ROSAT All Sky Survey Bright Source Catalog Research Articles

A LIMIT ON THE NUMBER OF ISOLATED NEUTRON STARS DETECTED IN THEROSATALL-SKY-SURVEY BRIGHT SOURCE CATALOG

Using new and archival observations made with the Swift satellite and other facilities, we examine 147 X-ray sources selected from the ROSAT All-Sky-Survey Bright Source Catalog (RASS/BSC) to produce a new limit on the number of isolated neutron stars (INSs) in the RASS/BSC, the most constraining such limit to-date. Independent of X-ray spectrum and variability, the number of INSs is <=48 (90% confidence). Restricting attention to soft (having an effective temperature of < 200 eV), non-variable X-ray sources -- as in a previous study -- yields an all-sky limit of <=31 INSs. In the course of our analysis, we identify five new high-quality INS candidates for targeted follow-up observations. A future all-sky X-ray survey with eROSITA, or another mission with similar capabilities, can be expected to increase the detected population of X-ray-discovered INSs from the 8 to 50 in the BSC, to (for a disk population) 240 to 1500, which will enable a more detailed study of neutron star population models.

XID II: STATISTICAL CROSS-ASSOCIATION OF ROSAT BRIGHT SOURCE CATALOG X-RAY SOURCES WITH 2MASS POINT SOURCE CATALOG NEAR-INFRARED SOURCES

The 18806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-ray sources are quantitatively cross-associated with near-infrared (NIR) sources from the Two Micron All Sky Survey Point Source Catalog (2MASS/PSC). An association catalog is presented, listing the most likely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquely associated, and the probability Pnoid that none of the 2MASS/PSC sources are associated with the X-ray source. The catalog includes 3853 high quality (Pid>0.98) X-ray--NIR matches, 2280 medium quality (0.98>Pid>0.9) matches, and 4153 low quality (0.9>Pid>0.5) matches. Of the high quality matches, 1418 are associations that are not listed in the SIMBAD database, and for which no high quality match with a USNO-A2 optical source was presented for the RASS/BSC source in previous work. The present work offers a significant number of new associations with RASS/BSC objects that will require optical/NIR spectroscopy for classification. For example, of the 6133 Pid>0.9 2MASS/PSC counterparts presented in the association catalog, 2411 have no classification listed in the SIMBAD database. These 2MASS/PSC sources will likely include scientifically useful examples of known source classes of X-ray emitters (white dwarfs, coronally active stars, active galactic nuclei), but may also contain previously unknown source classes. It is determined that all coronally active stars in the RASS/BSC should have a counterpart in the 2MASS/PSC, and that the unique association of these RASS/BSC sources with their NIR counterparts thus is confusion limited.

A Limit on the Number of Isolated Neutron Stars Detected in theROSATBright Source Catalogue

The challenge in searching for non-radio-pulsing isolated neutron stars (INSs) is in excluding association with objects in the very large error boxes (~13", 1 sigma radius) typical of sources from the largest X-ray all-sky survey, the ROSAT All-Sky-Survey/Bright Source Catalog (RASS/BSC). We search for candidate INSs using statistical analysis of optical (USNO-A2), infrared (IRAS), and radio (NVSS) sources near the ROSAT X-ray localization, and show that this selection would find 20% of the INSs in the RASS/BSC. This selection finds 32 candidates at declinations greater than -39 deg, among which are two previously known INSs, seventeen sources which we show are not INSs, and thirteen the classification of which are as yet undetermined. These results require a limit of <67 INSs (90% confidence, full sky, assuming isotropy) in the RASS/BSC. This limit modestly constrains a naive and optimistic model for cooling NSs in the galaxy.

A search for microquasar candidates at low galactic latitudes

Recent studies of relativistic jet sources in the Galaxy, also known as microquasars, have been very useful in trying to understand the accretion/ejection processes that take place near compact objects. However, the number of sources involved in such studies is still small. In an attempt to increase the number of known microquasars we have carried out a search for new Radio Emitting X-ray Binaries (REXBs). These sources are the ones to be observed later with VLBI techniques to unveil their possible microquasar nature. To this end, we have performed a cross-identification between the X-ray ROSAT all sky survey Bright Source Catalog (RBSC) and the radio NRAO VLA Sky Survey (NVSS) catalogs under very restrictive selection criteria for sources with |b|<5 degrees. We have also conducted a deep observational radio and optical study for six of the selected candidates. At the end of this process two of the candidates appear to be promising, and deserve additional observations aimed to confirm their proposed microquasar nature.

The Rate of Visual Binaries among the Brightest X-Ray Stars

Using the Tycho Double Star Catalogue and the Hipparcos Catalogue, I compare the rates of occurrence of resolved visual doubles among the brightest X-ray stars from the ROSAT All-Sky Survey Bright Source Catalog (RASS-BSC) and generally in these two catalogs. A strong correlation between visual binarity and brightness in X-rays is found. The rate of binarity, quantified as the fraction of resolved components, is 7 times higher for the RASS-BSC stars than generally for all Tycho-2 stars. In order to eliminate the distance bias in the sample, I compare the rates in a distance-limited sample of Hipparcos stars within 50 pc. The comparison has shown that the fraction of visual and unresolved orbiting binaries among stars with counterparts in the RASS-BSC, hence bright in X-rays, is 2.4 times higher than among fainter X-ray stars, i.e., the ones whose X-ray brightness is below the limit of the RASS-BSC. This difference is not caused by the enhancement of the observed flux due to the combined component's radiation. Previously found in the nearby open clusters, the correlation between binarity and X-ray luminosity exists for nearby field stars as well. Possible implications of the discrepancy are discussed, including the binarity-age and binarity-rotation velocity relations. The H-R diagram of components of nearby visual binaries indicates that the red clump giants luminous in X-rays may be unresolved binaries.

An AGN sample with high X-ray-to-optical flux ratio from RASS

We present the rst results of a program to identify the unknown bright active galactic nuclei (AGN) in the ROSAT All Sky Survey Bright Source Catalog (RASS-BSC). We have used logC +0 : 4Ras an alterna- tive expression for log(fX=fopt), where C is X-ray count rate and R stands for R magnitude. Then a high X-ray- to-optical flux ratio criterion has been used to select an AGN sample with 165 unidentied X-ray sources for op- tical spectroscopy. Those 165 X-ray sources have been identied in the following classes: 115 emission line AGN (QSOs and Seyferts), 2 BL Lac objects and 4 BL Lac can- didates, 22 clusters of galaxies, 12 Galactic stars and 10 objects remain unidentied. This represent a success rate of about 73% for detecting AGN using our selection cri- teria. Plausibility is based upon the optical classication and X-ray characteristics of the sources.

Observational Study of High X‐Ray–to–Optical Flux Ratio RASS Sources

We have performed a program to identify the unknown bright high Galactic latitude sources in the ROSAT All Sky Survey Bright Source Catalog (RASS-BSC) which have the highest X-rayEtoEoptical Nux ratios. Our goal is to establish a new bright X-rayEselected sample of active galactic nuclei (AGNs) for statistical and individual study. The results of the Einstein Medium Sensitivity Survey have shown that di†erent classes of X-ray sources represent di†erent narrow ranges in the X-rayEtoEoptical Nux ratios. We used log C] 0.4R as an alternative expression for where C is the X-ray count rate and R replog ( fX/fopt), resents the R magnitude from USNO-A1.0.1 According to our statistical analysis of known RASS-BSC X-ray sources, we found an apparent gap between Galactic stars and extragalactic objects except in the case of white dwarfs (Fig. 1) ; emission-line AGNs are concentrated in the region log Co[0.4R] 4.9. Then this high X-rayEtoEoptical Nux ratio criterion was used to select a sample with 165 unidentiÐed X-ray sources from RASS-BSC for optical spectroscopy. At present, over 93% of the X-ray sources in the sample have been fully identiÐed in the following proportions : emission-line AGNs (QSOs and Seyferts), 69.7%; BL Lacertae objects and their candidates, 3.6%; clusters of galaxies, 12.7%; Galactic stars, 7.3%; and 6.7% remain unidentiÐed because the proposed stellar counterparts are too faint to be the likely X-ray emitters. The plausibility of identiÐcation is based upon the optical classiÐcation, the

High X-ray-to-optical flux ratio RASS-BSC sources

We have used (where c is count rate, and E stands for E magnitude from POSS) as an alternative expression for to study the X-ray-to-optical flux ratio distribution for known sources in ROSAT All Sky Survey Bright Source Catalog (RASS-BSC). Attention focuses on which classes of X-ray emitters have the highest X-ray-to-optical flux ratios. Then a high X-ray-to-optical flux ratio criterion is set and used to select a sample with 34 unidentified X-ray sources from the RASS-BSC for optical spectroscopy. The optical identifications of the sample are presented. Those 34 X-ray sources have been identified in the following classes: 10 emission line active galactic nuclei, 7 BL Lacertae objects and 3 BL Lac candidates, 7 clusters of galaxies, 4 Galactic objects and 3 objects remain unidentified due to low signal to noise ratio of the spectra. Identification plausibility is based upon the optical spectroscopy, morphology in the DSS image and the X-ray properties of the sources. We find that preliminary classification can be achieved while extent and hardness ratio in X-ray band, optical morphology and X-ray-to-optical flux ratio are concerned. All of the X-ray sources in the sample are identified with previously known classes of X-ray emitters, and the result is in accord with the E magnitude vs. () diagram. The proportions of WD and CVs are much higher than that in the EMSS sample. We consider it is due to the lower bandpass of the ROSAT PSPC and the softness of the energy spectrum of these objects. AGN detected in this sample have extremely soft X-ray emission, which infers the presence of a soft X-ray excess in the AGN of this sample. The optical identification results also prove that to find BL Lacertae objects by high X-ray-to-optical flux ratio is an efficient way.