Abstract
We release the AllWISE counterparts and Gaia matches to 106 573 and 17 665 X-ray sources detected in the ROSAT 2RXS and XMMSL2 surveys with |b| > 15°. These are the brightest X-ray sources ...in the sky, but their position uncertainties and the sparse multi-wavelength coverage until now rendered the identification of their counterparts a demanding task with uncertain results. New all-sky multi-wavelength surveys of sufficient depth, like AllWISE and Gaia, and a new Bayesian statistics based algorithm, Nway, allow us, for the first time, to provide reliable counterpart associations. Nway extends previous distance and sky density based association methods and, using one or more priors (e.g. colours, magnitudes), weights the probability that sources from two or more catalogues are simultaneously associated on the basis of their observable characteristics. Here, counterparts have been determined using a Wide-field Infrared Survey Explorer (WISE) colour-magnitude prior. A reference sample of 4524 XMM/Chandra and Swift X-ray sources demonstrates a reliability of ∼94.7 per cent (2RXS) and 97.4 per cent (XMMSL2). Combining our results with Chandra-COSMOS data, we propose a new separation between stars and AGN in the X-ray/WISE flux-magnitude plane, valid over six orders of magnitude. We also release the Nway code and its user manual. Nway was extensively tested with XMM-COSMOS data. Using two different sets of priors, we find an agreement of 96 per cent and 99 per cent with published Likelihood Ratio methods. Our results were achieved faster and without any follow-up visual inspection. With the advent of deep and wide area surveys in X-rays (e.g. SRG/eROSITA, Athena/WFI) and radio (ASKAP/EMU, LOFAR, APERTIF, etc.) Nway will provide a powerful and reliable counterpart identification tool.
Lags measured from correlated X-ray/UV/optical monitoring of AGN allow us to determine whether UV/optical variability is driven by reprocessing of X-rays or X-ray variability is driven by UV/optical ...seed photon variations. We present the results of the largest study to date of the relationship between the X-ray, UV and optical variability in an AGN with 554 observations, over a 750 d period, of the Seyfert 1 galaxy NGC 5548 with Swift. There is a good overall correlation between the X-ray and UV/optical bands, particularly on short time-scales (tens of days). The UV/optical bands lag the X-ray band with lags which are proportional to wavelength raised to the power 1.23 ± 0.31. This power is very close to the power (4/3) expected if short time-scale UV/optical variability is driven by reprocessing of X-rays by a surrounding accretion disc. The observed lags, however, are longer than expected from a standard Shakura–Sunyaev accretion disc with X-ray heating, given the currently accepted black hole mass and accretion rate values, but can be explained with a slightly larger mass and accretion rate, and a generally hotter disc. Some long-term UV/optical variations are not paralleled exactly in the X-rays, suggesting an additional component to the UV/optical variability arising perhaps from accretion rate perturbations propagating inwards through the disc.
This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a ∼ 18 deg2 area of the equatorial XMM-XXL north field. A sample of 8445 ...point-like X-ray sources detected by XMM–Newton above a limiting flux of
$F_{\rm 0.5{\rm -}10\, keV} > 10^{-15} \rm \,erg\, cm^{-2}\, s^{-1}$
was matched to optical (Sloan Digital Sky Survey, SDSS) and infrared (IR; WISE) counterparts. We followed up 3042 sources brighter than r = 22.5 mag with the SDSS Baryon Oscillation Spectroscopic Survey (BOSS) spectrograph. The spectra yielded a reliable redshift measurement for 2578 AGNs in the redshift range z = 0.02–5.0, with 0.5-2 keV luminosities ranging from 1039-1046 erg s− 1. This is currently the largest published spectroscopic sample of X-ray-selected AGNs in a contiguous area. The BOSS spectra of AGN candidates show a distribution of optical line widths which is clearly bimodal, allowing an efficient separation between broad- and narrow-emission line AGNs. The former dominate our sample (70 per cent) due to the relatively bright X-ray flux limit and the optical BOSS magnitude limit. We classify the narrow-emission line objects (22 per cent of the full sample) using standard optical emission line diagnostics: the majority have line ratios indicating the dominant source of ionization is the AGN. A small number (8 per cent of the full sample) exhibit the typical narrow line ratios of star-forming galaxies, or only have absorption lines in their spectra. We term the latter two classes ‘elusive’ AGN, which would not be easy to identify correctly without their X-ray emission. We also compare X-ray (XMM–Newton), optical colour (SDSS) and and IR (WISE) AGN selections in this field. X-ray observations reveal, by far, the largest number of AGN. The overlap between the selections, which is a strong function of the imaging depth in a given band, is also remarkably small. We show using spectral stacking that a large fraction of the X-ray AGNs would not be selectable via optical or IR colours due to host galaxy contamination. A substantial fraction of AGN may therefore be missed by these longer wavelength selection methods.
We argue that the ‘changing look’ active galactic nucleus (AGN) recently reported by LaMassa et al. could be a luminous flare produced by the tidal disruption of a supersolar mass star passing just a ...few gravitational radii outside the event horizon of a ∼108 M⊙ nuclear black hole. This flare occurred in a massive, star-forming galaxy at redshift z = 0.312, robustly characterized thanks to repeated late-time photometric and spectroscopic observations. By taking difference photometry of the well-sampled multiyear Sloan Digital Sky Survey (SDSS) Stripe-82 light curve, we are able to probe the evolution of the nuclear spectrum over the course of the outburst. The tidal disruption event (TDE) interpretation is consistent with the very rapid rise and the decay time of the flare, which displays an evolution consistent with the well-known t
−5/3 behaviour (with a clear superimposed re-brightening flare). Our analysis places constraints on the physical properties of the TDE, such as the putative disrupted star's mass and orbital parameters, as well as the size and temperature of the emitting material. The properties of the broad and narrow emission lines observed in two epochs of SDSS spectra provide further constraints on the circum-nuclear structure, and could be indicative that the system hosted a moderate-luminosity AGN as recently as a few 104 yr ago, and is likely undergoing residual accretion as late as 10 yr after peak, as seen from the broad Hα emission line. We discuss the complex interplay between TDEs and gas accretion episodes in galactic nuclei, highlighting the implications for future TDE searches and for estimates of their intrinsic rates.
SPIDERS (The SPectroscopic IDentification of eROSITA Sources) is a programme dedicated to the homogeneous and complete spectroscopic follow-up of X-ray active galactic nuclei and galaxy clusters over ...a large area (~7500 deg super( 2)) of the extragalactic sky. SPIDERS is part of the Sloan Digital Sky Survey (SDSS)-IV project, together with the Extended Baryon Oscillation Spectroscopic Survey and the Time-Domain Spectroscopic Survey. This paper describes the largest project within SPIDERS before the launch of eROSITA: an optical spectroscopic survey of X-ray-selected, massive (~10 super( 14)-10 super( 15) M...) galaxy clusters discovered in ROSAT and XMM-Newton imaging. The immediate aim is to determine precise (... ~ 0.001) redshifts for 4000-5000 of these systems out to z ~ 0.6. The scientific goal of the program is precision cosmology, using clusters as probes of large-scale structure in the expanding Universe. We present the cluster samples, target selection algorithms and observation strategies. We demonstrate the efficiency of selecting targets using a combination of SDSS imaging data, a robust red-sequence finder and a dedicated prioritization scheme. We describe a set of algorithms and work-flow developed to collate spectra and assign cluster membership, and to deliver catalogues of spectroscopically confirmed clusters. We discuss the relevance of line-of-sight velocity dispersion estimators for the richer systems. We illustrate our techniques by constructing a catalogue of 230 spectroscopically validated clusters (0.031 < z < 0.658), found in pilot observations. We discuss two potential science applications of the SPIDERS sample: the study of the X-ray luminosity-velocity dispersion (L sub( X)-...) relation and the building of stacked phase-space diagrams. (ProQuest: ... denotes formulae/symbols omitted.)
Discerning the exact nature of the sub-mJy radio population has been historically difficult due to the low luminosity of these sources at most wavelengths. Using deep ground based optical follow-up ...and observations from the Spitzer Space Telescope we are able to disentangle the radio-selected active galactic nuclei (AGN) and star-forming galaxy (SFG) populations for the first time in a deep multifrequency VLA/MERLIN Survey of the 13HXMM–Newton/Chandra Deep Field. The discrimination diagnostics include radio morphology, radio spectral index, radio/near-infrared (near-IR) and mid-IR/radio flux density ratios. We are now able to calculate the extragalactic Euclidean normalized source counts separately for AGN and SFGs. We find that while SFGs dominate at the faintest flux densities and account for the majority of the upturn in the counts, AGN still make up around one quarter of the counts at ∼50 μJy (1.4 GHz). Using radio luminosity as an unobscured star formation rate (SFR) measure we are then able to examine the comoving SFR density of the Universe up to z= 3 which agrees well with measures at other wavelengths. We find a rough correlation of SFR with stellar mass for both the sample presented here and a sample of local radio-selected SFGs from the 6df-NVSS survey. This work also confirms the existence of, and provides alternative evidence for, the evolution of distribution of star formation by galaxy mass: ‘downsizing’. As both these samples are SFR-selected, this result suggests that there is a maximum SFR for a given galaxy that depends linearly on its stellar mass. The low ‘characteristic times’ (inverse specific SFR) of the SFGs in our sample are similar to those of the 6dF-NVSS sample, implying that most of these sources are in a current phase of enhanced star formation.
In the context of the upcoming SRG/eROSITA survey, we present an N-body simulation-based mock catalogue for X-ray-selected active galactic nucleus (AGN) samples. The model reproduces the observed ...hard X-ray AGN luminosity function (XLF) and the soft X-ray logN–logS from redshift 0 to 6. The XLF is reproduced to within |$\pm 5{{\ \rm per\ cent}}$| and the logN-logS to within |$\pm 20{{\ \rm per\ cent}}$|. We develop a joint X-ray – optical extinction and classification model. We adopt a set of empirical spectral energy distributions to predict observed magnitudes in the UV, optical, and NIR. With the latest eROSITA all sky survey sensitivity model, we create a high-fidelity full-sky mock catalogue of X-ray AGN. It predicts their distributions in right ascension, declination, redshift, and fluxes. Using empirical medium resolution optical spectral templates and an exposure time calculator, we find that 1.1 × 10^6 (4 × 10^5) fibre-hours are needed to follow-up spectroscopically from the ground the detected X-ray AGN with an optical magnitude 21 < r < 22.8 (22.8 < r < 25) with a 4-m (8-m) class multiobject spectroscopic facility. We find that future clustering studies will measure the AGN bias to the per cent level at redshift z < 1.2 and should discriminate possible scenarios of galaxy-AGN co-evolution. We predict the accuracy to which the baryon acoustic oscillation standard ruler will be measured using X-ray AGN: better than 3 per cent for AGN between redshift 0.5 to 3 and better than 1 per cent using the Ly α forest of X-ray QSOs discovered between redshift 2 and 3. eROSITA will provide an outstanding set of targets for future galaxy evolution and cosmological studies.
Abstract
SPIDERS (SPectroscopic IDentification of eROSITA Sources) is a Sloan Digital Sky Survey IV (SDSS-IV) survey running in parallel to the Extended Baryon Oscillation Spectroscopic Survey ...(eBOSS) cosmology project. SPIDERS will obtain optical spectroscopy for large numbers of X-ray-selected active galactic nuclei (AGN) and galaxy cluster members detected in wide-area eROSITA, XMM–Newton and ROSAT surveys. We describe the methods used to choose spectroscopic targets for two sub-programmes of SPIDERS X-ray selected AGN candidates detected in the ROSAT All Sky and the XMM–Newton Slew surveys. We have exploited a Bayesian cross-matching algorithm, guided by priors based on mid-IR colour–magnitude information from the Wide-field Infrared Survey Explorer survey, to select the most probable optical counterpart to each X-ray detection. We empirically demonstrate the high fidelity of our counterpart selection method using a reference sample of bright well-localized X-ray sources collated from XMM–Newton, Chandra and Swift-XRT serendipitous catalogues, and also by examining blank-sky locations. We describe the down-selection steps which resulted in the final set of SPIDERS-AGN targets put forward for spectroscopy within the eBOSS/TDSS/SPIDERS survey, and present catalogues of these targets. We also present catalogues of ∼12 000 ROSAT and ∼1500 XMM–Newton Slew survey sources that have existing optical spectroscopy from SDSS-DR12, including the results of our visual inspections. On completion of the SPIDERS programme, we expect to have collected homogeneous spectroscopic redshift information over a footprint of ∼7500 deg2 for >85 per cent of the ROSAT and XMM–Newton Slew survey sources having optical counterparts in the magnitude range 17 < r < 22.5, producing a large and highly complete sample of bright X-ray-selected AGN suitable for statistical studies of AGN evolution and clustering.
The narrow-line Seyfert 1 galaxy IRAS 13224−3809 has been observed with XMM-Newton for 500 ks. The source is rapidly variable on time-scales down to a few 100 s. The spectrum shows strong broad Fe − ...K
and L emission features which are interpreted as arising from reflection from the inner parts of an accretion disc around a rapidly spinning black hole. Assuming a power law emissivity for the reflected flux and that the innermost radius corresponds to the innermost stable circular orbit, the black hole spin is measured to be 0.989 with a statistical precision better than 1 per cent. Systematic uncertainties are discussed. A soft X-ray lag of 100 s confirms this scenario. The bulk of the power-law continuum source is located at a radius of 2-3 gravitational radii.
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