Observations of the extreme-ultraviolet transient RE J1255+266: a short outburst of a WZ Sge system?

We present a previously unpublished ROSAT Wide Field Camera observation of the transient source RE J1255+266 made just 4 d before the discovery observations. The source is not detected, limiting the duration of the outburst to be less than expected for a superoutburst of a WZ Sge system.We also present a marginal detection of X-ray emission from RE J1255+266 using ASCA. The most probable luminosity is 6x10(29) erg s(-1), which is very similar to WZ Sge itself.We discuss the nature of the source in the light of these observations, and conclude that it is most probably a WZ Sge system, but that the observed outburst must have been a normal dwarf nova outburst

Narrow-line x-ray-selected galaxies in the chandra-cosmos field. Ii. Optically elusive x-ray AGNS

In the Chandra-COSMOS (C-COSMOS) survey, we have looked for X-ray-selected active galactic nuclei (AGNs), which are not detected as such in the optical, the so-called elusive AGNs. A previous study based on XMM-Newton and Sloan Digital Sky Survey observations has found a sample of 31 X-ray AGNs optically misclassified as star-forming (SF) galaxies at $z\lt 0.4$, including 17 elusive Sy2s. Using Chandra observations provides a sample of fainter X-ray sources and so, for a given X-ray luminosity, extends to higher redshifts. To study the elusive Sy2s in the C-COSMOS field, we have removed the NLS1s that contaminate the narrow-line sample. Surprisingly, the contribution of NLS1s is much lower in the C-COSMOS sample (less than 10% of the optically misclassified X-ray AGNs) than in Pons & Watson. The optical misclassification of the X-ray AGNs (${L}_{{\rm{X}}}\gt {10}^{42}\;\mathrm{erg}\;{{\rm{s}}}^{-1}$) can be explained by the intrinsic weakness of these AGNs, in addition to, in some cases, optical dilution by the host galaxies. Interestingly, we found the fraction of elusive Sy2s (narrow emission-line objects) optically misclassified as SF galaxies up to $z\sim 1.4$ to be 10% ± 3% to 17% ± 4%, compared to the 6% ± 1.5% of the Pons & Watson work (up to $z\sim 0.4$). This result seems to indicate an evolution with redshift of the number of elusive Sy2s

The nature of the cataclysmic variable PT Per

We present a study of the cataclysmic variable star PT Per based on archival XMM-Newton X-ray data and new optical spectroscopy from the WHT with ISIS. The X-ray data show deep minima which recur at a period of 82 minutes and a hard, unabsorbed X-ray spectrum. The optical spectra of PT Per show a relatively featureless blue continuum. From an analysis of the X-ray and optical data we conclude that PT Per is likely to be a magnetic cataclysmic variable of the polar class in which the minima correspond to those phase intervals when the accretion column rotates out of the field of view of the observer. We suggest that the optical spectrum, obtained around 4 years after the X-ray coverage, is dominated by the white dwarf in the system, implying that PT Per was in a low accretion state at the time of the observations. An analysis of the likely system parameters for PT Per suggests a distance of ≈90 pc and a very low-mass secondary, consistent with the idea that PT Per is a “period-bounce” binary. Matching the observed absorption features in the optical spectrum with the expected Zeeman components constrains the white dwarf polar field to be Bp ≈ 25 − 27 MGauss

Searching for Compton-thick active galactic nuclei at z similar to 0.1

Using a suite of X-ray, mid-infrared (mid-IR) and optical active galactic nuclei (AGN) luminosity indicators, we search for Compton-thick AGNs with intrinsic L-X > 1042 erg s-1 at z similar to 0.03-0.2, a region of parameter space which is currently poorly constrained by deep narrow-field and high-energy (E > 10 keV) all-sky X-ray surveys. We have used the widest XMM-Newton survey (the serendipitous source catalogue) to select a representative subsample (14; approximate to 10 per cent) of the 147 X-ray undetected candidate Compton-thick AGNs in the Sloan Digital Sky Survey (SDSS) with f(X)/f([O III]) 1.5 x 10(24) cm(-2). Under the reasonable assumption that our 14 AGNs are representative of the overall X-ray undetected AGN population in the SDSS-XMM parent sample, we find that greater than or similar to 20 per cent of the optical type-2 AGN population are likely to be obscured by Compton-thick material. This implies a space density of log(Phi) greater than or similar to -4.9Mpc(-3) for Compton-thick AGNs with L-X greater than or similar to 10(42) erg s(-1) at z similar to 0.1, which we suggest may be consistent with that predicted by X-ray background synthesis models. Furthermore, using the 6-mu m continuum luminosity to infer the intrinsic AGN luminosity and the stellar velocity dispersion to estimate M-BH, we find that the most conservatively identified Compton-thick AGNs in this sample may harbour some of the most rapidly growing black holes (median M-BH approximate to 3 x 10(7) M-circle dot) in the nearby Universe, with a median Eddington ratio of eta approximate to 0.2

X-ray spectra of XMM-Newton serendipitous medium flux sources

We report on the results of a detailed analysis of the X-ray spectral properties of a large sample of sources detected serendipitously with the XMM-Newton observatory in 25 selected fields, for which optical identification is in progress. The survey covers a total solid angle of ~3.5 deg[superscript 2] and contains 1137 sources with ~10[superscript -15] 10[superscript 43] erg s[superscript -1], and therefore classified as type 2 AGNs) is significantly higher (40%), with a hint of moderately higher columns. After correcting for absorption, we do not find evidence for a redshift evolution of the underlying power law index of BLAGNs, which stays roughly constant at Γ ~ 1.9, with intrinsic dispersion of 0.4. A small fraction (~7%) of BLAGNs and NELGs require the presence of a soft excess, that we model as a black body with temperature ranging from 0.1 to 0.3 keV. Comparing our results on absorption to popular X-ray background synthesis models, we find absorption in only ~40% of the sources expected. This is due to a deficiency of heavily absorbed sources (with N[subscript H] ~ 10[superscript 22] – 10[superscript 24] cm[superscript -2]) in our sample in comparison with the models. We therefore conclude that the synthesis models require some revision in their specific parameters

The signature of supernova ejecta in the X-ray afterglow of the gamma-ray burst 011211

Now that γ-ray bursts (GRBs) have been determined to lie at cosmological distances, their isotropic burst energies are estimated to be as high as 10[superscript 54] erg (ref. 2), making them the most energetic phenomena in the Universe. The nature of the progenitors responsible for the bursts remains, however, elusive. The favoured models range from the merger of two neutron stars in a binary system[superscript 3-5] to the collapse of a massive star[superscript 6-8]. Spectroscopic studies of the afterglow emission could reveal details of the environment of the burst, by indicating the elements present, the speed of the outflow and an estimate of the temperature. Here we report an X-ray spectrum of the afterglow of GRB011211, which shows emission lines of magnesium, silicon, sulphur, argon, calcium and possibly nickel, arising in metal-enriched material with an outflow velocity of the order of one-tenth the speed of light. These observations strongly favour models[superscript 30] where a supernova explosion from a massive stellar progenitor precedes the burst event and is responsible for the outflowing matter

Extremely red objects in the UKIDSS Uultra Deep Survey Early Data Release

We construct a sample of extremely red objects (EROs) within the UKIDSS Ultra Deep Survey by combining the Early Data Release with optical data from the Subaru/XMM–Newton Deep Field. We find a total of 3715 objects over 2013 arcmin2 with R−K > 5.3 and K≤ 20.3, which is a higher surface density than found by previous studies. This is partly due to our ability to use a small aperture in which to measure colours, but is also the result of a genuine overdensity of objects compared to other fields. We separate our sample into passively evolving and dusty star-forming galaxies using their RJK colours and investigate their radio properties using a deep radio map. The dusty population has a higher fraction of individually detected radio sources and a higher mean radio flux density among the undetected objects, but the passive population has a higher fraction of bright radio sources, suggesting that active galactic nuclei are more prevalent among the passive ERO population

The evolution of the near-infrared galaxy luminosity function and colour bimodality up to z similar or equal to 2 rom the UKIDSS Ultra Deep Survey Early Data Release

We present new results on the cosmological evolution of the near-infrared (near-IR) galaxy luminosity function (LF), derived from the analysis of a new sample of similar to 22000K(AB) <= 22.5 galaxies selected over an area of 0.6 deg(2) from the Early Data Release of the UKIDSS Ultra Deep Survey (UDS). Our study has exploited the multiwavelength coverage of the UDS field provided by the new UKIDSS WFCAM K- and J- band imaging, the Subaru/XMM-Newton Deep Survey and the SpitzerWide-Area Infrared Extragalactic survey. The unique combination of large area and depth provided by this newsurvey minimizes the complicating effect of cosmic variance and has allowed us, for the first time, to trace the evolution of the brightest sources out to z similar or equal to 2 with good statistical accuracy.In agreement with previous studies, we find that the characteristic luminosity of the near-IR LF brightens by similar or equal to 1 mag between z = 0 and z similar or equal to 2, while the total density decreases by a factor of similar or equal to 2. Using the rest-frame (U - B) colour to split the sample into red and blue galaxies, we confirm the classic luminosity-dependent colour bimodality at z less than or similar to 1. However, the strength of the colour bimodality is found to be a decreasing function of redshift, and seems to disappear by z greater than or similar to 1.5. Due to the large size of our sample, we are able to investigate the differing cosmological evolution of the red and blue galaxy populations. It is found that the space density of the brightest red galaxies (MK <= - 23) stays approximately constant with redshift, and that these sources dominate the bright end of the LF at redshifts z less than or similar to 1. In contrast, the brightening of the characteristic luminosity and mild decrease in space density displayed by the blue galaxy population leads them to dominate the bright end of the LF at redshifts z greater than or similar to 1

The discovery of an evolving dust scattering 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

X-RAY ABSORPTION, NUCLEAR INFRARED EMISSION, AND DUST COVERING FACTORS OF AGNs: TESTING UNIFICATION SCHEMES

We present the distributions of the geometrical covering factors of the dusty tori (f2) of active galactic nuclei (AGNs) using an X-ray selected complete sample of 227 AGNs drawn from the Bright Ultra-hard XMM-Newton Survey. The AGNs have z from 0.05 to 1.7, 2–10 keV luminosities between 1042 and 1046 erg s−1, and Compton-thin X-ray absorption. Employing data from UKIDSS, 2MASS, and the Wide-field Infrared Survey Explorer in a previous work, we determined the rest-frame 1–20 μm continuum emission from the torus, which we model here with the clumpy torus models of Nenkova et al. Optically classified type 1 and type 2 AGNs are intrinsically different, with type 2 AGNs having, on average, tori with higher f2 than type 1 AGNs. Nevertheless, ~20% of type 1 AGNs have tori with large covering factors, while ~23%–28% of type 2 AGNs have tori with small covering factors. Low f2 are preferred at high AGN luminosities, as postulated by simple receding torus models, although for type 2 AGNs the effect is certainly small. f2 increases with the X-ray column density, which implies that dust extinction and X-ray absorption take place in material that share an overall geometry and most likely belong to the same structure, the putative torus. Based on our results, the viewing angle, AGN luminosity, and also f2 determine the optical appearance of an AGN and control the shape of the rest-frame ~1–20 μm nuclear continuum emission. Thus, the torus geometrical covering factor is a key ingredient of unification schemes