OPTICAL AND X-RAY CHARACTERISTICS OF STARS DETECTED IN THE EINSTEIN SLEW SURVEY

摘要

We detect X-rays for the first time from 63 cool (types A-M) stars. These stars are part of the 229 total stellar X-ray sources identified to date in the Einstein Slew Survey (hereafter Slew). We also list new X-ray data on one A star that may have a corona, five OB stars, and report discoveries 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°) is 26%. This agrees well with the Einstein Extended Medium Sensitivity Survey (EMSS) stellar sample, which only considered high latitudes. Because of the large solid angle covered by the Slew, and its shallow limiting flux, the sample will better probe the bright end of the stellar X-ray luminosity function (L_X > 10~(30) ergs s~(-1)). Presently (based on the 221 Slew stars with known spectral types), the sample is dominated by late-type systems (cooler than 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 Slew is assessed to be complete for spectral types earlier than K. Hence, K and M systems will be prominent in the ～ 50 as yet unidentified stars. We have embarked on an extensive program to (1) confirm the X-ray identifications with the optical counterparts; (2) search for Ca II H and K and Balmer line emission as activity signatures; and (3) search for supporting evidence of magnetic activity by measuring rotational velocities and relating them to X-ray luminosity level. Of 64 proposed coronal systems observed to date, we have confirmed 44 active stars, while in the remaining 20 we have been unable to find definitive activity. We have confirmed an additional 19 active stars from stellar database searches, and reclassified nine systems as having non-stellar optical counterparts from optical follow-up identification work and extragalactic database searches. We discuss notable new discoveries. From the sample of single active F7-M5 Slew stars with measured v sin i-values, we find a strong (99.9% confidence level) linear correlation of X-ray luminosity with v 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-M5 sample, we find the same 99.9% v sin i-L_X and R-L_X correlations. The L_X— v sin i relation for the combined sample appears to flatten with respect to the quadratic behavior seen for optically selected stellar samples at rotational velocities in excess of ～16 km s~(-1). For the unevolved subset of the Slew single star sample, we also find a correlation between L_X and Rossby number (R_0; more than 99% confidence). A least-squares fit gives L_X ～ R_0~(-0.4), which is similarly flatter than the quadratic dependence seen in optical samples. Using the stellar surface X-ray flux F_X versus B — V diagram, we interpret these results as saturation of the stellar surface by active regions at F_X/F_(bol) ≈ 10~(-3).