Abstract
This paper defines the UK Infra-Red Telescope (UKIRT) Hemisphere Survey (UHS) and release of the remaining ∼12 700 deg2 of J-band survey data products. The UHS will provide continuous J- and ...K-band coverage in the Northern hemisphere from a declination of 0° to 60° by combining the existing Large Area Survey, Galactic Plane Survey and Galactic Clusters Survey conducted under the UKIRT Infra-red Deep Sky Survey (UKIDSS) programme with this new additional area not covered by UKIDSS. The released data include J-band imaging and source catalogues over the new area, which, together with UKIDSS, completes the J-band UHS coverage over the full ∼17 900 deg2 area. 98 per cent of the data in this release have passed quality control criteria. The remaining 2 per cent have been scheduled for re-observation. The median 5σ point source sensitivity of the released data is 19.6 mag (Vega). The median full width at half-maximum of the point spread function across the data set
is 0.75 arcsec. In this paper, we outline the survey management, data acquisition, processing and calibration, quality control and archiving as well as summarizing the characteristics of the released data products. The data are initially available to a limited consortium with a world-wide release scheduled for 2018 August.
The polycomb repressive complex 2 (PRC2) regulates epigenetic gene repression in eukaryotes. Mechanisms controlling its developmental specificity and signal-responsiveness are poorly understood. ...Here, we identify an oxygen-sensitive N-terminal (N-) degron in the plant PRC2 subunit VERNALIZATION(VRN) 2, a homolog of animal Su(z)12, that promotes its degradation via the N-end rule pathway. We provide evidence that this N-degron arose early during angiosperm evolution via gene duplication and N-terminal truncation, facilitating expansion of PRC2 function in flowering plants. We show that proteolysis via the N-end rule pathway prevents ectopic VRN2 accumulation, and that hypoxia and long-term cold exposure lead to increased VRN2 abundance, which we propose may be due to inhibition of VRN2 turnover via its N-degron. Furthermore, we identify an overlap in the transcriptional responses to hypoxia and prolonged cold, and show that VRN2 promotes tolerance to hypoxia. Our work reveals a mechanism for post-translational regulation of VRN2 stability that could potentially link environmental inputs to the epigenetic control of plant development.
Aims. We investigate here the X-ray luminosity function (XLF) of absorbed (NH between 4 $\times$ 1021 and 1024 cm-2) and unabsorbed (NH < 4 $\times$ 1021 cm-2) AGN, the fraction of absorbed AGN as a ...function of LX (and z), the intrinsic NH distribution of the AGN population, and the XLF of Compton thick (NH > 1024 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 $\ga$ 7 $\times$ 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 < 4 $\times$ 1021 cm-2) and 22 are absorbed (NH between 4 $\times$ 1021 and ~1024 cm-2). Results. Absorbed and unabsorbed AGN are characterised by two different XLF with the absorbed AGN population being described by a steeper XLF, if compared with the unabsorbed ones, at all luminosities. The intrinsic fraction F of absorbed AGN (i.e., the fraction of sources with NH between 4 $\times$ 1021 and 1024 cm-2 divided the sources with NH below 1024 cm-2, corrected for the bias due to the photoelectric absorption) with $L_{2{-}10~{\rm keV}}$ $\ga$ 3 $\times$ 1042 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 1020 and 1024 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 ≥ 1024 cm-2) and Compton thin AGN (NH ≤ 1024 cm-2) decreases from 1.08 ± 0.44 at ~1043 erg s-1 to 0.57 ± 0.22 at ~1044 erg s-1 to 0.23 ± 0.15 at ~1045 erg s-1. Conclusions. The results presented here on the anti-correlation between F and $-L_{\rm x}$ 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.
We present here "The XMM-Newton Bright Serendipitous Survey", composed of two flux-limited samples: the XMM-Newton Bright Source Sample (BSS, hereafter) and the XMM-Newton "Hard" Bright Source Sample ...(HBSS, hereafter) having a flux limit of f sub(x) 7 x 10 super(-14) erg cm super(-2) s super(-1) in the 0.5-4.5 keV and 4.5-7.5 keV energy band, respectively. After discussing the main goals of this project and the survey strategy, we present the basic data on a complete sample of 400 X-ray sources (389 of them belong to the BSS, 67 to the HBSS with 56 X-ray sources in common) derived from the analysis of 237 suitable XMM-Newton fields (211 for the HBSS). At the flux limit of the survey we cover a survey area of 28.10 (25.17 for the HBSS) sq. deg. The extragalactic number-flux relationships (in the 0.5-4.5 keV and in the 4.5-7.5 keV energy bands) are in good agreement with previous and new results making us confident about the correctness of data selection and analysis. Up to now similar to 71% ( similar to 90%) of the sources have been spectroscopically identified making the BSS (HBSS) the sample with the highest number of identified XMM-Newton sources published so far. At the X-ray flux limits of the sources studied here we found that: a) the optical counterpart in the majority ( similar to 90%) of cases has a magnitude brighter than the POSS II limit (R similar to 21 super(mag)); b) the majority of the objects identified so far are broad line AGN both in the BSS and in the HBSS. No obvious trend of the source spectra (as deduced from the Hardness Ratios analysis) as a function of the count rate is measured and the average spectra of the "extragalactic" population corresponds to a (0.5-4.5 keV) energy spectral index of similar to 0.8 ( similar to 0.64) for the BSS (HBSS) sample. Based on the hardness ratios we infer that about 13% (40%) of the sources in the BSS (HBSS) sample are described by an energy spectral index flatter than that of the cosmic X-ray background. Based on previous X-ray spectral results on a small subsample of objects we speculate that all these sources are indeed absorbed AGN with the N sub(H) ranging from a few times 10 super(21) up to few times 10 super(23) cm super(-2). We do not find strong evidence that the 4.5-7.5 keV survey is sampling a completely different source population if compared with the 0.5-4.5 keV survey; rather we find that, as expected from the CXB synthesis models, the hard survey is simply picking up a larger fraction of absorbed AGN. At the flux limit of the HBSS sample we measure surface densities of optically type 1 and type 2 AGN of 1.63 plus or minus 0.25 deg super(-2) and 0.83 plus or minus 0.18 deg super(-2), respectively; optically type 2 AGN represent 34 plus or minus 9% of the total AGN population. Finally, we have found a clear separation, in the hardness ratio diagram and in teh (hardness ratio) vs. (X-ray to optical flux ratio) diagram, between Galactic "coronal emitting" stars and extragalactic sources. The information and "calibration" reported in this paper will make the existing and incoming XMM-Newton catalogs a unique resource for astrophysical studies.
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 $|b|>20{^\circ}$ covering a total sky area of 132.3 ${\rm deg^2}$ to compile the largest complete samples of X-ray selected objects to date both in the 0.5-1 keV, 1-2 keV, 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 ~${\rm 10^{-15}}$ to ${\rm 10^{-12}~erg~cm^{-2}~s^{-1}}$ below 2 keV and from ~${\rm 10^{-14}}$ to ${\rm 10^{-12}~erg~cm^{-2}~s^{-1}}$ 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 $\log N$(>S) - $\log S$ 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 ≥${\rm 10^{-13}~erg~cm^{-2}~s^{-1}}$, 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.
Context. Recent results have revised upwards the total X-ray background (XRB) intensity below similar to 10 keV, therefore an accurate determination of the source counts is needed. There are also ...contradictory results on the clustering of X-ray selected sources. 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) in order to evaluate the contribution of sources 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 latitude XMM-Newton observations covering 4.8 deg super(2) in the Northern sky and containing 1433 serendipitous X-ray sources detected with 5-\sigma significance. This survey has similar depth to the XMM-Newton catalogues and therefore serves as a pathfinder to explore their possibilities. We have combined this survey with shallower and deeper surveys, and fitted the source counts with a Maximum 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 hard 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 is not sufficient to saturate the XRB intensity. Only galaxies and/or absorbed AGN could contribute the remaining unresolved XRB intensity. Our results are compatible, within the errors, with recent revisions of the XRB intensity in the soft and hard bands. The maximum fractional contribution to the XRB comes from fluxes within about a decade of the break in the source counts ( similar to 10 super(-14) cgs), reaching similar to 50% of the total in the soft and hard bands. Angular clustering (widely distributed over the sky and not confined to a few deep 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 of significantly stronger clustering in the hard- spectrum hard 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.
We study a sample of six X-ray-selected broad absorption line (BAL) quasi-stellar objects (QSOs) from the XMM-Newton Wide Angle Survey. All six objects are classified as BALQSOs using the classic ...balnicity index, and together they form the largest sample of X-ray-selected BALQSOs. We find evidence for absorption in the X-ray spectra of all six objects. An ionized absorption model applied to an X-ray spectral shape that would be typical for non-BAL QSOs (a power law with energy index a = 0.98) provides acceptable fits to the X-ray spectra of all six objects. The optical to X-ray spectral indices, aOX, of the X-ray-selected BALQSOs, have a mean value of = 1.69 plus or minus 0.05, which is similar to that found for X-ray-selected and optically selected non-BAL QSOs of a similar ultraviolet luminosity. In contrast, optically selected BALQSOs typically have much larger aOX and so are characterized as being X-ray weak. The results imply that X-ray selection yields intrinsically X-ray bright BALQSOs, but their X-ray spectra are absorbed by a similar degree to that seen in optically selected BALQSO samples; X-ray absorption appears to be ubiquitous in BALQSOs, but X-ray weakness is not. We argue that BALQSOs sit at one end of a spectrum of X-ray absorption properties in QSOs related to the degree of ultraviolet absorption in C IV 1550 A.
Aims. We discuss the broad band X-ray properties of one of the largest samples of X-ray selected type-1 AGN to date (487 objects in total), drawn from the XMM-Newton Wide Angle Survey (XWAS). The ...objects presented in this work cover 2-10 keV (rest-frame) luminosities from ~1042-1045 erg s-1 and are detected up to redshift ~4. We constrain the overall properties of the broad band continuum, soft excess and X-ray absorption, along with their dependence on the X-ray luminosity and redshift. We discuss the implications for models of AGN emission. Methods. We fitted the observed 0.2-12 keV broad band spectra with various models to search for X-ray absorption and soft excess. The F-test was used with a significance threshold of 99% to statistically accept the detection of additional spectral components. Results. We constrained the mean spectral index of the broad band X-ray continuum to $\langle$Γ$\rangle$ = 1.96 ± 0.02 with intrinsic dispersion ${\rm \sigma_{\langle \Gamma \rangle}}$ = $0.27_{-0.02}^{+0.01}$. The continuum becomes harder at faint fluxes and at higher redshifts and hard (2-10 keV) luminosities. The dependence of Γ with flux is likely due to undetected absorption rather than to spectral variation. We found a strong dependence of the detection efficiency of objects on the spectral shape. We expect this effect to have an impact on the measured mean continuum shapes of sources at different redshifts and luminosities. We detected excess absorption in $\ga$3% of our objects, with rest-frame column densities ~a few $\times$1022 cm-2. The apparent mismatch between the optical classification and X-ray properties of these objects is a challenge for the standard orientation-based AGN unification model. We found that the fraction of objects with detected soft excess is ~36%. Using a thermal model, we constrained the soft excess mean rest-frame temperature and intrinsic dispersion to kT ~ 100 eV and $\sigma_{kT}$ ~ 34 eV. The origin of the soft excess as thermal emission from the accretion disk or Compton scattered disk emission is ruled out on the basis of the temperatures detected and the lack of correlation of the soft excess temperature with the hard X-ray luminosity over more than 2 orders of magnitude in luminosity. Furthermore, the high luminosities of the soft excess rule out an origin in the host galaxy.
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