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The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| A complete sample of Seyfert galaxies selected at 0.25 keV
We have used the ROSAT Bright Source Catalogue to extract a completesample of sources selected in the band from 0.1-0.4keV. This0.25keV-selected sample is composed of 54 Seyfert galaxies, 25 BLLacertae objects, 4 clusters and 27 Galactic stars or binaries.Seyfert-type galaxies with `ultrasoft' X-ray spectra can very often beclassed optically as narrow-line Seyfert 1s (NLS1s). Such objects arereadily detected in 0.25keV surveys; the sample reported here contains20 NLS1s, corresponding to a 40 per cent fraction of the Seyferts.Optical spectra of the Seyfert galaxies were gathered for correlativeanalysis, which confirmed the well-known relations between X-ray slopeand optical spectral properties (e.g. [Oiii]/Hβ ratio; Feiistrength, Hβ width). The various intercorrelations are most likelydriven, fundamentally, by the shape of the photoionizing continuum inSeyfert nuclei. We argue that a steep X-ray spectrum is a betterindicator of an `extreme' set of physical properties in Seyfert galaxiesthan is the narrowness of the optical Hβ line. The correlationstudies were also used to isolate a number of Seyfert galaxies withapparently `anomalous' properties. Of particular interest are the sixobjects with relatively weak permitted line emission (Hβ and Feii)and weak optical continua. Such objects are rare in most surveys, buttwo of these (IC 3599 and NGC 5905) are known to be transient activegalactic nuclei in which the X-ray flux has faded by factors ~100. Ifthe other four objects also turn out to be transient, this woulddemonstrate that 0.25keV surveys provide an efficient way of finding aninteresting class of object. Finally, the luminosity function of the0.25keV-selected Seyfert galaxies was determined and broken down intosubsamples to investigate the relative space densities of Seyferts whenseparated on the basis of either X-ray slope or Hβ linewidth.
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| Optical and X-Ray Characteristics of Stars Detected in the Einstein Slew Survey
We detect X-rays for the first time from 63 cool (types AM) stars. Thesestars are part of the 229 total stellar X-ray sources identified to datein the Einstein Slew Survey (hereafter Slew). We also list new X-raydata on one A star that may have a corona, five OB stars, and reportdiscoveries of two new T Tauri stars and two new cataclysmic variables.The stellar content of the Slew high-latitude subset (currently 93%identified for |b_II_| > 20^deg^) is 26%. This agrees well with theEinstein Extended Medium Sensitivity Survey (EMSS) stellar sample, whichonly considered high latitudes. Because of the large solid angle coveredby the Slew, and its shallow limiting flux, the sample will better probethe bright end of the stellar X-ray luminosity function (L_X_ >10^30^ ergs s^-1^). Presently (based on the 221 Slew stars with knownspectral types), the sample is dominated by late-type systems (coolerthan F; 68%). These include dMe's, BY Dra, RS CVn, and FK Comae systems.Based on the limiting magnitudes of catalogs searched to date, the Slewis assessed to be complete for spectral types earlier than K. Hence, Kand M systems will be prominent in the ~50 as yet unidentified stars. Wehave embarked on an extensive program to (1) confirm the X-rayidentifications with the optical counterparts; (2) search for Ca II Hand K and Balmer line emission as activity signatures; and (3) searchfor supporting evidence of magnetic activity by measuring rotationalvelocities and relating them to X-ray luminosity level. Of 64 proposedcoronal systems observed to date, we have confirmed 44 active stars,while in the remaining 20 we have been unable to find definitiveactivity. We have confirmed an additional 19 active stars from stellardatabase searches, and reclassified nine systems as having nonstellaroptical counterparts from optical follow-up identification work andextragalactic database searches. We discuss notable new discoveries.From the sample of single active F7-MS Slew stars with measured ν sini-values, we find a strong (99.9% confidence level) linear correlationof X-ray luminosity with ν sin i and with stellar radius (R).However, L_X_ is uncorrelated with angular rotation speed at the 99%level. For the combined Slew and EMSS single star F7-MS sample, we findthe same 99.9% ν sin i-L_X_ and R-L_X_ correlations. The L_X_-νsin i relation for the combined sample appears to flatten with respectto the quadratic behavior seen for optically selected stellar samples atrotational velocities in excess of ~16 km s^-1^. For the unevolvedsubset of the Slew single star sample, we also find a correlationbetween L_X_ and Rossby number (R_0_; more than 99% confidence). Aleast-squares fit gives L_X_ ~ R_0_^-0.4^, which is similarly flatterthan the quadratic dependence seen in optical samples. Using the stellarsurface X-ray flux F_X_ versus B - V diagram, we interpret these resultsas saturation of the stellar surface by active regions at F_X_/F_bol_ ~10^-3^.
| The Einstein Slew Survey
A catalog of 819 sources detected in the Einstein IPC Slew Survey of theX-ray sky is presented; 313 of the sources were not previously known asX-ray sources. Typical count rates are 0.1 IPC count/s, roughlyequivalent to a flux of 3 x 10 exp -12 ergs/sq cm s. The sources havepositional uncertainties of 1.2 arcmin (90 percent confidence) radius,based on a subset of 452 sources identified with previously knownpointlike X-ray sources (i.e., extent less than 3 arcmin).Identifications based on a number of existing catalogs of X-ray andoptical objects are proposed for 637 of the sources, 78 percent of thesurvey (within a 3-arcmin error radius) including 133 identifications ofnew X-ray sources. A public identification data base for the Slew Surveysources will be maintained at CfA, and contributions to this data baseare invited.
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