The stellar content of soft all-sky X-ray surveys

Wide angle soft X-ray surveys such as the ROSAT all-sky survey, the XMM slew survey, or the upcoming eROSITA all-sky survey(s) produce - more or less homogeneous - data sets with tens and hundreds of thousands of X-ray sources. The counterparts of typically about a third of these X-ray sources are stars, mostly of late spectral type. With the availability of genuine all-sky surveys at optical (GAIA) and infrared wavebands (2MASS) with reliable positions and multiband fluxes and in particular with the (eventual) availability of GAIA parallax information down to v = 15 mag and below, the automatic extraction and identification of the stellar content of soft X-ray surveys becomes feasible and doable. Distance information and hence accurate X-ray luminosities are available for the full data set, the counterparts can be accurately placed in the HR diagram and the local stellar volume X-ray emissivity can be measured. We discuss optimal identification strategies, the potential arising from future GAIA data releases and apply our methods to the XMM slew survey data. Our results suggest that 30.7% of the XMM slew survey entries can be identified with (non-accreting) stars.