfert 1 galaxy. Using XMM-Newton we have observed 1H0707-495 and find a drop in flux by a factor of more than 2 at a rest-frame energy of ~7 keV without any detectable narrow Fe K alpha line emission. The energy of this feature suggests a connection with the neutral iron K photoelectric edge, but the lack of any obvious absorption in the spectrum at lower energies makes the interpretation challenging. We explore two alternative explanations for this unusual spectral feature: (i) partial covering absorption by clouds of neutral material and (ii) ionised disc reflection with lines and edges from different ionisation stages of iron blurred together by relativistic effects. We note that both models require an iron overabundance to explain the depth of the feature. The X-ray light curve shows strong and rapid variability, changing by a factor of four during the observation. The source displays modest spectral variability which is uncorrelated with flux.

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arXiv2001

Reddening, Emission-Line, and Intrinsic Absorption Properties in the Narrow-Line Seyfert 1 Galaxy Akn 564

We use Hubble Space Telescope UV and optical spectra of the narrow-line Seyfert 1 (NLS1) galaxy Akn 564 to investigate its internal reddening and properties of its emission-line and intrinsic UV absorption gas. We find that the extinction curve of Akn 564, derived from a comparison of its UV/optical continuum to that of an unreddened NLS1, lacks a 2200 A bump and turns up towards the UV at a longer wavelength (4000 A) than the standard Galactic, LMC, and SMC curves. However, it does not show the extremely steep rise to 1200 A that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The emission-lines and continuum experience the same amount of reddening, indicating the presence of a dust screen that is external to the narrow-line region (NLR). Echelle spectra from the Space Telescope Imaging Spectrograph show intrinsic UV absorption lines due to Ly-alpha, N V, C IV, Si IV, and Si III, centered at a radial velocity of -190 km/s (relative to the host galaxy). Photoionization models of the UV absorber indicate that it has a sufficient columnand is at a sufficient distance from the nucleus (D > 95 pc) to be the source of the dust screen. Thus, Akn 564 contains a dusty ``lukewarm absorber'' similar to that seen in NGC 3227.

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arXiv2001

J-Band Spectroscopy of the z = 5.74 BAL QSO SDSSp J104433.04-012502.2

We use the NIRSPEC near-IR spectrometer on Keck II to obtain moderate resolution (R = 1540) spectroscopy that shows conclusively that the C IV emission line in the z = 5.74 quasar SDSSp J104433.04-012502.2 is accompanied by broad, blueshifted C IV absorption. The line has a "balnicity index" of 900 km/s and a rest-frame equivalent width of 13.1 +/- 1.3 relative to the continuum. This confirms its membership in the class of objects called "broad-absorption line" (BAL) QSOs. SDSSP J1044-0125 was previously suggested to be a BAL QSO based on its larger UV-to-X-ray flux ratio than most non-BAL QSOs. The C IV emission is of normal strength, implying a metallicity similar to that found in other, lower-redshift QSOs.

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arXiv2001

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

Most galactic nuclei are now believed to harbour supermassive black holes. Studies of stellar motions 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 million solar masses. This object is spatially coincident with Sagittarius A* (Sgr A*), the unique compact radio source located at the dynamical centre of our Galaxy. By analogy with distant quasars and nearby active galactic nuclei (AGN), Sgr A* is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. However, Sgr A* is much fainter than expected in all wavebands, especially in X-rays, casting some doubt on this model. Recently, we reported the first strong evidence of X-ray emission from Sgr A*. Here we report the discovery of rapid X-ray flaring from the direction of Sgr A*. These data provide compelling evidence that the X-ray emission is coming from accretion onto a supermassive black hole at the Galactic Centre, and the nature of the variations provides strong constraints on the astrophysical processes near the event horizon of the black hole.

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arXiv2001

The Chandra Deep Field North Survey. VI. The Nature of the Optically Faint X-ray Source Population

We provide constraints on the nature of the optically faint (I>=24) X-ray source population from a 1 Ms Chandra exposure of a 8.4x8.4 arcmin region containing the Hubble Deep Field North. We detect 47 (2,400 deg^-2) optically faint X-ray sources. The fraction of optically faint X-ray sources is approximately constant (at approx 35%) for 0.5-8.0 keV fluxes from 3x10^-14 cgs down to the X-ray flux limit. A considerable fraction (approx 30%) of the sources are Very Red Objects (I-K>=4). Analysis of the optical and X-ray properties suggests a large number of optically faint X-ray sources are likely to host obscured AGN activity at z= 1-3. From these results we calculate that a significant fraction (approx 5-45%) could be obscured QSOs at z<=3. Given the number of X-ray sources without I-band counterparts, there are unlikely to be more than approx 15 sources at z>6. We provide evidence that the true number of z>6 sources is considerably lower. Nine optically faint X-ray sources have micro-Jy radio counterparts; approx 53% of the optically faint micro-Jy radio sources in this region. The most likely origin of the X-ray emission in these X-ray detected, optically faint micro-Jy radio sources is obscured AGN activity. However, two of these sources have been previously detected at sub-millimeter wavelengths and the X-ray emission from these sources could be due to luminous star formation activity. Assuming the SED of NGC 6240, we estimate the 175 micron flux of a typical optically faint X-ray source to be <10 mJy. As expected for sources with AGN activity, the two optically faint X-ray sources within the most sensitive area of the ISOCAM region have faint (approx 50 micro-Jy) 15 micron counterparts. (ABRIDGED).

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arXiv2001

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

An extremely deep X-ray survey (about 1 Ms) of the Hubble Deep Field North and its environs (about 450 arcmin^2) 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 keV and 2-8 keV flux limits of 3 x 10^{-17} erg/cm^2/s and 2 x 10^{-16} erg/cm^2/s, 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 0.6-1.7 arcsec (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^{+1100}_{-940} deg^{-2} (at 4.2 x 10^{-17} erg/cm^2/s) and 4200^{+670}_{-580} deg^{-2} (at 3.8 x 10^{-16} erg/cm^2/s) are observed in the soft and hard bands, respectively.

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arXiv2001

Exploratory Chandra Observations of the Highest-Redshift Quasars: X-rays from the Dawn of the Modern Universe

We report exploratory Chandra observations of 14 high-redshift (z=4.06-5.27), optically selected quasars. Ten of these quasars are detected, increasing the number of z>4 X-ray detected quasars by 71%. Our detections include four of the five highest-redshift X-ray detected quasars to date, among them SDSSp J021043.17-001818.4, the highest-redshift (z=4.77) radio-loud quasar detected in the X-ray band. The four undetected objects are the Broad Absorption Line quasars SDSSp J112956.10-014212.4 and SDSSp 160501.21-011220.0, the weak emission-line quasar SDSSp J153259.96-003944.1, and the quasar PSS 1435+3057. A comparison of the quasars' spectral energy distributions (by means of the optical-to-X-ray spectral index, alpha_ox) with those of lower-redshift samples indicates that the Chandra quasars are X-ray fainter by a factor of approx 2. X-ray faintness could be associated with the presence of large amounts of gas in the primeval galaxies harboring these high-redshift quasars, as suggested by recent studies conducted on z>4 quasars in other bands. Using the current Chandra data, predictions for the next generation of X-ray observatories, Constellation-X and Xeus, are also provided.

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arXiv2001

HST STIS Observations of PG 0946+301: the Highest Quality UV Spectrum of a BALQSO

We describe deep (40 orbits) HST/STIS observations of the BALQSO PG 0946+301 and make them available to the community. These observations are the major part of a multi-wavelength campaign on this object aimed at determining the ionization equilibrium and abundances (IEA) in broad absorption line (BAL) QSOs. We present simple template fits to the entire data set, which yield firm identifications for more than two dozen BALs from 18 ions and give lower limits for the ionic column densities. We find that the outflow's metalicity is consistent with being solar, while the abundance ratio of phosphorus to other metals is at least ten times solar. These findings are based on diagnostics that are not sensitive to saturation and partial covering effects in the BALs, which considerably weakened previous claims for enhanced metalicity. Ample evidence for these effects is seen in the spectrum. We also discuss several options for extracting tighter IEA constraints in future analyses, and present the significant temporal changes which are detected between these spectra and those taken by the HST/FOS in 1992.

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arXiv2001

The Chandra Deep Field North Survey. VII. X-ray Emission from Lyman Break Galaxies

We present results from stacking analyses, using the 1 Ms Chandra Deep Field North data, that constrain the X-ray emission of Lyman break galaxies at z = 2-4. Stacking the counts from 24 individually undetected Lyman break galaxies located within the Hubble Deep Field North, we have obtained average detections of these objects in the resulting 0.5-8.0 keV and 0.5-2.0 keV images; these images have effective exposure times of 22.4 Ms (260 days). Monte Carlo testing empirically shows the detections to be highly significant. The average rest-frame 2-8 keV luminosity of a Lyman break galaxy is derived to be 3.2 x 10^{41} erg s^{-1}, comparable to that of the most X-ray luminous starbursts in the local Universe. The observed ratio of X-ray to B-band luminosity is somewhat, but not greatly, higher than that seen from local starbursts. The X-ray emission probably arises from a combination of high-mass X-ray binaries, "super-Eddington" X-ray sources, and low-luminosity active galactic nuclei.

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arXiv2001

The Submillimeter Properties of the 1 Ms Chandra Deep Field North X-ray Sample

We present submillimeter observations for 136 of the 370 X-ray sources detected in the 1 Ms exposure of the Chandra Deep Field North. Ten of the X-ray sources are significantly detected in the submillimeter. The average X-ray source in the sample has a significant 850 micron flux of 1.69+/-0.27 mJy. This value shows little dependence on the 2-8 keV flux from 5e-16 erg/cm^2/s to 1e-14 erg/cm^2/s. The ensemble of X-ray sources contribute about 10% of the extragalactic background light at 850 microns. The submillimeter excess is found to be strongest in the optically faint X-ray sources that are also seen at 20 cm, which is consistent with these X-ray sources being obscured and at high redshift (z>1).

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