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HD 61396: An unusual mass-transfer RS CVn binary
HD 61396 has been identified as an X-ray and radio source and has shownHα emission and photometric variability which have marked it outas an RS CVn star. Although it has been termed an RS CVn binary, hardevidence of its duplicity has been lacking. It is now shown to be adouble-lined binary consisting of two giant stars, probably of almostequal size and luminosity but with a mass ratio of more than eight toone, in a 34-day orbit that is circular or very nearly so. Mass transferis probably still continuing. We have not been able to produce asatisfactory evolutionary model.
| Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources. VIII. The late-type stellar component
We present results of an investigation of the X-ray properties, agedistribution, and kinematical characteristics of a high-galacticlatitude sample of late-type field stars selected from the ROSAT All-SkySurvey (RASS). The sample comprises 254 RASS sources with opticalcounterparts of spectral types F to M distributed over six study areaslocated at |b| 20 °, and Dec ≥ -9 °. A detailed studywas carried out for the subsample of ~200 G, K, and M stars. Lithiumabundances were determined for 179 G-M stars. Radial velocities weremeasured for most of the 141 G and K type stars of the sample. Combinedwith proper motions these data were used to study the age distributionand the kinematical properties of the sample. Based on the lithiumabundances half of the G-K stars were found to be younger than theHyades (660 Myr). About 25% are comparable in age to the Pleiades (100Myr). A small subsample of 10 stars is younger than the Pleiades. Theyare therefore most likely pre-main sequence stars. Kinematically the PMSand Pleiades-type stars appear to form a group with space velocitiesclose to the Castor moving group but clearly distinct from the LocalAssociation.Based on observations collected at the German-Spanish AstronomicalCentre, Calar Alto, operated by the Max-Planck-Institut fürAstronomie, Heidelberg, jointly with the Spanish National Commission forAstronomy, and at the European Southern Observatory, La Silla, Chile.Tables A2-A4 are only available in electronic form athttp://www.edpsciences.org
| BVR photometry of a newly identified RS CVn binary star HD 61396
BVR photometry of a recently identified RS CVn binary star HD 61396,carried out during 2001, is presented. The new photometry revealsignificant evolution in the shape and amplitude of light curve whencompared with those reported earlier by Parihar et al. [MNRAS 314 (2000)733]. The traditional two-starspot model has been used to obtain thespot parameters from the observed light curve. Changes in the spot areaand their location on the stellar surface are discernible from theextracted parameters from the new photometry.
| The HRX-BL Lac sample - Evolution of BL Lac objects
The unification of X-ray and radio selected BL Lacs has been anoutstanding problem in the blazar research in the past years. Recentinvestigations have shown that the gap between the two classes can befilled with intermediate objects and that apparently all differences canbe explained by mutual shifts of the peak frequencies of the synchrotronand inverse Compton component of the emission. We study the consequencesof this scheme using a new sample of X-ray selected BL Lac objectscomprising 104 objects with z<0.9 and a mean redshift bar {z} = 0.34.77 BL Lacs, of which the redshift could be determined for 64 (83%)objects, form a complete sample. The new data could not confirm ourearlier result, drawn from a subsample, that the negative evolutionvanishes below a synchrotron peak frequency log nupeak =16.5. The complete sample shows negative evolution at the 2sigma level(< Ve/Va > = 0.42 +/- 0.04). We concludethat the observed properties of the HRX BL Lac sample show typicalbehaviour for X-ray selected BL Lacs. They support an evolutionarymodel, in which flat-spectrum radio quasars (FSRQ) with high energeticjets evolve towards low frequency peaked (mostly radio-selected) BL Lacobjects and later on to high frequency peaked (mostly X-ray selected) BLLacs.Appendix (Tables 8 and 9, Fig. 8) is only available in electronic format http://www.edpsciences.org
| RBSC-NVSS Sample. I. Radio and Optical Identifications of a Complete Sample of 1556 Bright X-Ray Sources
We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAOVLA Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightestX-ray sources (>=0.1 counts s-1~10-12 ergscm-2 s-1 in the 0.1-2.4 keV band) that are alsoradio sources (S>=2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalacticsky with |b|>15deg and δ>-40deg. Thesky density of NVSS sources is low enough that they can be reliablyidentified with RBSC sources having rms positional uncertainties>=10". We used the more accurate radio positions to make reliableX-ray/radio/optical identifications down to the POSS plate limits. Weobtained optical spectra for many of the bright identifications lackingpublished redshifts. The resulting X-ray/radio sample is unique in itssize (1557 objects), composition (a mixture of nearly normal galaxies,Seyfert galaxies, quasars, and clusters), and low average redshift[~0.1].
| Optical, X-ray and radio observations of HD 61396: a probable new RS CVn-type binary
We have carried out BVR photometric and Hα spectroscopicobservations of the star HD 61396 during 1998 March 20 to 1999 April 3.We have discovered regular optical photometric variability from thisstar, with an inferred period of 31.95+/-0.10d, and an amplitude of~0.18mag. A possible period of 35.34+/-0.12d, as determined withHipparcos, cannot be completely ruled out, however. Modelling of itsphotometric light curve with two circular spots indicates that 5-21 percent of the stellar surface is covered by dark starspots which are ~830Kcooler than the surrounding photosphere, and produce the observedrotational modulation of the optical flux. Optical spectroscopy revealsa variable Hα emission feature, indicating that it is an unusuallyactive star. In addition, we have analysed archival X-ray data of HD61396, obtained from serendipitous observations with the ROSAT X-rayobservatory, and we also discuss the radio properties of this star,based on both published Green Bank and unpublished VLA observations. Thestrong photometric variability and Hα emission, the relativelyhard X-ray spectrum, and the high X-ray and radio luminosities implythat HD 61396 is most likely to be a member of the RS CVn class ofevolved active binary stars. Its X-ray and radio luminosities place itamong the five most luminous active binaries detected so far.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| Identification of a Complete Sample of Northern ROSAT All-Sky Survey X-Ray Sources. III. The Catalog
We present a catalog of optical identifications of a representativesample of northern ( delta > -9 deg) ROSAT All-Sky Survey (RASS)sources. A full identification has been carried out for a count-rate-and area-limited complete RASS subsample comprising 674 sources. Allsources are within six study areas outside the Galactic plane (| b |> 19.dg6), one area being near the north Galactic pole and one nearthe north ecliptic pole.
| Radio-loud active galaxies in the northern ROSAT All-Sky Survey. II. Multi-frequency properties of unidentified sources.
We present the broad band, radio - to - X-ray, properties of a largesample of mostly previously optically unidentified radio-loud X-raysources from the correlation of a ROSAT All-Sky Survey source list withthe 5GHz Green Bank Survey of the northern sky (RGB sample) which is oneof the largest well-defined flux-limited surveys of AGN ever obtained.Further, the RGB pushes 1-2 orders of magnitude deeper in both X-ray andradio flux compared to previous unbiased wide-area AGN surveys. Followup VLA observations of the candidate objects yielded positions witharcsec accuracy which were used to find optical counterparts to thesources from digitized POSS plates. The sources are divided into threeclasses according to the positional offset between the X-ray and radiocandidates and the spatial resolution of the radio observations,reflecting the various degrees of confidence about the correctness ofthe proposed association. Although the nature of the sources as well astheir redshifts remain to be determined in spectroscopic follow upobservations, the derived flux ratios lead to the conclusion that themajority of them are quasars. Hardly any correlations could be foundbetween different source parameters, possibly due to the fact that mostof the objects are found in a relatively small flux range near thesensitivity limit of the radio catalogue. The majority of the new RGBsources have broad-band properties between those of traditionalradio-selected and X-ray selected AGN. There is no bimodal distributionin the radio-loudness distribution, and the traditional division betweenradio-quiet and radio-loud AGN may not be warranted.
| 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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