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Register a new userFinding counterparts for All-sky X-ray surveys with Nway: a Bayesian algorithm for cross-matching multiple catalogues
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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., colors, 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 WISE color-magnitude prior. A reference sample of 4524 XMM/Chandra and Swift X-ray sources demonstrates a reliability of ~ 94.7% (2RXS) and 97.4% (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% and 99% 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.
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SPIDERS (SPectroscopic IDentification of eROSITA Sources) is an SDSS-IV survey running in parallel to the eBOSS cosmology project. SPIDERS will obtain optical spectroscopy for large numbers of X-ray-selected 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 WISE 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-localised 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 ~12000 ROSAT and ~1500 XMM-Newton Slew survey sources which have existing optical spectroscopy from SDSS-DR12, including the results of our visual inspections. On completion of the SPIDERS program, we expect to have collected homogeneous spectroscopic redshift information over a footprint of ~7500 deg$^2$ for >85 percent 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 on-going X-ray all-sky survey with the eROSITA instrument will yield large galaxy cluster samples, which will bring strong constraints on cosmological parameters. In particular, the survey holds great promise to investigate the tension between CMB and low-redshift measurements. The current bottleneck preventing the full exploitation of the survey data is the systematics associated with the relation between survey observable and halo mass. Numerous recent studies have shown that gas mass and core-excised X-ray luminosity exhibit very low scatter at fixed mass. We propose a new method to reconstruct these quantities from low photon count data and validate the method using extensive eROSITA-like simulations. We find that even near the detection threshold of ~50 counts the core-excised luminosity and the gas mass can be recovered with 20-30% precision, which is substantially less than the scatter of the full integrated X-ray luminosity at fixed mass. When combined with an accurate calibration of the absolute mass scale (e.g. through weak gravitational lensing), our technique reduces the systematics on cosmological parameters induced by the mass calibration.
Prompted by the H I Ly$alpha$ absorption associated with the X-ray ultra-fast outflow at -17,300 $rm km~s^{-1}$ in the quasar PG~1211+143, we have searched archival UV spectra at the expected locations of H I Ly$alpha$ absorption for a large sample of ultra-fast outflows identified in XMM-Newton and Suzaku observations. Sixteen of the X-ray outflows have predicted H I Ly$alpha$ wavelengths falling within the bandpass of spectra from either the Far Ultraviolet Spectroscopic Explorer or the Hubble Space Telescope, although none of the archival observations were simultaneous with the X-ray observations in which UFOs were detected. In our spectra broad features with full-width at half-maximum of 1000 $rm km~s^{-1}$ have 2-$sigma$ upper limits on the H I column density of generally <$2times10^{13}~rm cm^{-2}$. Using grids of photoionization models covering a broad range of spectral energy distributions, we find that producing Fe XXVI Ly$alpha$ X-ray absorption with equivalent widths $>30$ eV and associated H I Ly$alpha$ absorption with $rm N_{HI}<2times10^{13}~cm^{-2}$ requires total absorbing column densities $rm N_{H}>5times10^{22}~cm^{-2}$ and ionization parameters log $xi$ > 3.7. Nevertheless, a wide range of SEDs would predict observable H I Ly$alpha$ absorption if ionization parameters are only slightly below peak ionization fractions for Fe XXV and Fe XXVI. The lack of Ly$alpha$ features in the archival UV spectra indicates that either the UFOs have very high ionization parameters, very hard UV-ionizing spectra, or that they were not present at the time of the UV spectral observations due to variability.
We outline scientific objectives for monitoring X-ray sources and transients with wide-angle, coded mask cameras. It is now possible to instantaneously view half of the sky over long time intervals, gaining access to events of extraordinary interest. Solid state detectors can raise the quality of data products for bright sources to levels associated with pointed instruments. There are diverse ways to advance high energy astrophysics and quantitative applications for general relativity.
The AKARI All-Sky Catalogues are an important infrared astronomical database for next-generation astronomy that take over the IRAS catalog. We have developed an online service, AKARI Catalogue Archive Server (AKARI-CAS), for astronomers. The service includes useful and attractive search tools and visual tools. One of the new features of AKARI-CAS is cached SIMBAD/NED entries, which can match AKARI catalogs with other catalogs stored in SIMBAD or NED. To allow advanced queries to the databases, direct input of SQL is also supported. In those queries, fast dynamic cross-identification between registered catalogs is a remarkable feature. In addition, multiwavelength quick-look images are displayed in the visualization tools, which will increase the value of the service. In the construction of our service, we considered a wide variety of astronomers requirements. As a result of our discussion, we concluded that supporting users SQL submissions is the best solution for the requirements. Therefore, we implemented an RDBMS layer so that it covered important facilities including the whole processing of tables. We found that PostgreSQL is the best open-source RDBMS products for such purpose, and we wrote codes for both simple and advanced searches into the SQL stored functions. To implement such stored functions for fast radial search and cross-identification with minimum cost, we applied a simple technique that is not based on dividing celestial sphere such as HTM or HEALPix. In contrast, the Web application layer became compact, and was written in simple procedural PHP codes. In total, our system realizes cost-effective maintenance and enhancements.
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