ROSAT Bright Source Catalogue Research Articles

Diffuse radio sources in a statistically complete sample of high-redshift galaxy clusters

Aims. Non-thermal properties of galaxy clusters have been studied using detailed and deep radio images in comparison with X-ray data. While much progress has been made in this area, most of the studied clusters are at a relatively low redshift (z < 0.3). Here we investigate the evolutionary properties of the non-thermal cluster emission using two statistically complete samples at z > 0.3. Methods. We obtained short JVLA observations at the L-band of the statistically complete sample of very X-ray luminous clusters from the Massive Cluster Survey (MACS), namely 34 clusters in the redshift range of 0.3–0.5 and with nominal X-ray fluxes in excess of 2 × 10−12 erg s−1 cm−2 (0.1–2.4 keV) in the ROSAT Bright Source Catalogue. We add to this list the complete sample of the 12 most distant MACS clusters (z > 0.5). Results. Most clusters show evidence of emission in the radio regime. We present the radio properties of all clusters in our sample and show images of newly detected diffuse sources. A radio halo is detected in 19 clusters and five clusters contain a relic source. Most of the brightest cluster galaxies (BCG) in relaxed clusters show radio emission with powers typical of FRII radio galaxies and some are surrounded by a radio mini-halo. Conclusions. The high frequency of radio emission from the BCG in relaxed clusters suggests that BCG feedback mechanisms are already in place at z ∼ 0.6. The properties of radio halos and the small number of detected relics suggest redshift evolution in the properties of diffuse sources. The radio power (and size) of radio halos could be related to the number of past merger events in the history of the system. In this scenario, the presence of a giant and high-power radio halo is indicative of an evolved system with a large number of past major mergers, whereas small low-power halos are found in less evolved clusters.

Clusters of Galaxies Masquerading as X-Ray Quasars

Abstract Inspired by the discovery of the Phoenix cluster by the South Pole Telescope team, we initiated a search for other massive clusters of galaxies missing from the standard X-ray catalogs. We began by identifying 25 cluster candidates not included in the Meta-Catalog of X-ray Clusters of galaxies cluster compilation through cross-identification of the central galaxies of optically identified clusters in the Sloan Digital Sky Survey GMBCG catalog with bright X-ray sources in the ROSAT Bright Source Catalog. Those candidates were mostly unidentified or previously classified as X-ray active galactic nucleus (AGN). We analyzed brief Chandra X-ray Observatory observations of 14 of these X-ray sources and found that eight are X-ray luminous clusters of galaxies, only one showing evidence for a central X-ray point source. The remaining six candidates turned out to be point-source dominated, with faint detections or upper limits on any extended emission. We were not able to rule out the presence of extended X-ray emission from any of the point sources. The levels of extended emission around the six point sources are consistent with expectations based on optical richness, but could also be contaminated by scattered X-ray light from the central point source or extended nonthermal emission from possible radio lobes. We characterize the extended components of each of the well-detected cluster sources, finding that six of the eight X-ray clusters are consistent with being compact cool-core clusters. One of the newly identified low-luminosity X-ray clusters may have had an X-ray-luminous AGN 20 yr prior to the recent Chandra observations, based on the 4σ difference between its Chandra and ROSAT fluxes.

Tidal disruption events: Past, present, and future

The tidal disruption of a star by a nuclear supermassive black hole was predicted in the 1970s and first confirmed by soft X‐ray flares seen from quiescent galaxies in the ROSAT all‐sky survey. We report here on the discovery of a new candidate tidal disruption event, 1RXS J075908.8 + 074835, from the ROSAT bright source catalog, which faded by a factor > 100 in 20 years. Similar events have continued to be found in XMM‐Newton and Chandra data and more recently, flares in large‐area hard X‐ray, ultraviolet (UV) and optical transient surveys have also been attributed to the same phenomenon. Each of these spectral components, sometimes complemented by radio and IR activity, have their own timescale and possibly physical mechanism. Current models and simulations revolve around trying to explain how the energy released by the fall back of stellar debris in the gravitational well of the black hole is converted into radiation in each wave band. We look at the connection between optical, UV, and X‐ray emission and illustrate a dichotomy between X‐ray and optically dominated tidal disruption event (TDE), which is currently unexplained. We look at the observations that are needed to make further breakthroughs in the field and assess the optimum follow‐up strategies needed for events discovered in future large survey missions such as extended Roentgen Survey with an Imaging Telescope Array (eROSITA), the Einstein Probe, and Large Synoptic Survey Telescope (LSST). HIgh‐energy LIght‐curve GeneraTor (HILIGT), a useful new tool for comparing fluxes and upper limits between missions, is presented.

Studying the Flux Density of Bright Active Galaxies at Different Spectral Bands

Statistical studies are reported in this article for an active galactic nuclei sample of different type of active galaxies Seyferts 1, Seyferts 2, and Quasars. These sources have been selected from a Catalogue for bright X-ray galaxies. The name of this index is ROSAT Bright Source Catalogue (RBSC) and the NRAO VLA Sky Survey (NVSS). In this research, multi-wavelength observational bands Radio at 1.4 GHz, Optical at 4400 A0, and X-ray at energy 0.1-2.4 KeV have been adopted in this study. The behavior of flux density ratios has been studied , with respect to the absolute magnitude . Furthermore, the Seyfert1 and Seyfert 2 objects are combined in one group and the QSOs are collectest in another group. Also, it has been found that the ratios , are increasing towards fainter optical absolute magnitude especially in Quasars.

Studying the Flux Density of Bright Active Galaxies at Different Spectral Bands

Statistical studies are reported in this article for an active galactic nuclei sample of different type of active galaxies Seyferts 1, Seyferts 2, and Quasars. These sources have been selected from a Catalogue for bright X-ray galaxies. The name of this index is ROSAT Bright Source Catalogue (RBSC) and the NRAO VLA Sky Survey (NVSS). In this research, multi-wavelength observational bands Radio at 1.4 GHz, Optical at 4400 A0, and X-ray at energy 0.1-2.4 KeV have been adopted in this study. The behavior of flux density ratios has been studied , with respect to the absolute magnitude . Furthermore, the Seyfert1 and Seyfert 2 objects are combined in one group and the QSOs are collectest in another group. Also, it has been found that the ratios , are increasing towards fainter optical absolute magnitude especially in Quasars.

Stellar tidal disruption candidates found by cross-correlating the ROSAT Bright Source Catalogue and XMM–Newton observations

We performed a systematic search for stellar tidal disruption events (TDE) by looking for X-ray sources that were detected during the ROSAT All Sky Survey and faded by more than an order of magnitude over the next two decades according to XMM-Newton serendipitous observations. Besides a number of highly variable persistent X-ray sources (like active galactic nuclei and cataclysmic variables), we found three sources that are broadly consistent with the TDE scenario: 1RXS J114727.1+494302, 1RXS J130547.2+641252, and 1RXS J235424.5-102053. A TDE association is also acceptable for the fourth source, 1RXS J112312.7+012858, but an AGN origin cannot be ruled out either. This statistics implies a TDE rate of $ \sim 3\times 10^{-5} $ yr$ ^{-1} $ per galaxy in the Universe within $z\sim 0.18$, which is broadly consistent with the estimates of the TDE rate in the more local Universe obtained previously.

A New Cluster of Galaxies Towards the Galactic Bulge, Suzaku J1759–3450

Abstract We observed an extended X-ray source, designated as Suzaku J1759–3450, with Suzaku and Chandra observations towards 1RXS J175911.0–344921, which is an unidentified X-ray source listed in the ROSAT Bright Source Catalogue. A conspicuous emission line at 6 keV was also found in the Suzaku J1759–3450 spectrum. Assuming the emission line to be K emission from He-like Fe ions, we inferred Suzaku J1759–3450 to be an extragalactic object located at z = 0.13. The radial profile of the surface brightness in the 0.5–10 keV band was explained well with an isothermal β-model of $r_\textrm c=1{.^\prime}61$ and $\beta=0.78$. The X-ray spectrum was well reproduced by an opticallythin thermal plasma with the electron temperature of $kT_e = 5.3$ keV, attenuated by the photoelectric absorption of $N_\textrm H = 2.3 \times 10^{21} \textrm c\textrm m^{-2}$. The bolometric X-ray luminosity of $L_\textrm X(r < r_{500}) = 4.3 \times 10^{44} \textrm {erg} \textrm s^{-1}$ is consistent with that expected from the LX–T relation of clusters of galaxies. In terms of the spatial extent, the X-ray spectrum, and the bolometric luminosity of the X-ray emitting gas, we concluded that Suzaku J1759–3450 is a new cluster of galaxies.

The ROSAT/NVSS AGN sample

AbstractWe attempt to create an X-ray/radio AGN catalog and make its multiwavelength studies. ROSAT Bright Source Catalogue (BSC) contains 18,806 and ROSAT Faint Source Catalogue (FSC), 105,922 X-ray sources giving the total number of ROSAT X-ray sources 124,727 (one source is listed twice). On the other hand, NVSS radio catalogue contains 1,773,484 sources. Taking into account that X-ray sources contain AGN, bright stars and galaxies, clusters, white dwarfs (WD), cataclysmic variables (CV), etc., the cross-identification with radio catalogue may distinguish the extragalactic sources. We have cross-correlated ROSAT catalogs with NVSS one with a search radius 30 arcsec. 9,193 associations have been found. To distinguish AGN from the normal bright galaxies and clusters, Veron-Cetty & Veron AGN catalog (v.13, 2010; VCV-13) containing 168,940 objects have been used. A cross-correlation of the 9,193 ROSAT/NVSS sources with the VCV-13 with a search radius 30 arcsec resulted in 3,094 associations. Thus we are left with more 6,099 X-ray/radio sources without an optical identification. Brighter objects are normal bright galaxies, while we believe that all faint ones are candidate AGN with some contamination of distant clusters. SDSS spectroscopic survey allows us classify objects by activity types, and a number of our candidate AGN is found to be present in SDSS. We attempt to find connections between the fluxes in different wavelength ranges, which will allow us to confirm AGN and blazars candidates and in some cases find new ones.

BL LAC CANDIDATES FOR TeV OBSERVATIONS

BL Lac objects are the most numerous class of extragalactic TeV-detected sources. One of the biggest difficulties in investigating their TeV emission resides in their limited number, since only 47 BL Lacs are known as TeV emitters. In this paper, we propose new criteria to select TeV BL Lac candidates based on the infrared (IR) and X-ray observations. We apply our selection criteria to the BL Lac objects listed in the ROMA-BZCAT catalog so identifying 41 potential TeV emitters. We then consider a search over a more extended sample combining the ROSAT bright source catalog and the WISE all-sky survey revealing 54 additional candidates for TeV observations. Our investigation also led to a tentative classification of 16 unidentified X-ray sources as BL Lac candidates. This analysis provides new interesting BL Lac targets for future observations with ground based Cherenkov telescopes.

The X-ray brightest clusters of galaxies from the Massive Cluster Survey

We present a statistically complete sample of very X-ray luminous galaxy clusters detected in the MAssive Cluster Survey (MACS). This second MACS release comprises all 34 MACS clusters with nominal X-ray fluxes in excess of 2x10^(-12) erg/s/cm^2 (0.1-2.4 keV) in the ROSAT Bright Source Catalogue; two thirds of them are new discoveries. Extending over the redshift range from 0.3 to 0.5, this subset complements the complete sample of the 12 most distant MACS clusters (z>0.5) published in 2007 and further exemplifies the efficacy of X-ray selection for the compilation of samples of intrinsically massive galaxy clusters. Extensive follow-up observations with Chandra/ACIS led to three additional MACS cluster candidates being eliminated as (predominantly) X-ray point sources. For another four clusters --- which, however, remain in our sample of 34 --- the point-source contamination was found to be about 50%. The median X-ray luminosity of 1.3x10^45 erg/s (0.1-2.4 keV, Chandra, within r_500) of the clusters in this subsample demonstrates the power of the MACS survey strategy to find the most extreme and rarest clusters out to significant redshift. A comparison of the optical and X-ray data for all clusters in this release finds a wide range of morphologies with no obvious bias in favour of either relaxed or merging systems.

ROSAT's view of the X-ray sky

All-sky X-ray surveys with satellites started in the early 1970s with relatively crude, non-imaging instrumentation. The resulting survey catalogs, for example those from Uhuru & Ariel 5, contained only a few hundred sources, rising only modestly to ∼1000 entries by the end of the decade with the HEAO-1 (A-1) survey. Since then the most important and influential X-ray sky survey has undoubtedly been that carried out by ROSAT in the early 1990s. The ROSAT Bright Source Catalogue paper by Voges et al., when it was published 10 years ago, thus represented not only the culmination of the ROSAT project’s primary aim of surveying the whole sky at X-ray wavelengths with an unprecedented sensitivity, but also a major step forward in our knowledge of the X-ray sky. ROSAT (ROntgen SATellit; Trumper 1982) was a German national project with an international partnership with the UK and US. From its origins as a mission concept in the middle of the 1970s to its realization as ROSAT, the main aim of the project was to perform the first all-sky survey with an imaging X-ray telescope, but the scope of the project was expanded through the international partnerships to encompass a program of pointed observations (with the provision of an additional focal plane detector by the US) and to include an extreme ultraviolet (EUV) capability with a separate EUV telescope supplied by the UK. Originally planned as a space shuttle launch, the Challenger tragedy in 1986 delayed the project until an alternative launch – by a Delta II rocket – could be secured. ROSAT was eventually launched on 1990 June 1, just over a month after the Hubble Space Telescope. ROSAT’s main X-ray telescope operated in the soft X-ray band (0.1–2.4 keV) with two different, interchangeable, detectors in the focal plane: two Position Sensitive Proportional Counters (PSPCs), used for the all-sky survey, and the High Resolution Imager (HRI), provided by the US and used only for pointed observations. The ROSAT EUV telescope had a separate micro-channel plate detector, the ROSAT Wide Field Camera (WFC). The ROSAT All-Sky Survey (RASS) took place in late 1990/early 19911. Rather remarkably, in 6 months observing time, ROSAT surveyed essentially the whole sky to a sensitivity a factor 20 better than previous missions, while simultaneously

An X‐Ray Search for Compact Central Sources in Supernova Remnants. II. Six Large‐Diameter SNRs

We present the second in a series of studies in which we have searched for undiscovered neutron stars in supernova remnants (SNRs). This paper deals with the six largest SNRs in our sample, too large for Chandra or XMM-Newton to cover in a single pointing. These SNRs are nearby, with typical distances of <1 kpc. We therefore used the ROSAT Bright Source Catalog and past observations in the literature to identify X-ray point sources in and near the SNRs. Out of 54 sources, we were immediately able to identify optical/IR counterparts to 41 from existing data. We obtained Chandra snapshot images of the remaining 13 sources. Of these, 10 were point sources with readily identified counterparts, two were extended, and one was not detected in the Chandra observation but is likely a flare star. One of the extended sources may be a pulsar wind nebula, but if so it is probably not associated with the nearby SNR. We are then left with no identified neutron stars in these six SNRs down to luminosity limits of ~1032 ergs s-1. These limits are generally less than the luminosities of typical neutron stars of the same ages, but are compatible with some lower luminosity sources such as the neutron stars in the SNRs CTA 1 and IC 443.

Using the ROSAT Bright Source Catalogue to find counterparts for IBIS/ISGRI survey sources

The IBIS/ISGRI first year galactic plane survey has produced a catalogue containing 123 hard X-ray sources visible down to a flux limit of a few milliCrabs. The point source location accuracy of typically 1-3 arcmin has allowed the counterparts for 95 of these sources to be found at other wavelengths. In order to identify the remaining 28 objects, we have cross-correlated the ISGRI catalogue with the ROSAT All Sky Survey Bright Source Catalogue. In this way, for ISGRI sources which have a counterpart in soft X-rays, we can use the, much smaller, ROSAT error box to search for identifications. As expected, we find a strong correlation between the two catalogues and calculate that there are 75 associations with the number expected by chance to be almost zero. Of these 75 sources, ten are in the list of unidentified objects. Using the ROSAT error boxes we provide tentative associations for 8 of these.

Galaxy and hot gas distributions in the z = 0.52 galaxy cluster RBS 380 from CHANDRA and NTT observations

We present CHANDRA X-ray and NTT optical observations of the distant z= 0:52 galaxy cluster RBS 380 - the most distant cluster of the ROSAT Bright Source (RBS) catalogue. We find diuse, non-spherically symmetric X-ray emission with a X-ray luminosity of LX(0:3 10 keV) = 1:6 10 44 erg/s, which is lower than expected from the RBS. The reason is a bright AGN in the centre of the cluster contributing considerably to the X-ray flux. This AGN could not be resolved with ROSAT. In optical wavelength we identify several galaxies belonging to the cluster. The galaxy density is at least 2 times higher than expected for such a X-ray faint cluster, which is another confirmation of the weak correlation between X-ray luminosity and optical richness. The example of the source confusion in this cluster shows how important high-resolution X-ray imaging is for cosmological research.

The Hamburg/RASS Catalogue of optical identifications

We present the Hamburg/RASS Catalogue (HRC) of optical identifications of X-ray sources at high-galactic latitude. The HRC includes all X-ray sources from the ROSAT Bright Source Catalogue (RASS-BSC) with galactic latitude and declination . In this part of the sky covering ~10 000 deg2 the RASS-BSC contains 5341 X-ray sources. For the optical identification we used blue Schmidt prism and direct plates taken for the northern hemisphere Hamburg Quasar Survey (HQS) which are now available in digitized form. The limiting magnitudes are 18.5 and 20, respectively. For 82% of the selected RASS-BSC an identification could be given. For the rest either no counterpart was visible in the error circle or a plausible identification was not possible. With ~42% AGN represent the largest group of X-ray emitters, ~31% have a stellar counterpart, whereas galaxies and cluster of galaxies comprise only ~4% and ~5%, respectively. In ~3% of the RASS-BSC sources no object was visible on our blue direct plates within 40´´ around the X-ray source position. The catalogue is used as a source for the selection of (nearly) complete samples of the various classes of X-ray emitters.

Further identification of ROSAT all-sky survey sources in Orion

We report on the identification of 22 ROSAT All-Sky Survey (RASS) X-ray sources distributed in the general direc- tion of the Orion star-forming region. The X-ray sample contains sources from the ROSAT bright source catalogue and from previous detections. The optical identifications are based on intermediate-resolution spectroscopy and UBV Johnson photom- etry using a 1m-class telescope. The strengths of the Hα ,N a D2 and lithium lines for the stellar counterparts are evaluated applying the spectral subtraction technique, using templates of the same spectral type. Radial velocities of the optical coun- terparts are also reported. Thirteen of the optical counterparts show the lithium absorption line in their spectra and have radial velocities consistent with the Orion star forming region. Four of these objects can be classified as new bona-fide T Tauri stars.

Optical and X-Ray Clusters as Tracers of the Supercluster-Void Network. II. The Spatial Correlation Function

We study the spatial distribution of Abell and X-ray–selected clusters of galaxies from the ROSAT Bright Source Catalog and determine correlation functions for both cluster samples. We find that on small scales the correlation functions depend on the cluster environment: clusters in rich superclusters have a larger correlation length and amplitude than clusters of the whole sample. On large scales the correlation functions depend on the distribution of superclusters, but for both X-ray and Abell clusters they are oscillating with a period of ~115 h-1 Mpc, indicating the presence of a local peak in the power spectrum at an effective wavenumber k = 0.055 h Mpc-1.

A complete sample of Seyfert galaxies selected at 0.25 keV

We have used the ROSAT Bright Source Catalogue to extract a complete sample of sources selected in the band from 0.1-0.4 keV. This 0.25 keV-selected sample is composed of 54 Seyfert galaxies, 25 BL Lacertae objects, 4 clusters and 27 Galactic stars or binaries. Seyfert-type galaxies with 'ultrasoft' X-ray spectra can very often be classed optically as narrow-line Seyfert 1s (NLS1s). Such objects are readily detected in 0.25 keV surveys; the sample reported here contains 20 NLS1s, corresponding to a 40 per cent fraction of the Seyferts. Optical spectra of the Seyfert galaxies were gathered for correlative analysis, which confirmed the well-known relations between X-ray slope and optical spectral properties (e.g. [QIII]/Hβ ratio; Fe II strength, Hβ width). The various intercorrelations are most likely driven, fundamentally, by the shape of the photoionizing continuum in Seyfert nuclei. We argue that a steep X-ray spectrum is a better indicator of an 'extreme' set of physical properties in Seyfert galaxies than is the narrowness of the optical Hβ line. The correlation studies were also used to isolate a number of Seyfert galaxies with apparently 'anomalous' properties. Of particular interest are the six objects with relatively weak permitted line emission (Hβ and Fe II) and weak optical continua. Such objects are rare in most surveys, but two of these (IC 3599 and NGC 5905) are known to be transient active galactic nuclei in which the X-ray flux has faded by factors ∼100. If the other four objects also turn out to be transient, this would demonstrate that 0.25 keV surveys provide an efficient way of finding an interesting class of object. Finally, the luminosity function of the 0.25 keV-selected Seyfert galaxies was determined and broken down into subsamples to investigate the relative space densities of Seyferts when separated on the basis of either X-ray slope or Hβ linewidth.

MACS: A Quest for the Most Massive Galaxy Clusters in the Universe

We describe the design and current status of a new X-ray cluster survey aimed at the compilation of a statistically complete sample of very X-ray luminous (and thus, by inference, massive), distant clusters of galaxies. The primary goal of the Massive Cluster Survey (MACS) is to increase the number of known massive clusters at z > 0.3 from a handful to hundreds. Upon completion of the survey, the MACS cluster sample will greatly improve our ability to study quantitatively the physical and cosmological parameters driving cluster evolution at redshifts and luminosities poorly sampled by all existing surveys. To achieve these goals, we apply an X-ray flux and X-ray hardness ratio cut to select distant cluster candidates from the ROSAT Bright Source Catalogue. Starting from a list of more than 5000 X-ray sources within the survey area of 22,735 deg2, we use positional cross-correlations with public catalogs of Galactic and extragalactic objects, reference to Automated Plate Measuring Machine (APM) colors, visual inspection of Digitized Sky Survey images, extensive CCD imaging, and finally spectroscopic observations with the University of Hawaii's 2.2 m and the Keck 10 m telescopes to compile the final cluster sample. We discuss in detail the X-ray selection procedure and the resulting selection function and present model predictions for the number of distant clusters expected to emerge from MACS. At the time of this writing the MACS cluster sample comprises 101 spectroscopically confirmed clusters at 0.3 ≤ z ≤ 0.6; more than two-thirds of these are new discoveries. Our preliminary sample is already 15 times larger than that of the EMSS in the same redshift and X-ray luminosity range.

X-ray variability in a complete sample of Soft X-ray selected AGN

We present ROSAT All-Sky Survey and ROSAT pointed observations (PSPC and HRI) of a complete sample of 113 bright soft X-ray AGN selected from the ROSAT Bright Source Catalog. We compare these observations in order to search for extreme cases of flux and spectral X-ray variability - X-ray transient AGN. Three definite transients and one transient candidate are found. The other sources show amplitude variations typically by factors of 2-3 on timescales of years. We found that the variability strength on timescales of days is a function of the steepness of the X-ray spectrum: steeper X-ray objects show stronger variability than flat X-ray spectrum sources. We also present new HRI measurements of our extreme X-ray transients IC 3599 and WPVS007. We discuss possible models to explain the X-ray transience and the variabilities observed in the non-transient sources.