Medium-Deep Observations of QSOs/AGNs in the Field of SA 57

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.

2002

The 13hr XMM-Newton/Chandra deep survey is the first of two extremely deep XMM-Newton fields observed by the XMM-OM consortium. A 120 ks Chandra mosaic, covering 0.2 square degrees, provides sensitive, confusion-free point source detection with sub-arcsecond positions, while the 200 ks XMM-Newton observation provides high quality X-ray spectroscopy over the same sky area. We have optical spectroscopic identifications for 70 X-ray sources. Of these, 42 are broad emission-line AGN with a wide range of redshifts. The optical counterparts of a further 23 sources are narrow emission line galaxies and absorption line galaxies. These 23 sources all lie at z < 1 and typically have lower X-ray luminosities than the broad-line AGN. About half of them show significant X-ray absorption and are almost certainly intrinsically absorbed AGN. However some of them have unabsorbed, AGN-like, power-law components in their X-ray spectra, but do not show broad emission lines in their optical spectra. These sources may be weak, unobscured AGN in bright galaxies and their existence at low redshifts could be a consequence of the strong cosmological evolution of AGN characteristic luminosities.

Monthly Notices of the Royal Astronomical Society, 2013

The Astrophysical Journal Supplement Series, 2007

We present a detailed spectral analysis of point-like X-ray sources in the XMM-COSMOS field. Our sample of 135 sources only includes those that have more than 100 net counts in the 0.3-10 keV energy band and have been identified through optical spectroscopy. The majority of the sources are well described by a simple power-law model with either no absorption (76%) or a significant intrinsic, absorbing column (20%). The remaining ∼ 4% of the sources require a more complex modeling by incorporating additional components to the power-law. For sources with more than 180 net counts (bright sample), we allowed both the photon spectral index Γ and the equivalent hydrogen column N H to be free parameters. For fainter sources, we fix Γ to the average value and allow N H to vary. The mean spectral index of the 82 sources in the bright sample is < Γ >= 2.06±0.08, with an intrinsic dispersion of ∼ 0.24. Each of these sources have fractional errors on the value of Γ below 20%. As expected, the distribution of intrinsic absorbing column densities is markedly different between AGN with or without broad optical emission lines. We find within our sample four Type-2 QSOs candidates (L X > 10 44 erg s −1 , N H > 10 22 cm −2 ), with a spectral energy distribution well reproduced by a composite Seyfert-2 spectrum, that demonstrates the strength of the wide field XMM/COSMOS survey to detect these rare and underrepresented sources. In addition, we have identified a Compton-thick (N H > 1.5 × 10 24 cm −2 ) AGN at z=0.1248. Its X-ray spectrum is well fitted by a pure reflection model and a significant Fe Kα line at rest-frame energy of 6.4 keV.

Astronomy & Astrophysics, 2013

Context. The main element of the observing program of the Spectrum-Roentgen-Gamma orbital observatory is a fouryear all-sky survey, in the course of which the entire sky will be scanned eight times. Aims. We analyze the statistical properties of AGN (active galactic nuclei) and QSOs (quasars/quasi-stellar objects) that are expected to be detected in the course of the eROSITA all-sky survey (eRASS). Methods. According to the currently planned survey strategy and based on the parameters of the Galactic and extragalactic X-ray background as well as on the results of the recent calculations of the eROSITA instrumental background, we computed a sensitivity map of the eRASS. Using the best available redshift-dependent AGN X-ray luminosity function (XLF), we computed various characteristics of the eRASS AGN sample, such as their luminosity-and redshift distributions, and the brightness distributions of their optical counterparts. Results. After four years of the survey, a sky-average sensitivity of about 1 × 10 −14 erg s −1 cm −2 will be achieved in the 0.5 − 2.0 keV band. With this sensitivity, eROSITA is expected to detect about 3 million AGN on the extragalactic sky (|b| > 10 •). The median redshift of the eRASS AGN will be z ≈ 1 with approximately 40 % of the objects in the z = 1 − 2 redshift range. About 10 4 − 10 5 AGN are predicted beyond redshift z = 3 and about 2 000 − 30 000 AGN beyond redshift z = 4, the exact numbers depend on the poorly known behavior of the AGN XLF in the high-redshift and luminosity regimes. Of the detected AGN, the brightest 10 % will be detected with more than ≈ 38 counts per point-spread-function (half-energy width), while the faintest 10 % will have fewer than ≈ 9 counts. The optical counterparts of approximately 95 % of the AGN will be brighter than IAB = 22.5 mag. The planned scanning strategy will allow one to search for transient events on a timescale of half a year and a few hours with a 0.5 − 2.0 keV sensitivity of ≈ 2 × 10 −14 to ≈ 2 × 10 −13 erg s −1 cm −2 , respectively.

Astrophysical Journal Supplement Series, 2007

[ABRIGED] We present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg^2 of the XMM-COSMOS survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ~10^-15 erg cm-2 s-1 (~5x10^-15 erg cm^-2 s-1). We were able to associate a candidate optical counterpart to ~90% (626) of the X-ray sources, while for the remaining ~10% of the sources we were not able to provide a unique optical association due to the faintness of the possible optical counterparts (I_AB>25) or to the presence of multiple optical sources within the XMM-Newton error circles. We also cross-correlated the candidate optical counterparts with the Subaru multicolor and ACS catalogs and with the Magellan/IMACS, zCOSMOS and literature spectroscopic data; the spectroscopic sample comprises 248 objects (~40% of the full sample). Our analysis reveals that for ~80% of the counterparts there is a very good agreement between the spectroscopic classification, the morphological parameters as derived from ACS data, and the optical to near infrared colors. About 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the ``extended'' BL AGN lie at redshift <1.5, while the redshift distribution of the full BL AGN population peaks at z~1.5. Our analysis also suggests that the Type 2/Type 1 ratio decreases towards high luminosities, in qualitative agreement with the results from X-ray spectral analysis and the most recent modeling of the X-ray luminosity function evolution.

Astronomy & Astrophysics

We present a first catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory in the 4-12 keV energy band during its ongoing all-sky survey. The catalog comprises 867 sources detected on the combined map of the first two 6-month scans of the sky (December 2019-December 2020), ART-XC sky surveys 1 and 2, or ARTSS12. The achieved sensitivity to point sources varies between ∼ 4 × 10 −12 erg s −1 cm −2 near the ecliptic plane and ∼ 8 × 10 −13 erg s −1 cm −2 (4-12 keV) near the ecliptic poles, and the typical localization accuracy is ∼ 15. Of the 750 sources of known or suspected origin in the catalog, 56% are extragalactic (mostly active galactic nuclei, AGN; and clusters of galaxies) and the rest are Galactic (mostly cataclysmic variables, CVs; and low-and high-mass X-ray binaries). For 114 sources, ART-XC has detected X-rays for the first time. Although the majority of these (∼ 80) are expected to be spurious (given the adopted detection threshold), there can be a significant number of newly discovered astrophysical objects. We have started a program of optical follow-up observations of the new and previously unidentified X-ray sources, which has already led to the identification of several AGN and CVs. With the SRG all-sky survey planned to continue for a total of four years, we can expect the ART-XC survey in the 4-12 keV band to significantly surpass previous surveys that were carried out in similar (medium X-ray) energy bands in terms of the combination of angular resolution, sensitivity, and sky coverage.

Astronomy and Astrophysics, 2010

We investigate the possibility of using the ratio between the 2-10 keV flux and the [Ne V]3426 emission line flux (X/NeV) as a diagnostic diagram to discover heavily obscured, possibly Compton-Thick Active Galactic Nuclei (AGN) in the distant Universe. While being on average about one order of magnitude fainter than the more commonly used [O III]5007 emission line, the [Ne V]3426 line can be observed with optical spectroscopy up to z ∼ 1.5, whereas the [O III]5007 line is redshifted out of the optical bands already at z ∼ 0.8. First, we calibrate a relation between X/NeV and the cold absorbing column density N H using a sample of 74 bright, nearby Seyferts with both X-ray and [Ne V] data available in the literature, and for which the column density is determined unambiguously. Similarly to what is found for the X-ray to [O III]5007 flux ratio (X/OIII), we found that the X/NeV ratio decreases towards large column densities, as expected if [Ne V]3426 emission is a good tracer of the AGN intrinsic power. Essentially all local Seyferts with X/NeV values below 15 are found to be Compton-Thick objects. At X/NeV values below 100, the percentage of Compton-Thick nuclei decreases to ∼ 50%, but still ∼ 80% of the considered sample is absorbed with N H > 10 23 cm −2 . Second, we apply this diagnostic diagram to different samples of distant obscured and unobscured QSOs in the Sloan Digital Sky Survey (SDSS). SDSS blue, unobscured, type-1 QSOs in the redshift range z = [0.1 − 1.5] indeed show X/NeV values typical of unobscured Seyfert 1s in the local Universe. Conversely, SDSS type-2 QSOs at z ∼ 0.5 classified either as Compton-Thick or Compton-Thin on the basis of their X/OIII ratio, would have been mostly classified in the same way based on the X/NeV ratio. We apply the X/NeV diagnostic diagram to 9 SDSS obscured QSOs in the redshift range z = [0.85 − 1.31], selected by means of their prominent [Ne V]3426 line (rest EW > 4Å) and observed with Chandra ACIS-S for 10ks each (8 of them as part of our proprietary program). Based on the X/NeV ratio, complemented by X-ray spectral analysis, 2 objects appear good Compton-Thick QSO candidates, 4 objects appear as Compton-Thin QSOs, while 3 have an ambiguous classification. When excluding from the sample broad lined QSOs with a red continuum and thus considering only genuine narrow-line objects, the efficiency in selecting Compton-Thick QSOs through the [Ne V] line is about 50% (with large errors, though), more similar to what is achieved with [O III] selection. We discuss the possibility of applying the X/NeV diagnostic to deep X-ray surveys to search for Compton-Thick Seyferts at z ∼ 1, i.e. those objects which are thought to be responsible for the "missing" X-ray background. Finally, we compare the optical spectral properties of [Ne V]-selected QSOs with those of other SDSS populations of obscured and unobscured QSOs. By restricting the analysis to objects in the same redshift (and luminosity) range z=[0.4-1.5], we found evidence that, at any given [Ne V] luminosity, increasing obscuration is accompanied by increasing [O II]3727 emission. This correlation is interpreted as evidence for enhanced star formation in obscured QSOs, which is consistent with current popular scenarios of BHgalaxy coevolution.

The Astrophysical Journal, 2007

The Swift Burst Alert Telescope (BAT) is performing a high Galactic latitude survey in the 14-195 keV band at a flux limit of ∼10 −11 erg cm −2 s −1 , leading to the discovery of new high energy sources, most of which have not so far been properly classified. A similar work has also been performed with the RXTE slew survey leading to the discovery of 68 sources detected above 8 keV, many of which are still unclassified. Follow-up observations with the Swift X-ray Telescope (XRT) provide, for many of these objects, source localization with a positional accuracy of few arcsec, thus allowing the search for optical counterparts to be more efficient and reliable. We present the results of optical/X-ray followup studies of 11 Swift BAT detections and one AGN detected in the RXTE Slew Survey, aimed at identifying their longer-wavelength counterparts and at assessing their nature. These data allowed, for the first time, the optical classification of 8 objects and a distance determination for 3 of them. For another object, a more refined optical classification than that available in the literature is also provided. For the remaining sources, optical spectroscopy provides a characterization of the source near in time to the X-ray measurement. The sample consists of 6 Seyfert 2 galaxies, 5 Seyferts of intermediate type 1.2-1.8, and one object of Galactic nature -an Intermediate Polar (i.e., magnetic) Cataclysmic Variable. Out of the 11 AGNs, 8 (∼70%) including 2 Seyferts of type 1.2 and 1.5, are absorbed with N H > 10 22 cm −2 . Up to 3 objects could be Compton thick (i.e. N H > 1.5 × 10 24 cm −2 ), but only in one case (Swift J0609.1-8636) does all -2the observational evidence strongly suggests this possibility. The present data demonstrate the capability of coordinated hard X-ray and optical observations to discover absorbed AGNs.

Astronomy &amp; Astrophysics, 2007

Aims. Our goal is to probe the populations of obscured and unobscured AGN investigating their optical-IR and X-ray properties as a function of X-ray flux, luminosity and redshift within a hard X-ray selected sample with wide multiwavelength coverage. Methods. We selected a sample of 136 X-ray sources detected at a significance of ≥3σ in the 2−10 keV band (F 2−10 > ∼ 10 −14 erg cm −2 s −1) in a ∼1 deg 2 area in the XMM Medium Deep Survey (XMDS). The XMDS area is covered with optical photometry from the VVDS and CFHTLS surveys and infrared Spitzer data from the SWIRE survey. Based on the X-ray luminosity and X-ray to optical ratio, 132 sources are likely AGN, of which 122 have unambiguous optical-IR identification. The observed optical and IR spectral energy distributions of all identified sources are fitted with AGN/galaxy templates in order to classify them and compute photometric redshifts. X-ray spectral analysis is performed individually for sources with a sufficient number of counts and using a stacking technique for subsamples of sources at different flux levels. Hardness ratios are used to estimate X-ray absorption in individual weak sources. Results. 70% of the AGN are fitted by a type 2 AGN or a star forming galaxy template. We group them together in a single class of "optically obscured" AGN. These have "red" optical colors and in about 60% of cases show significant X-ray absorption (N H > 10 22 cm −2). Sources with SEDs typical of type 1 AGN have "blue" optical colors and exhibit X-ray absorption in about 30% of cases. The stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and has an average spectral slope of Γ = 1.6. The subsample of objects fitted by a star forming galaxy template has an even harder stacked spectrum, with Γ ∼ 1.2−1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower luminosities. X-ray absorption is less common than "optical" obscuration and its incidence is nearly constant with redshift and luminosity. This implies that at high luminosities X-ray absorption is not necessarily related to optical obscuration. The estimated surface densities of obscured, unobscured AGN and type 2 QSOs are respectively 138, 59 and 35 deg −2 at F > 10 −14 erg cm −2 s −1 .

Astronomy and Astrophysics, 2009

Aims. We present a sample of candidate quasars selected using the KX-technique. The data cover 0.68 deg 2 of the X-ray Multi-Mirror (XMM) Large-Scale Structure (LSS) survey area where overlapping multi-wavelength imaging data permits an investigation of the physical nature of selected sources. Methods. The KX method identifies quasars on the basis of their optical (R and z ) to near-infrared (K s ) photometry and point-like morphology. We combine these data with optical (u * , g r , i , z ) and mid-infrared (3.6 − 24 µm) wavebands to reconstruct the spectral energy distributions (SEDs) of candidate quasars. Results. Of 93 sources selected as candidate quasars by the KX method, 25 are classified as quasars by the subsequent SED analysis. Spectroscopic observations are available for 12/25 of these sources and confirm the quasar hypothesis in each case. Even more, 90% of the SED-classified quasars show X-ray emission, a property not shared by any of the false candidates in the KX-selected sample. Applying a photometric redshift analysis to the sources without spectroscopy indicates that the 25 sources classified as quasars occupy the interval 0.7 ≤ z ≤ 2.5. The remaining 68/93 sources are classified as stars and unresolved galaxies.

The Astrophysical Journal Supplement Series, 2009

We present catalogs of QSO candidates selected using photometry from GALEX combined with SDSS in the Stripe 82 region and Blanco Cosmology Survey (BCS) near declination −55 degrees. The SDSS region contains ≃ 700 objects with magnitude i < 20 and ≃ 3600 objects with i < 21.5 in a ≃ 60 square degree sky region, while the BCS region contains ≃ 280 objects with magnitude i < 20 and ∼ 2000 objects with i < 21.5 for a 11 square degree sky region that is being observed by three current microwave Sunyaev-Zeldovich surveys. Our QSO catalog is the first one in the BCS region. Deep GALEX exposures ( 2000 seconds in F UV and N UV , except in three fields) provide high signal-to-noise photometry in the GALEX bands (F UV , N UV < 24.5 mag). From this data, we select QSO candidates using only GALEX and optical r-band photometry, using the method given by Atlee and Gould (2008). In the Stripe 82 field, 60% (30%) of the GALEX selected QSO's with optical magnitude i < 20 (i < 21.5) also appear in the Richards et al. QSO catalog constructed using 5-band optical SDSS photometry. Comparison with the same catalog by Richards et al. shows that the completeness of the sample is approximately 40%(25%). However, for regions of the sky with very low dust extinction, like the BCS 23hr field and the Stripe 82 between 0 and 10 degrees in RA, our completeness is close to 95%, demonstrating that deep GALEX observations are almost as efficient as multi-wavelength observations at finding QSO's. GALEX observations thus provide a viable alternate route to QSO catalogs in sky regions where u-band optical photometry is not available. The full catalog is available at http://www.ice.csic.es/personal/jimenez/PHOTOZ.

Astrophysics and Space Science, 2012

We undertake a spectral study of a sample of bright X-ray sources taken from six XMM-Newton fields at high galactic latitudes,where AGN are the most populous class. These six fields were chosen such that the observation had an exposure time more than 60 ksec, had data from the EPIC-pn detector in the full-Frame mode and lying at high galactic latitude $|b| > 25^o$ . The analysis started by fitting the spectra of all sources with an absorbed power-law model, and then we fitted all the spectra with an absorbed power-law with a low energy black-body component model.The sources for which we added a black body gave an F-test probability of 0.01 or less (i.e. at 99\% confidence level), were recognized as sources that display soft excess. We perform a comparative analysis of soft excess spectral parameters with respect to the underlying power-law one for sources that satisfy this criterion. Those sources, that do not show evidence for a soft excess, based on the F-test probability at a 99\% confidence level, were also fitted with the absorbed power-law with a low energy black-body component model with the black-body temperature fixed at 0.1 and 0.2 keV. We establish upper limits on the soft excess flux for those sources at these two temperatures. Finally we have made use of Aladdin interactive sky atlas and matching with NASA/IPAC Extragalactic Database (NED) to identify the X-ray sources in our sample. For those sources which are identified in the NED catalogue, we make a comparative study of the soft excess phenomenon for different types of systems.

The Astronomical Journal, 1991

This is the fifth and final paper in a series reporting the results of a program aimed at selecting-1000 QSOs brighter than Bj-18.75 using machine-scanned direct and objective-prism plates from the UK Schmidt Telescope. The plate material is scanned at the Automated Plate Measuring facility. The candidate list is derived using a number of complementary selection algorithms ranging from "traditional" criteria, such as the presence of strong emission features, to criteria designed to select objects whose objective-prism spectra cannot be classified as normal stars, although they may possess no obvious QSO-like features. Follow-up spectroscopy at the 2.5 m duPont telescope of the Las Campanas Observatory is used to classify each candidate. In this paper we present results from three UK Schmidt Telescope fields in which 200 QSOs have been found, 182 of which form part of the LBQS sample. Coordinates, magnitudes, redshifts, and spectra of moderate resolution and signal-to-noise ratio are given for all 200 QSOs as well as for 3 additional extragalactic objects which fail to meet our absolute magnitude criterion as QSOs. Three pairs of direct and objective-prism United Kingdom Schmidt Telescope (UKST) plates, details of which are presented in Table 1, were searched for QSO candidates. The Automated Plate Measuring facility scans an area of ~ 5.8° X 5.8° on each plate, but the exclusion of areas around bright sources and near calibration wedges from further processing reduces the total area for the present fields to 94.9° sq. Furthermore, approximately 10% of the spectra cannot be processed because of plate flaws, satellite trails, or because they overlap with those of nearby sources. This results in an effective area of 85.5° sq., the figure that should be used for surface density and luminosity function calculations. Candidates fainter than Bj = 16.0 and brighter than the adopted limiting magnitude for each field (Table 1) were observed, irrespective of their direct image morphology. Candidate selection procedures have been described in Papers II and III.

1999

We present the X-ray data and the optical identifications for a deep ROSAT PSPC observation in the "Marano field". In the inner region of the ROSAT field (15 ′ radius) we detected 50 X-ray sources with S x ≥ 3.7 × 10 −15 erg cm −2 s −1. When corrected for the different sensitivity over the field, the estimated observed surface density at S x ≥ 4 × 10 −15 erg cm −2 s −1 is 272 ± 40 sources/sq.deg. Four X-ray sources, corresponding to 8% of the total sample, have been detected in radio images with a flux limit of about 0.2 mJy. Careful statistical analysis of multi-colour CCD data in the error boxes of the 50 X-ray sources has led to the identification of 42 sources, corresponding to 84% of the Xray sample. These 42 reliable identifications are 33 AGNs (including two radio galaxies and one BL Lac candidate; 79% of the identified sources), 2 galaxies, 3 groups or clusters of galaxies and 4 stars. If we divide our sample into two equally populated sub-samples as a function of flux, at S x = 6.5 × 10 −15 erg cm −2 s −1 , we find that the percentage of identifications remains approximately constant (88% and 80% in the high and low flux sub-samples, respectively). AGNs are the dominant class of objects in both sub-samples (90% of the optical identifications in the high flux sub-sample and 65% in the low flux subsample), while the few identifications with clusters and galaxies are all in the low flux sub-sample. We also show that it is likely that a few of the 8 unidentified sources are such because the derived X-ray positions may be offset with respect to the real ones due to confu