Jonathan A. Tedds

The XMM-Newton serendipitous survey: V - The Second XMM-Newton serendipitous source catalogue

Aims. Pointed observations with XMM-Newton provide the basis for creating catalogues of X-ray sources detected serendipitously in each field. This paper describes the creation and characteristics of the 2XMM catalogue. Methods. The 2XMM catalogue has been compiled from a new processing of the XMM-Newton EPIC camera data. The main features of the processing pipeline are described in detail. Results. The catalogue, the largest ever made at X-ray wavelengths, contains 246 897 detections drawn from 3491 public XMM-Newton observations over a 7-year interval, which relate to 191 870 unique sources. The catalogue fields cover a sky area of more than 500 deg(2). The non-overlapping sky area is similar to 360 deg(2) (similar to 1% of the sky) as many regions of the sky are observed more than once by XMM-Newton. The catalogue probes a large sky area at the flux limit where the bulk of the objects that contribute to the X-ray background lie and provides a major resource for generating large, well-defined X-ray selected source samples, studying the X-ray source population and identifying rare object types. The main characteristics of the catalogue are presented, including its photometric and astrometric properties

The Herschel ATLAS

The Herschel ATLAS is the largest open-time key project that will be carried out on the Herschel Space Observatory. It will survey 570 deg2 of the extragalactic sky, 4 times larger than all the other Herschel extragalactic surveys combined, in five far-infrared and submillimeter bands. We describe the survey, the complementary multiwavelength data sets that will be combined with the Herschel data, and the six major science programs we are undertaking. Using new models based on a previous submillimeter survey of galaxies, we present predictions of the properties of the ATLAS sources in other wave bands.

The XMM-Newton serendipitous survey - IV. Optical identification of the XMM-Newton medium sensitivity survey (XMS)

Aims. X-ray sources at intermediate fluxes (a few x 10(-14) erg cm(-2) s(-1)) with a sky density of similar to 100 deg(-2) are responsible for a significant fraction of the cosmic X-ray background at various energies below 10 keV. The aim of this paper is to provide an unbiased and quantitative description of the X-ray source population at these fluxes and in various X-ray energy bands. Methods. We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited. Results. We report on the optical identification of the XMS samples, complete to 85-95%. At the flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (similar to 35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN.

The cosmological properties of AGN in the XMM-Newton Hard Bright Survey

Aims. We investigate here the X-ray luminosity function (XLF) of absorbed (NH between 4 × 10 21 and 10 24 cm −2 ) and unabsorbed (NH 10 24 cm −2 ) AGN. Methods. To carry out this investigation, we have used the XMM-Newton Hard Bright Serendipitous Sample (HBSS), a complete sample of bright X-ray sources (fx > 7 × 10 −14 erg cm −2 s −1 ) at high galactic latitude (|b| > 20 ◦ ) selected in the 4.5−7.5 keV energy band. The HBSS sample is now almost completely identified (97% spectroscopic identifications) and it can be safely used for a statistical investigation. The HBSS contains 62 AGN out of which 40 are unabsorbed (or marginally absorbed; NH 3 × 10 42 erg s −1 is 0.57 ± 0.11; we find that F decreases with the intrinsic luminosity, and probably, increases with the redshift. Our data are consistent with a flat Log NH distribution for NH between 10 20 and 10 24 cm −2 . Finally, by comparing the results obtained here with those obtained using an optically-selected sample of AGN we derive, in an indirect way, the XLF of Compton thick AGN; the latter is well described by a XLF similar, in shape, to that of absorbed AGN, but having a normalization of about a factor of 2 above. The density ratio between Compton thick AGN (NH ≥ 10 24 cm −2 )a nd Compton thin AGN (NH ≤ 10 24 cm −2 ) decreases from 1.08 ± 0.44 at ∼10 43 erg s −1 to 0.57 ± 0.22 at ∼10 44 erg s −1 to 0.23 ± 0.15 at ∼10 45 erg s −1 . Conclusions. The results presented here on the anti-correlation between F and −Lx are fully consistent with the hypothesis of a reduction of the covering factor of the gas as a function of the luminosity and are clearly inconsistent with the simplest unified scheme of AGN. These results strongly support the recently proposed radiation-limited clumpy dust torus model although alternative physical models are also consistent with the observations.

High precision X-ray log N - Log S distributions: Implications for the obscured AGN population

Context. Our knowledge of the properties of AGN, especially those of optical type-2 objects, is very incomplete. Because extragalactic source count distributions are dependent on the cosmological and statistical properties of AGN, they provide a direct method of investigating the underlying source populations. Aims. We aim to constrain the extragalactic source count distributions over a broad range of X-ray fluxes and in various energy bands to test whether the predictions from X-ray background synthesis models agree with the observational constraints provided by our measurements. Methods. We have used 1129 XMM-Newton observations at |b| > 20 ◦ covering a total sky area of 132.3 deg 2 to compile the largest complete samples of X-ray selected objects to date both in the 0.5− 1k eV, 1− 2k eV, 2−4.5 keV, 4.5−10 keV bands employed in standard XMM-Newton data processing and in the 0.5−2 keV and 2−10 keV energy bands more usually considered in source count studies. Our survey includes in excess of 30 000 sources and spans fluxes from ∼10 −15 to 10 −12 erg cm −2 s −1 below 2 keV and from ∼10 −14 to 10 −12 erg cm −2 s −1 above 2 keV where the bulk of the cosmic X-ray background energy density is produced. Results. The very large sample size we obtained means our results are not limited by cosmic variance or low counting statistics. A break in the source count distributions was detected in all energy bands except the 4.5−10 keV band. We find that an analytical model comprising 2 power-law components cannot adequately describe the curvature seen in the source count distributions. The shape of the logN(>S ) − logS is strongly dependent on the energy band with a general steepening apparent as we move to higher energies. This is due to the fact that non-AGN populations, comprised mainly of stars and clusters of galaxies, contribute up to 30% of the source population at energies <2 keV and at fluxes ≥10 −13 erg cm −2 s −1 , and these populations of objects have significantly flatter source count distributions than AGN. We find a substantial increase in the relative fraction of hard X-ray sources at higher energies, from ≥55% below 2 keV to ≥77% above 2 keV. However, the majority of sources detected above 4.5 keV still have significant flux below 2 keV. Comparison with predictions from the synthesis models suggest that the models might be overpredicting the number of faint absorbed AGN, which would call for fine adjustment of some model parameters such as the obscured to unobscured AGN ratio and/or the distribution of column densities at intermediate obscuration.

The Discovery of an Evolving Dust-scattered X-Ray Halo around GRB 031203

We report the first detection of a time-dependent dust-scattered X-ray halo around a gamma-ray burst (GRB). GRB 031203 was observed by XMM-Newton starting 6 hr after the burst. The halo appeared as concentric ringlike structures centered on the GRB location. The radii of these structures increased with time as t1/2, consistent with small-angle X-ray scattering caused by a large column of dust along the line of sight to a cosmologically distant GRB. The rings are due to dust concentrated in two distinct slabs in the Galaxy located at distances of 880 and 1390 pc, consistent with known Galactic features. The halo brightness implies an initial soft X-ray pulse consistent with the observed GRB.

A very low luminosity X-ray flash: XMM newton observations of GRB 031203

GRB 031203 was observed by XMM-Newton twice, first with an observation beginning 6 hr after the burst and again after 3 days. The afterglow had average 0.2-10.0 keV fluxes for the first and second observations of × 10-13 and × 10-13 ergs cm-2 s-1, respectively, decaying very slowly according to a power law with an index of -0.55 ± 0.05. The prompt soft X-ray flux, inferred from a detection of the dust echo of the prompt emission, strongly implies that this burst is very soft and should be classified as an X-ray flash (XRF) and further, implies a steep temporal slope (-1.7) between the prompt and afterglow phases or in the early afterglow, very different from the later afterglow decay slope. A power law (Γ = 1.90 ± 0.05) with absorption at a level consistent with the Galactic foreground absorption fits the afterglow spectrum well. A bright low-redshift (z = 0.105) galaxy lies within 05 of the X-ray position and is likely to be the gamma-ray burst (GRB) host. At this redshift, GRB 031203 is the closest GRB or XRF known after GRB 980425. It has a very low equivalent isotropic gamma-ray energy in the burst (~3 × 1049 ergs) and X-ray luminosity in the afterglow (9 × 1042 ergs s-1 at 10 hr), 3-4 orders of magnitude less than typical bursts, though higher than either the faint XRF 020903 or GRB 980425. The rapid initial decline and subsequent very slow fading of the X-ray afterglow is also similar to that observed in GRB 980425, indicating that GRB 031203 may be representative of low-luminosity bursts.

The XMM-Newton serendipitous survey - III. The AXIS X-ray source counts and angular clustering

Context. Recent results have revised upwards the total X-ray background (XRB) intensity below∼10 keV, therefore an accurate determination of the source counts is needed. There are also contradicting results on the clustering of X-ray selected sou rces. Aims. We have studied the X-ray source counts in four energy bands soft (0.5-2 keV), hard (2-10 keV), XID (0.5-4.5 keV) and ultra-hard (4.5-7.5 keV), to evaluate the contribution of s ources at different fluxes to the X-ray background. We have also studied the angular clustering of X-ray sources in those bands. Methods. AXIS (An XMM-Newton International Survey) is a survey of 36 high Galactic latitu de XMM-Newton observations covering 4.8 deg 2 and containing 1433 serendipitous X-ray sources detected with 5-σ significance. This survey has similar depth to the XMM-Newton catalogues and can serve as a pathfinder to explore their poss ibilities. We have combined this survey with shallower and deeper surveys, and fitted the source counts with a Maximu m Likelihood technique. Using only AXIS sources, we have studied the angular correlation using a novel robust technique. Results. Our source counts results are compatible with most previous samples in the soft, XID, ultra-hard and hard bands. We have improved on previous results in the latter band. The fractions of the XRB resolved in the surveys used in this work are 87%, 85%, 60% and 25% in the soft, hard, XID and ultra-hard bands, respectively. Extrapolation of our source counts to zero flux are n ot enough to saturate the XRB intensity. Only galaxies and/or absorbed AGN may be able contribute the remaining unresolved XRB intensity. Our results are compatible, within the errors, with recent r evisions of the XRB intensity in the soft and hard bands. The maximum fractional contribution to the XRB comes from fluxes within a bout a decade of the break in the source counts (∼ 10 −14 cgs), reaching ∼50% of the total in the soft and hard bands. Angular clustering (widely distributed over the sky and not confined to a few dee p fields) is detected at 99-99.9% significance in the soft and XID bands , with no detection in the hard and ultra-hard band (probably due to the smaller number of sources). We cannot confirm the detection o f significantly stronger clustering in the hard-spectrum ha rd sources. Conclusions. Medium depth surveys such as AXIS are essential to determine the evolution of the X-ray emission in the Universe below 10 keV.

The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme

Aims.Our goal is to characterize AGN populations by comparing their X-ray and optical classifications within the framework of the standard orientation-based unified scheme. Methods: We present a sample of 99 spectroscopically identified (R ? 22 mag) X-ray selected point sources in the XMM-LSS survey which are significantly detected (?3?) in the [ 2-10] keV band with fluxes between 8 × 10-15 and 8 × 10-14 erg s-1 cm-2, and which have more than 80 counts. We have compared their X-ray and optical classifications. To this end, we performed an X-ray spectral analysis for all of these 99 X-ray sources in order to assess whether they are intrinsically absorbed or not. The X-ray classification is based on the measured intrinsic column density. The optical classification is based on the measured FWHM of the permitted emission lines, the absence of broad lines being due to obscuration within the framework of the standard AGN unified scheme. Results: Introducing the fourfold point correlation coefficient r, we find a mild correlation between the X-ray and the optical classifications (r = 0.28), as up to 32 X-ray sources out of 99 have differing X-ray and optical classifications: on one hand, 10% of the type 1 sources (7/32) present broad emission lines in their optical spectra and strong absorption (N{H}int ? 1022 cm-2) in the X-rays. These objects are highly luminous AGN lying at high redshift and thus dilution effects by the host galaxy light are totally ruled out, their discrepant nature being an intrinsic property instead. Their X-ray luminosities and redshifts distributions are consistent with those of the unabsorbed X-ray sources with broad emission lines (L{2-10} 4 × 1044 erg s-1; z 1.9). On the other hand, 25/32 are moderate luminosity (L{2-10} ? 5 × 1043 erg s-1) AGN, which are both unabsorbed in the X-rays and only present narrow emission lines in their optical spectra. Based on their line ratios in the optical, the majority of them have an optical spectrum which is more representative of the host galaxy rather than of a reddened AGN. We finally infer that dilution of the AGN by the host galaxy seems to account for their nature. 5/25 have been defined as Seyfert 2 based on their optical spectra. In conclusion, most of these 32 discrepant cases can be accounted for by the standard AGN unified scheme, as its predictions are not met for only 12% of the 99 X-ray sources. Table 4 and Figs. [see full text]-[see full text] are only available in electronic form at http://www.aanda.org

X-ray absorption in distant type II QSOs

Aims. We present the results of the X-ray spectral analysis of an XMM-Newton-selected type II QSO sample with z≥ 0.5 and 0.510 keV flux of 0.3− 33× 10 −14 erg/s/cm 2 . The distribution of absorbing column densities in type II QSOs is investigated and the dependence of absorption on X-ray luminosity and redshift is studied. Methods. We inspected 51 spectroscopically classified type II QSO can didates from the XMM-Newton Marano field survey, the XMM-Newton-2dF wide angle survey (XWAS), and the AXIS survey to set-up a well-defined sample with secure optical type II identifications. Fourteen type II QSOs were classified and an X-ra y spectral analysis performed. Since most of our sources have only∼40 X-ray counts (PN-detector), we carefully studied the fit res ults of the simulated X-ray spectra as a function of fit statis tic and binning method. We determined that fitting the spectra with the Cash- statistic and a binning of minimum one count per bin recovers the input values of the simulated X-ray spectra best. Above 100 PN counts, the free fits of the spectrum’s slope and absorbing hydrog en column density are reliable. Results. We find only moderate absorption ( NH = (2− 10)× 10 22 cm −2 ) and no obvious trends with redshift and intrinsic X-ray luminosity. In a few cases a Compton-thick absorber cannot be excluded. Two type II objects with no X-ray absorption were discovered. We find no evidence for an intrinsic separation between type I I AGN and high X-ray luminosity type II QSO in terms of absorption. The stacked X-ray spectrum of our 14 type II QSOs shows no iron Kα line. In contrast, the stack of the 8 type II AGN reveals a very prominent iron Kα line at an energy of∼ 6.6 keV and an EW∼ 2 keV.