ROSAT Blank Field Sources. I. Sample Selection and Archival Data (original) (raw)
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Chandra Observations of Unresolved X-Ray Sources around Two Clusters of Galaxies
The Astrophysical …, 2002
We have searched for unresolved X-ray sources in the vicinity of two rich clusters of galaxies: Abell 1995 (A1995) and MS 0451.6-0305 (MS0451), using the Chandra X-ray observatory. We detected significantly more unresolved sources around A1995 than expected based on the number of X-ray sources to the same flux limit detected in deep Chandra observations of blank fields. Previous studies have also found excess X-ray sources in the vicinity of several nearby clusters of galaxies using ROSAT , and recently in more distant (z ≈ 0.5) clusters (RXJ0030 and 3C295) using Chandra . In contrast, we detect only 14 unresolved X-ray sources near MS0451, which is consistent with the number expected from a cluster-free background. We determine the luminosity functions of the extra sources under the assumption that they are at the distance of their respective clusters. The characteristic luminosity of the extra sources around A1995 must be an order of magnitude fainter than that of the extra sources around RXJ0030 and 3C295. The apparent lack of extra sources around MS0451 is consistent with its greater distance and the same characteristic luminosity as the A1995 sources. Hardness ratios suggest that, on average, the extra sources in A1995 may have harder spectra than those of RXJ0030 and 3C295. These results indicate that different classes of objects may dominate in different clusters, perhaps depending on the formation history and/or dynamical state of the accompanying cluster.
The rosat deep survey: vi. x-ray sources and optical identifications of the ultra deep survey
Arxiv preprint astro-ph/ …, 2001
The ROSAT Deep Surveys in the direction of the Lockman Hole are the most sensitive X-ray surveys performed with the ROSAT satellite. About 70-80% of the X-ray background has been resolved into discrete sources at a flux limit of ∼10 −15 erg cm −2 s −1 in the 0.5-2.0 keV energy band. A nearly complete optical identification of the ROSAT Deep Survey (RDS) has shown that the great majority of sources are AGNs. We describe in this paper the ROSAT Ultra Deep Survey (UDS), an extension of the RDS in the Lockman Hole. The Ultra Deep Survey reaches a flux level of 1.2 10 −15 erg cm −2 s −1 in 0.5-2.0 keV energy band, a level ∼4.6 times fainter than the RDS. We present nearly complete spectroscopic identifications (90%) of the sample of 94 X-ray sources based on low-resolution Keck spectra. The majority of the sources (57) are broad emission line AGNs (type I), whereas a further 13 AGNs show only narrow emission lines or broad Balmer emission lines with a large Balmer decrement (type II AGNs) indicating significant optical absorption. The second most abundant class of objects (10) are groups and clusters of galaxies (∼11%). Further we found five galactic stars and one "normal" emission line galaxy. Eight X-ray sources remain spectroscopically unidentified. We see no evidence for any change in population from the RDS survey to the UDS survey. The photometric redshift determination indicates in three out of the eight sources the presence of an obscured AGN. Their photometric redshifts, assuming that the spectral energy distribution (SED) in the optical/near-infrared is due to stellar processes, are in the range of 1.2 ≤ z ≤ 2.7. These objects could belong to the long-sought population of type 2 QSOs, which are predicted by the AGN synthesis models of the X-ray background. Finally, we discuss the optical and soft X-ray properties of the type I AGN, type II AGN, and groups and clusters of galaxies, and the implication to the X-ray background.
X-ray luminous galaxies I. Chandra observations of IRAS00317-2142
2002
We present Chandra observations of the enigmatic galaxy IRAS00317-2142, which is classified as a star-forming galaxy on the basis of the ionization level of its emission lines. However, a weak broad Hα wing and a high X-ray luminosity give away the presence of an active nucleus. The Chandra image reveals a nuclear point source (L (2−10keV) ∼ 6 × 10 41 erg s −1 ), contributing over 80 % of the galaxy X-ray counts in the 0.3-8 keV band. This is surrounded by some fainter nebulosity extending up to 6 kpc. The nucleus does not show evidence for short-term variability. However, we detect long term variations between the ROSAT, ASCA and Chandra epoch. Indeed, the source has decreased its flux by over a factor of 25 in a period of about 10 years. The nuclear X-ray spectrum is well represented by a power-law with a photon index of Γ = 1.91 +0.17 −0.15 while the extended emission by a Raymond-Smith component with a temperature of ∼ 0.6keV. We find no evidence for the presence of an Fe line. The nucleus is absorbed by an intrinsic column density of N H ∼ 8 × 10 20 cm −2 . Thus the Chandra observations suggest that at least the X-ray emission is dominated by a type-1 AGN. Then the observed optical spectrum can be explained due to the masking of the nucleus by the powerful star-forming component. These together with previous X-ray observations of galaxies with no clear signs of AGN activity in the optical, (eg NGC6240) cast doubt on the optical classification scheme and have implications for the nature of the 'normal' galaxies detected in deep Chandra X-ray surveys.
The Astronomical Journal, 2005
We have cross-correlated X-ray catalogs derived from archival Chandra ACIS observations with a Sloan Digital Sky Survey (SDSS) Data Release 2 (DR2) galaxy catalog to form a sample of 42 serendipitously X-ray detected galaxies over the redshift interval 0.03 < z < 0.25. This pilot study will help fill in the "redshift gap" between local X-raystudied samples of normal galaxies and those in the deepest X-ray surveys. Our chief purpose is to compare optical spectroscopic diagnostics of activity (both star-formation and accretion) with X-ray properties of galaxies. Our work supports a normalization value of the X-ray-star-formation-rate (X-ray-SFR) correlation consistent with the lower values published in the literature. The difference is in the allocation of X-ray emission to high-mass X-ray binaries relative to other components such as hot gas, low-mass X-ray binaries, and/or AGN. We are able to quantify a few pitfalls in the use of lowerresolution, lower signal-to-noise optical spectroscopy to identify X-ray sources (as has necessarily been employed for many X-ray surveys). Notably, we find a few AGN that likely would have been misidentified as non-AGN sources in higher-redshift studies. However, we do not find any X-ray hard, highly X-ray-luminous galaxies lacking optical spectroscopic diagnostics of AGN activity. Such sources are members of the "X-ray Bright, Optically Normal Galaxy" (XBONG) class of AGN.
The Astrophysical Journal, 2019
We present a search for nuclear X-ray emission in the Brightest Cluster Galaxies (BCGs) of a sample of groups and clusters of galaxies extracted from the Chandra archive. The exquisite angular resolution of Chandra allows us to obtain robust photometry at the position of the BCG, and to firmly identify unresolved X-ray emission when present, thanks to an accurate characterization of the extended emission at the BCG position. We consider two redshift bins (0.2<z<0.3 and 0.55<z<0.75) and analyze all the clusters observed by Chandra with exposure time larger than 20 ks. Our samples have 81 BCGs in 73 clusters and 51 BCGs in 49 clusters in the low- and high-redshift bin, respectively. X-ray emission in the soft (0.5-2 keV) or hard (2-7 keV) band is detected only in 14 and 9 BCGs (∼18% of the total samples), respectively. The X-ray photometry shows that at least half of the BCGs have a high hardness ratio, compatible with significant intrinsic absorption. This is confirmed by the spectral analysis with a power law model plus intrinsic absorption. We compute the fraction of X-ray bright BCGs above a given hard X-ray luminosity, considering only sources with positive photometry in the hard band (12/5 sources in the low/high-z sample). In the 0.2<z<0.3 interval the hard X-ray luminosity ranges from 10^42 to 7×10^43 erg s−1, with most sources found below 10^43 erg s−1. In the 0.55<z<0.75 range, we find a similar distribution of luminosities below ∼10^44 erg s−1, plus two very bright sources of a few 10^45 erg s−1 associated to two radio galaxies. We also find that X-ray luminous BCGs tend to be hosted by cool core clusters, despite the majority of cool cores do not host nuclear X-ray emission.
2002
We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae galaxies. We derive the average X-ray spectral properties of sources in different luminosity ranges, and we correlate the X-ray positions with radio, IR, and optical (HST) data. The X-ray sources are predominantly associated with young stellar clusters, indicating that they belong to the young stellar population. Based on both their co-added X-ray spectrum, and on the lack of associated radio emission, we conclude that the Ultra Luminous X-ray sources (ULXs), with L X ≥ 10 39 ergs s −1 , are not young compact Supernova Remnants (SNR), but accretion binaries. While their spectrum is consistent with those of ULXs studied in nearby galaxies, and interpreted as the counterparts of intermediate mass black-holes (M> 10 − 1000 M ⊙ ), comparison with the position of star-clusters suggests that some of the ULXs may be runaway binaries, thus suggesting lower-mass binary systems. The co-added spectrum of the sources in the 3 × 10 38 − 10 39 erg s −1 luminosity range is consistent with those of Galactic black-hole candidates. These sources are also on average displaced from neighboring star clusters. The softer spectrum of the less luminous sources suggests the presence of SNRs or of hot interstellar medium (ISM) in the Chandra source extraction area. Comparison with HI and CO observations shows that most sources are detected in the outskirts of large concentrations of gas. The absorbing columns inferred from these observations would indeed absorb X-rays up to 5 keV, so there may be several hidden X-ray sources. Associated with these obscured regions we find 6 sources with heavily absorbed X-ray spectra and absorption-corrected luminosities in the ULX range. We detect the nuclei of both galaxies with luminosities in the 10 39 ergs s −1 range and soft, possibly thermal, X-ray spectra.
The Astrophysical Journal, 2009
We present new identifications of infrared counterparts to the population of hard X-ray sources near the Galactic center detected by the Chandra X-ray Observatory. We have spectroscopically confirmed 16 new massive stellar counterparts to the X-ray population, including nitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars, and O supergiants. These discoveries increase the total sample of massive stellar X-ray sources in the Galactic center region to 30 (possibly 31). For the majority of these sources, the X-ray photometry is consistent with thermal emission from plasma having temperatures in the range of kT = 1-8 keV or non-thermal emission having power-law indices in the range of −1 Γ 3, and X-ray luminosities in the range of L X ∼ 10 32-10 34 erg s −1 (0.5-8.0 keV). Several sources have exhibited X-ray variability of several factors between observations. These X-ray properties are not a ubiquitous feature of single massive stars but are typical of massive binaries, in which the high-energy emission is generated by the collision of supersonic winds, or by accretion onto a compact companion. However, without direct evidence for companions, the possibility of intrinsic hard X-ray generation from single stars cannot be completely ruled out. The spectral energy distributions of these sources exhibit significant infrared excess, attributable to free-free emission from ionized stellar winds, supplemented by hot dust emission in the case of the WC stars. With the exception of one object located near the outer regions of the Quintuplet cluster, most of the new stars appear isolated or in loose associations. Seven hydrogen-rich WN and
Heavily Obscured AGN in Star-Forming Galaxies at z sime 2
Astrophysical Journal, 2009
We study the properties of a sample of 211 heavily-obscured Active Galactic Nucleus (AGN) candidates in the Extended Chandra Deep Field-South selecting objects with f 24µm /f R >1000 and R-K>4.5. Of these, 18 were detected in X-rays and found to be obscured AGN with neutral hydrogen column densities of ∼10 23 cm −2 . In the X-ray undetected sample, the following evidence suggests a large fraction of heavily-obscured (Compton Thick) AGN: (i) The stacked X-ray signal of the sample is strong, with an observed ratio of soft to hard X-ray counts consistent with a population of ∼90% heavily obscured AGN combined with 10% star-forming galaxies. (ii) The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ∼2 orders of magnitude smaller than for the general AGN population, suggesting column densities of N H 5×10 24 cm −2 . (iii) The Spitzer near-and mid-IR colors of these sources are consistent with those of the X-ray-detected sample if the effects of dust self-absorption are considered. Spectral fitting to the rest-frame UV/optical light (dominated by the host galaxy) returns stellar masses of ∼10 11 M ⊙ and <E(B-V)>=0.5, and reveals evidence for a significant young stellar population, indicating that these sources are experiencing considerable star-formation. This sample of heavily-obscured AGN candidates implies a space density at z∼2 of ∼10 −5 Mpc −3 , finding a strong evolution in the number of L X >10 44 erg/s sources from z=1.5 to 2.5, possibly consistent with a short-lived heavily-obscured phase before an unobscured quasar is visible.
Spectroscopic identification of ten faint hard X-ray sources discovered by Chandra
New Astronomy, 2000
We report optical spectroscopic identifications of 10 hard (2-10 keV) X-ray selected sources discovered by Chandra. The X-ray flux of the sources ranges between 1.5 and 25 ×10 −14 erg cm −2 s −1 , the lower value being 3 times fainter than in previous BeppoSAX and ASCA surveys. Their R band magnitudes are in the range 12.8-22. Six of the Chandra sources are broad line quasars with redshifts between 0.42 and 1.19, while the optical identification of the remaining four is quite varied: two are Xray obscured, emission line AGN at z=0.272 and z=0.683, one is a starburst galaxy at z=0.016 and one, most unusually, is an apparently normal galaxy at z=0.158. These findings confirm and extend down to fainter X-ray fluxes the BeppoSAX results, in providing samples with a wide range of X-ray and optical properties. The ratio between the soft X-ray and the optical luminosity of the z=0.158 galaxy is a factor at least 30 higher than that of normal galaxies, and similar to those of AGN. The high X-ray luminosity and the lack of optical emission lines suggest an AGN in which either continuum beaming dominates, or emission lines are obscured or not efficiently produced.
HEAVILY OBSCURED AGN IN STAR-FORMING GALAXIES AT z ≃ 2
The Astrophysical Journal, 2009
We study the properties of a sample of 211 heavily-obscured Active Galactic Nucleus (AGN) candidates in the Extended Chandra Deep Field-South selecting objects with f 24µm /f R >1000 and R-K>4.5. Of these, 18 were detected in X-rays and found to be obscured AGN with neutral hydrogen column densities of ∼10 23 cm −2 . In the X-ray undetected sample, the following evidence suggests a large fraction of heavily-obscured (Compton Thick) AGN: (i) The stacked X-ray signal of the sample is strong, with an observed ratio of soft to hard X-ray counts consistent with a population of ∼90% heavily obscured AGN combined with 10% star-forming galaxies. (ii) The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ∼2 orders of magnitude smaller than for the general AGN population, suggesting column densities of N H 5×10 24 cm −2 . (iii) The Spitzer near-and mid-IR colors of these sources are consistent with those of the X-ray-detected sample if the effects of dust self-absorption are considered. Spectral fitting to the rest-frame UV/optical light (dominated by the host galaxy) returns stellar masses of ∼10 11 M ⊙ and <E(B-V)>=0.5, and reveals evidence for a significant young stellar population, indicating that these sources are experiencing considerable star-formation. This sample of heavily-obscured AGN candidates implies a space density at z∼2 of ∼10 −5 Mpc −3 , finding a strong evolution in the number of L X >10 44 erg/s sources from z=1.5 to 2.5, possibly consistent with a short-lived heavily-obscured phase before an unobscured quasar is visible.