arXiv subjects

G. Chartas

Publications and source records attributed to G. Chartas.

At least 73 records · Page 4Linked to original sources

arXiv2004

The Power of Exploratory Chandra Observations

With its excellent spatial resolution, low background, and hard-band response, the Chandra X-ray Observatory is ideal for performing exploratory surveys. These efficient, sensitive observations can place constraints on fundamental properties of a quasar continuum including the X-ray luminosity, the ratio of X-ray to UV power, and the X-ray spectral shape. To demonstrate the power of such surveys to provide significant insight, we consider two examples, a Large Bright Quasar Survey sample of broad absorption line quasars and a sample of Sloan Digital Sky Survey (SDSS) quasars with extreme CIV blueshifts. In both cases, exploratory Chandra observations provide important information for a physical understanding of UV spectroscopic differences in quasars.

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arXiv2004

A survey of extended radio jets with Chandra and HST

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 jet's 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.

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arXiv2004

Chandra Observations of the Cloverleaf Quasar H 1413+117: A Unique Laboratory for Microlensing Studies of a Lobal Quasar

We present new results uncovered by a re-analysis of a Chandra observation of the gravitationally lensed, low-ionization broad absorption line (LoBAL) quasar H 1413+117. Previous analyses of the same Chandra observation led to the detection of a strong, redshifted Fe Kalpha line from the combined spectrum of all images. We show that the redshifted Fe Kalpha line is only significant in the brighter image A. The X-ray flux fraction of image A is larger by a factor of 1.55 +/- 0.17 than the optical R-band flux fraction, indicating that image A is significantly enhanced in the X-ray band. We also find that the Fe Kalpha line and the continuum are enhanced by different factors. A microlensing event could explain both the energy-dependent magnification and the significant detection of Fe Kalpha line emission in the spectrum of image A only. In the context of this interpretation we provide constraints on the spatial extent of the inferred scattered continuum and reprocessed Fe Kalpha line emission regions in a LoBAL quasar.

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arXiv2003

Dramatic X-ray Spectral Variability of the Broad Absorption Line Quasar PG2112+059

With a 1999 ASCA observation, PG2112+059 became notable as the first Broad Absorption Line (BAL) quasar found to exhibit a typical radio-quiet quasar X-ray continuum underlying a large amount of intrinsic absorption. We present a recent Chandra ACIS-S3 observation of PG2112+059 that demonstrates remarkable spectral and luminosity variability since that time. In addition to a decrease in the continuum normalization by a factor of ~3.5, the absorption column density has apparently increased substantially, and a strong feature in the Fe K alpha region has appeared. Concurrent HST STIS data compared with archival HST data from earlier epochs show evidence for variability of the continuum (up to a factor of ~1.7 in the ultraviolet), and in some absorption features of the CIV 1549 BAL since 1992; however, the OVI BAL structure is consistent with a 1995 observation. We also present evidence for Ly beta--OVI 1037.62 and Ly alpha--NV 1242.80 line-locked absorption systems, supporting the assumption that ultraviolet line pressure is driving the BAL outflow. Whereas ultraviolet BALs typically exhibit only modest equivalent-width variability over timescales of years, the dramatic X-ray variability of PG2112+059 suggests that X-ray spectral variability studies of BAL quasars have great potential for probing the physics of quasar winds.

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arXiv2003

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. The spectrum of PG1115+080 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 keV and 9.5 keV. Assuming that these two lines are produced by resonant absorption due to Fe xxv Kalpha, 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 ~ 10^(16)(M_BH/M_8)^(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.

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arXiv2003

The Chandra Deep Field-North Survey. XIII. 2 Ms Point-Source Catalogs

We present catalogs for the ~2 Ms Chandra Deep Field-North, currently the deepest X-ray observation of the Universe in the 0.5-8.0 keV band. Five hundred and three (503) X-ray sources are detected over an ~448 sq.arcmin area in up to seven bands; 20 of these X-ray sources lie in the Hubble Deep Field-North. Source positions are determined using matched-filter and centroiding techniques; the median positional uncertainty is ~0.3 arcsecs. The X-ray colors of the detected sources indicate a broad variety of source types, although absorbed AGNs (including some possible Compton-thick sources) are clearly the dominant type. We also match lower significance X-ray sources to optical counterparts and provide a list of 79 optically bright R<~23) lower significance Chandra sources. The majority of these sources appear to be starburst and normal galaxies. We investigate the source-free background, determine the maximum photon-limited exposures, and investigate source confusion. These analyses directly show that Chandra can achieve significantly higher sensitivities in an efficient nearly photon-limited manner and be largely free of source confusion. To allow consistent comparisons, we have also produced point-source catalogs for the ~1 Ms Chandra Deep Field-South (CDF-S). Three hundred and twenty-six (326) X-ray sources are included in the main Chandra catalog, and an additional 42 optically bright X-ray sources are included in a lower significance Chandra catalog. We find good agreement with the photometry of the previously published CDF-S catalogs; however, we provide significantly improved positional accuracy (ABRIDGED).

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arXiv2003

Chandra Observations of Nuclear X-ray Emission from a Sample of Radio Sources

We present the X-ray properties of a sample of 17 radio sources observed with the Chandra X-ray Observatory as part of a project aimed at studying the X-ray emission from their radio jets. In this paper, we concentrate on the X-ray properties of the unresolved cores. The sample includes 16 quasars (11 core-dominated and 5 lobe-dominated) in the redshift range z=0.30--1.96, and one low-power radio-galaxy at z=0.064. No diffuse X-ray emission is present around the cores of the quasars, except for the nearby low-power galaxy that has diffuse emission on a scale and with a luminosity consistent with other FRIs. No high-amplitude, short-term variability is detected within the relatively short Chandra exposures. However, 1510-089 shows low-amplitude flux changes with a timescale of $\sim$25 minutes. The X-ray spectra of the quasar cores are generally well described by a single power law model with Galactic absorption. However, in six quasars we find soft X-ray excess emission below 1.6 keV. Interestingly, we detect an Fe K-shell emission line, consistent with fluorescent Kalpha emission from cold Iron, in one lobe- and two core-dominated sources. The average X-ray photon index for the quasars in the sample is Gamma=1.66 and dispersion, sigma=0.23. The average spectral slope for our sample is flatter than the slope found for radio-quiet quasars and for radio-loud AGNs with larger jet orientations; this indicates that beaming affects the X-ray emission from the cores in our sample of quasars.

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arXiv2003

Chandra Observations of QSO 2237+0305

We present the observations of the gravitationally lensed system QSO 2237+0305 performed with the Chandra X-ray Observatory on 2000 Sept. 6, and on 2001 Dec. 8 for 30.3 ks and 9.5 ks, respectively. Imaging analysis resolves the four X-ray images of the Einstein Cross. A possible fifth image is detected; however, this detection less certain. Fits to the combined spectrum of all images of the Einstein Cross assuming a simple power law with Galactic and intervening absorption at the lensing galaxy yield a photon index of 1.90(+0.05,-0.05). For the first observation, this spectral model yields a 0.4-8.0 keV X-ray flux of 4.6e-13 erg cm-2 s-1 and a 0.4-8.0 keV lensed luminosity of 1.0e46 erg s-1. The source exhibits variability both over long and short time scales. The X-ray flux has dropped by 20% between the two observations, and the K-S test showed that image A is variable at the 97% confidence level within the first observation. Furthermore, a possible time-delay of 2.7(+0.5,-0.9) hours between images A and B with image A leading is detected in the first observation. The X-ray flux ratios of the images are consistent with the optical flux ratios which are affected by microlensing suggesting that the X-ray emission is also microlensed. A comparison between our measured column densities and those inferred from extinction measurements suggests a higher dust-to-gas ratio in the lensing galaxy than the average value of our Galaxy. Finally, we report the detection at the 99.99% confidence level of a broad emission feature near the redshifted energy of the Fe Kαline in only the spectrum of image A. The rest frame energy, width, and equivalent width of this feature are E = 5.7(+0.2,-0.3) keV, sigma = 0.87(+0.30,-0.15) keV, and EW = 1200(+300,-200) eV, respectively.

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arXiv2002

Probing Broad Absorption Line Quasar Outflows: X-ray Insights

Energetic outflows appear to occur in conjunction with active mass accretion onto supermassive black holes. These outflows are most readily observed in the approximately 10% of quasars with broad absorption lines, where the observer's line of sight passes through the wind. Until fairly recently, the paucity of X-ray data from these objects was notable, but now sensitive hard-band missions such as Chandra and XMM-Newton are routinely detecting broad absorption line quasars. The X-ray regime offers qualitatively new information for the understanding of these objects, and these new results must be taken into account in theoretical modeling of quasar winds.

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arXiv2002

The Chandra Deep Field-North Survey. XIV. X-ray detected Obscured AGNs and Starburst Galaxies in the Bright Submm Source Population

We provide X-ray constraints and perform the first X-ray spectral analyses for bright (f_850>=5mJy; S/N>=4) SCUBA sources in an 8.4'x8.4' area of the 2 Ms Chandra Deep Field-North survey containing the Hubble Deep Field-North. X-ray emission is detected from 7 of the 10 bright submm sources in this region, corresponding to an X-ray detected submm source density of ~360 deg^-2 (>~36% of the bright submm source population). Two of the X-ray detected sources have nearby (within 3") X-ray companions, suggesting merging/interacting sources or gravitational lensing effects, and 3 lie within the approximate extent of a proto-cluster candidate. Five of the X-ray detected sources have flat X-ray spectral slopes, suggesting obscured AGN activity. X-ray spectral analyses suggest that one of these AGNs may be a Compton-thick source; of the other 4 AGNs, 3 appear to be Compton-thin sources and one has poor constraints. The rest-frame unabsorbed X-ray luminosities of these AGNs are more consistent with those of Seyfert galaxies than QSOs. Thus, the low X-ray detection rate of bright submm sources by moderately deep X-ray surveys appears to be due to the relatively low luminosities of the AGNs rather than Compton-thick absorption. A comparison of these sources to the well-studied heavily obscured AGN NGC6240 shows that the average AGN contribution is negligible at submm wavelengths. The X-ray properties of the other 2 X-ray detected sources are consistent with those expected from luminous star formation; however, we cannot rule out the possibility that low-luminosity AGNs are present. The 3 X-ray undetected sources appear to lie at high redshift (z>4) and could be either AGNs or starbust galaxies.

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arXiv2002

Chandra Detects Relativistic Broad Absorption Lines from APM 08279+5255

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).

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arXiv2002

An Exploratory Chandra survey of Large Bright Quasar Survey broad absorption line QSOs

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.

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arXiv2002

Deep Near Infrared Observations of the X-ray Emitting Class 0 Protostar Candidates in the Orion Molecular Cloud-3

We obtained near infrared (NIR) imaging with the Subaru Telescope of the class 0 protostar candidates in the Orion Molecular Cloud-3, two of which were discovered to have X-ray emission by the Chandra X-ray Observatory. We found strong evidence for the class~0 nature of the X-ray sources. First, our deep K-band image shows no emission brighter than 19.6 mag from both of these X-ray sources. Since class I protostars or class II T Tauri stars should be easily detected in the NIR with this sensitivity, the lack of K-band detection suggests that they are likely much more obscured than class I protostars. Second, our H2 v=1-0 S(1) image shows a bubble-like feature from one of the X-ray class 0 protostar candidates, which reinforces the idea that this is a class 0 protostar. We also discuss the nature of nine NIR sources found in our deep image based on their colors, spatial coincidence with millimeter cores, and the properties of their X-ray counterparts.

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arXiv2002

A survey of extended radio jets in AGN with Chandra and HST: First Results

We present the first results from an X-ray and optical survey of a sample of AGN radio jets with Chandra and HST. We focus here on the first six sources observed at X-rays, in four of which a bright X-ray jet was detected for the first time. In three out of four cases optical emission from the jet is also detected in our HST images. We compare the X-ray morphology with the radio as derived from improved processing of archival VLA data and we construct spectral energy distributions (SED) for the most conspicuous emission knots. In most cases the SEDs, together with the similarity of the X-ray and radio morphologies, favor an inverse Compton origin of the X-rays. The most likely origin of the seed photons is the Cosmic Microwave Background, implying the jets are still relativistic on kiloparsec scales. However, in the first knot of the PKS 1136-135 jet, X-rays are likely produced via the synchrotron process. In all four cases bulk Lorentz factors of a few are required. The radio maps of the two jets not detected by either Chandra or HST suggest that they are less beamed at large scales than the other four detected sources. Our results demonstrate that, at the sensitivity and resolution of Chandra, X-ray emission from extragalactic jets is common, yielding essential information on their physical properties.

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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

Caught in the Act; Chandra Observations of Microlensing of the Radio-Loud Quasar MG J0414+0534

We present results from monitoring of the distant (z = 2.64), gravitationally lensed quasar MG J0414+0534 with the Chandra X-ray Observatory. An Fe Ka line at 6.49 +/- 0.09 keV (rest-frame) with an equivalent width of ~ 190eV consistent with fluorescence from a cold medium is detected at the 99 percent confidence level in the spectrum of the brightest image A. During the last two observations of our monitoring program we detected a five-fold increase of the equivalent width of a narrow Fe Ka line in the spectrum of image B but not in the brighter image A whereas image C is too faint to resolve the line. The continuum emission component of image B did not follow the sudden enhancement of the iron line in the last two observations. We propose that the sudden increase in the iron line strength from ~ 190eV to 900eV only in image B can be explained with a caustic crossing due to microlensing that selectively enhances a strip of the line emission region of the accretion disk. The non-enhancement of the continuum emission in the spectrum of image B suggests that the continuum emission region is concentrated closer to the center of the accretion disk than the iron line emission region and the magnification caustic has not reached close enough to the former region to amplify it. A model of a caustic crossing event predicts discontinuities in the light-curve of the magnification and provides an upper limit of ~ 5 x 10^(-4) pc on the outer radius of the Fe Ka emission region. The non-detection of any relativistic or Doppler shifts of the iron line in the spectrum of image B implies that the magnification caustic for the last two observations was located at a radius greater than ~ 100 gravitational radii.

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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

Extreme X-ray Behavior in the Low-Luminosity Active Nucleus of NGC 4395

We present the results of a 17 ks Chandra observation of the nearby dwarf spiral galaxy NGC 4395, focusing on the X-ray properties of the moderate-mass black hole that resides in its nucleus. Chandra affords the first high-quality, broadband X-ray detection of this object that is free of contamination from nearby sources in the field. We find that the nuclear X-ray emission is unresolved in the Chandra image and confirm the rapid, large-amplitude X-ray variability reported in previous studies. The spectrum of the nuclear source shows evidence for absorption by an ionized medium. There is also evidence for spectral variability over the course of the Chandra observation, although contrary to prior reports, it appears to be uncorrelated with fluctuations in the hard X-ray count rate. It is possible that the short-term spectral variability results from column density changes in the ionized absorber. By far the most unusual high-energy property of NGC 4395 is the shape of its spectrum above 1 keV. The Chandra data indicate a power-law photon index of Gamma \approx 0.6, which is much flatter than the Gamma \approx 1.8 X-ray spectra typical of active galactic nuclei and the slope of the nuclear X-ray spectrum measured from an earlier ASCA observation of NGC 4395. This extreme flatness and dramatic long-term variability of the X-ray spectrum are unprecedented among active galactic nuclei. A variety of possibilities for the origin of the flat continuum slope are considered; none provides a fully satisfactory explanation.

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