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The ROSAT International X-ray/Optical Survey (RIXOS): source catalogue
We describe the ROSAT International X-ray/Optical Survey (RIXOS), amedium-sensitivity survey and optical identification of X-ray sourcesdiscovered in ROSAT high Galactic latitude fields (|b|>28 deg) andobserved with the Position Sensitive Proportional Counter (PSPC)detector. The survey made use of the central 17arcmin of each ROSATfield. A flux limit of3x10-14ergcm-2s-1 (0.5-2keV) wasadopted for the survey, and a minimum exposure time of 8000s wasrequired for qualifying ROSAT observations. X-ray sources in the surveyare therefore substantially above the detection threshold of each fieldused, and many contain enough counts to allow the X-ray spectral slopeto be estimated. Spectroscopic observations of potential counterpartswere obtained of all sources down to the survey limit in 64 fields,totalling a sky area of 15.77deg2. Positive opticalidentifications are made for 94per cent of the 296 sources thusexamined. A further 18 fields (4.44deg2), containing 105sources above the 3x10-14ergcm-2s-1survey limit, are completely optically identified to a higher flux of8x10-14ergcm-2s-1 (0.5-2keV). Opticalspectroscopic data are supplemented by deep CCD imaging of many sourcesto reveal the morphology of the optical counterparts, and objects toofaint to register on Sky Survey plates. The faintest opticalcounterparts have R~22. This paper describes the survey method, andpresents a catalogue of the RIXOS sources and their opticalidentifications. Finding charts based on Sky Survey data are given foreach source, supplemented by CCD imaging where necessary.
| X-ray spectra of the RIXOS source sample
We present results of an extensive study of the X-ray spectralproperties of sources detected in the RIXOS survey, which is a large,nearly complete sample of objects detected serendipitously in ROSAT PSPCfields down to a flux limit of 3x10^-14ergcm^-2s^-1 (0.5-2keV). We showthat for X-ray surveys containing sources with low count rate, such asRIXOS, spectral slopes estimated using simple hardness ratios in theROSAT band can be biased. Instead, we analyse three-colour X-ray datausing statistical techniques appropriate to the Poisson regime whichremove the effects of this bias. We also show that the use ofthree-colour data enables some discrimination between thermal andnon-thermal spectra. We have then applied this technique to the RIXOSsurvey to study the spectral properties of the sample. For the AGN wefind an average energy index of 1.05+/-0.05, with no evidence forspectral evolution with redshift. Individual AGN are shown to have arange of properties, including soft X-ray excesses and intrinsicabsorption. Narrow-emission-line galaxies (NELGs) also seem to fit to apower-law spectrum, which may indicate a non-thermal origin for theirX-ray emission. We infer that most of the clusters in the sample have abremsstrahlung temperature >3keV, although some show evidence for acooling flow. The stars deviate strongly from a power-law model but fitto a thermal model. Finally, we have analysed the whole RIXOS sample(extending the flux cut-off to the sensitivity threshold of eachindividual observation) containing 1762 sources to study therelationship between spectral slope and flux. We find that the meanspectral slope of the sources hardens at lower fluxes, in agreement withresults from other samples. However, a study of the individual sourcesdemonstrates that the majority have relatively soft spectra even atfaint flux levels, and the hardening of the mean is caused by theappearance of a population of very hard sources at the lowest fluxes.This has implications for the nature of the soft X-ray background.
| Spectroscopic Observations of X-Ray-Selected Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987AJ.....93..869S&db_key=AST
| Einstein Observations of X-ray emission from A stars
Results are reported from the combined CfA Stellar Survey of selectedbright A stars and an Einstein Guest Observer program for Ap and Amstars. In an initial report of results from the CfA Stellar Surveys byVaiana et al. (1981) it was noted that the spread in observed X-rayluminosities among the few A stars observed was quite large. The reasonsfor this large spread was studied by Pallavicini et al. (1981). It wasfound that the X-ray emission from normal stars is related very stronglyto bolometric luminosity for early-type stars and to rotation rate forlate-type stars. However, an exception to this rule has been theapparently anomalous behavior of A star X-ray emission, for which thelarge spread in luminosity showed no apparent correlation with eitherbolometric luminosity or stellar rotation rate. In the present study, itis shown that the level of emission from normal A stars agrees with thecorrelation observed for O and B stars.
| A magnitude limited stellar X-ray survey and the F star X-ray luminosity function
An X-ray survey has been conducted of stars brighter than visualmagnitude 8.5 that have serendipitously fallen into the fields of viewof the Imaging Proportional Counter of the Einstein Observatory. Thesurvey includes 227 separate 1 x 1 deg fields, containing 274 stars witha visual magnitude of no more than 8.5 and covering a wide range ofspectral types and luminosity classes. X-ray emission was detected from33 stars, and upper limits have been determined for the remainder of thesample. F type stars dominate the detected sample, and most of these areshown to be dwarfs. An X-ray luminosity function for dF stars has beendeduced, and reveals that the average 0.2-4.0 keV luminosity of thesestars is around 10 to the 29th erg/sec. Constraints have been placed onthe high luminosity tails and medians of the X-ray luminosity functionsfor other types of stars.
| A medium sensitivity X-ray survey using the Einstein Observatory - The log N-log S relation for extragalactic X-ray sources
The paper presents results of an X-ray survey of about 50 sq deg of thehigh galactic latitude sky at sensitivities in the range of 7 x 10 tothe -14 to 5 x 10 to the -12 ergs/sq cm/s. The number-flux relation isderived for the extragalactic population to yield a best-fit power-lawslope of 1.53 + or - 0.16, and the content of the sample is analyzed interms of types of sources, appearing to be significantly different fromthe content of similar samples selected at higher fluxes. The mediumsensitivity sample of extragalactic sources is dominated by activegalactic nuclei, while samples selected at higher fluxes and higherenergies are dominated by clusters of galaxies. Thus, the number-fluxrelation for extragalactic sources may be interpreted to a firstapproximation as the sum of the two different distributions with flatterand steeper slopes describing clusters and AGNs, respectively.
| Results from an extensive Einstein stellar survey
The preliminary results of the Einstein Observatory stellar X-ray surveyare presented. To date, 143 soft X-ray sources have been identified withstellar counterparts, leaving no doubt that stars in general constitutea pervasive class of low-luminosity galactic X-ray sources. Stars alongthe entire main sequence, of all luminosity classes, pre-main sequencestars as well as very evolved stars have been detected. Early type OBstars have X-ray luminosities in the range 10 to the 31st to 10 to the34th ergs/s; late type stars show a somewhat lower range of X-rayemission levels, from 10 to the 26th to 10 to the 31st ergs/s. Late typemain-sequence stars show little dependence of X-ray emission levels uponstellar effective temperature; similarly, the observations suggest weak,if any, dependence of X-ray luminosity upon effective gravity. Instead,the data show a broad range of emission levels (about three orders ofmagnitude) throughout the main sequence later than F0.
| Physical investigations of star chains.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975A&A....41..447H&db_key=AST
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Observation and Astrometry data
| Constellation: | Krebs |
| Right ascension: | 08h53m45.74s |
| Declination: | +13°49'49.5" |
| Apparent magnitude: | 7.069 |
| Distance: | 125.471 parsecs |
| Proper motion RA: | -30.7 |
| Proper motion Dec: | -5.8 |
| B-T magnitude: | 7.287 |
| V-T magnitude: | 7.087 |
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