Aims. We compile a sample of X-ray-selected galaxy groups and clusters from the XMM-Newton serendipitous source catalogue (2XMMi-DR3) with optical confirmation and redshift measurement from the Sloan Digital Sky Survey (SDSS). We present an analysis of the X-ray properties of this new sample with particular emphasis on the X-ray luminosity-temperature (L X − T ) relation. Methods. The X-ray cluster candidates were selected from the 2XMMi-DR3 catalogue in the footprint of the SDSS-DR7. We developed a finding algorithm to search for overdensities of galaxies at the positions of the X-ray cluster candidates in the photometric redshift space and to measure the redshifts of the clusters from the SDSS data. For optically confirmed clusters with good quality X-ray data we derived the X-ray flux, luminosity, and temperature from proper spectral fits, while the X-ray flux for clusters with low-quality X-ray data was obtained from the 2XMMi-DR3 catalogue. Results. The detection algorithm provides the photometric redshift of 530 galaxy clusters. Of these, 310 clusters have a spectroscopic redshift for at least one member galaxy. About 75 percent of the optically confirmed cluster sample are newly discovered X-ray clusters. Moreover, 301 systems are known as optically selected clusters in the literature while the remainder are new discoveries in X-ray and optical bands. The optically confirmed cluster sample spans a wide redshift range 0.03−0.70 (median z = 0.32). In this paper, we present the catalogue of X-ray-selected galaxy groups and clusters from the 2XMMi/SDSS galaxy cluster survey. The catalogue has two subsamples: (i) a cluster sample comprising 345 objects with their X-ray spectroscopic temperature and flux from the spectral fitting; and (ii) a cluster sample consisting of 185 systems with their X-ray flux from the 2XMMi-DR3 catalogue, because their X-ray data are insufficient for spectral fitting. For each cluster, the catalogue also provides the X-ray bolometric luminosity and the cluster mass at R 500 based on scaling relations and the position of the likely brightest cluster galaxy (BCG). The updated L X − T relation of the current sample with X-ray spectroscopic parameters is presented. We found the slope of the L X − T relation to be consistent with published ones. We see no evidence for evolution in the slope and intrinsic scatter of the L X − T relation with redshift when excluding the low-luminosity groups.
We present a catalogue of X-ray selected galaxy clusters and groups as a first release of the 2XMMi/SDSS Galaxy Cluster Survey. The survey is a search for galaxy clusters detected serendipitously in observations with XMM-Newton in the footprint of the Sloan Digital Sky Survey (SDSS). The main aims of the survey are to identify new X-ray galaxy clusters, investigate their X-ray scaling relations, identify distant cluster candidates, and study the correlation of the X-ray and optical properties. In this paper, we describe the basic strategy to identify and characterize the X-ray cluster candidates that currently comprise 1180 objects selected from the second XMM-Newton serendipitous source catalogue (2XMMi-DR3). Cross-correlation of the initial catalogue with recently published optically selected SDSS galaxy cluster catalogues yields photometric redshifts for 275 objects. Of these, 182 clusters have at least one member with a spectroscopic redshift from existing public data (SDSS-DR8). We developed an automated method to reprocess the XMM-Newton X-ray observations, determine the optimum source extraction radius, generate source and background spectra, and derive the temperatures and luminosities of the optically confirmed clusters. Here we present the X-ray properties of the first cluster sample, which comprises 175 clusters, among which 139 objects are new X-ray discoveries while the others were previously known as X-ray sources. For each cluster, the catalogue provides: two identifiers, coordinates, temperature, flux [0.5−2] keV, luminosity [0.5−2] keV extracted from an optimum aperture, bolometric luminosity L 500 , total mass M 500 , radius R 500 , and the optical properties of the counterpart. The first cluster sample from the survey covers a wide range of redshifts from 0.09 to 0.61, bolometric luminosities L 500 = 1.9×10 42 −1.2×10 45 erg s −1 , and masses M 500 = 2.3×10 13 −4.9×10 14 M . We extend the relation between the X-ray bolometric luminosity L 500 and the X-ray temperature towards significantly lower T and L and still find that the slope of the linear L − T relation is consistent with values published for high luminosities.
We present our discovery of dramatic variability in SDSS J1100+4421 by the high-cadence transient survey Kiso Supernova Survey (KISS). The source brightened in the optical by at least a factor of three within about half a day. Spectroscopic observations suggest that this object is likely a narrow-line Seyfert 1 galaxy (NLS1) at z = 0.840, however with unusually strong narrow emission lines. The estimated black hole mass of ∼ 10 7 M ⊙ implies bolometric nuclear luminosity close to the Eddington limit. SDSS J1100+4421 is also extremely radio-loud, with a radio loudness parameter of R ≃ 4 × 10 2 − 3 × 10 3 , which implies the presence of relativistic jets. Rapid and large-amplitude optical variability of the target, reminiscent of that found in a few radio-and γ-ray loud NLS1s, is therefore produced most likely in a blazar-like core. The 1.4 GHz radio image of the source shows an extended structure with a linear size of about 100 kpc. If SDSS J1100+4421 is a genuine NLS1, as suggested here, this radio structure would then be the largest ever discovered in this type of active galaxies.
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