1998A&A...332..825S

We have searched for low-mass pre-main-sequence stars in the nearby Upper Sco-Cen star-forming region using ROSAT observations analyzed with an innovative source detection method based on wavelet transforms. In a 5.2ksec ROSAT PSPC image we find 32 sources down to a limiting flux of 3.9x10^–14erg/cm²/s. In two HRI observations that cover with greater sensitivity approximately one-third of the same region we find 17 sources, down to a limiting flux of 1.4x10^–14erg/cm²/s. Half of the HRI sources were not detected either in the PSPC pointed observation nor are they reported in the ROSAT Bright Source Catalog, nor in the final analysis of Einstein IPC data of the same region, which resulted in the detection of 18 sources down to a limiting flux of 9.1x10^–14erg/cm²/s. Considering all the available X-ray data together, we have a total of 50 distinct X-ray sources, of which only 13 were previously reported as Einstein IPC sources in a sky region of nearly the same area as that surveyed in the present work. Our new analysis allows us to largely increase the census of likely PMS stars in the Upper Sco-Cen region and shows that the IPC survey of PMS stars in this region by Walter and collaborators is more severely incomplete than previously reported. The likely PMS nature of the stellar counterparts of the faint X-ray sources is supported by follow-up optical spectroscopy of some of these sources, as well as by the presence of variability of X-ray emission in a substantial fraction of them, and by the characteristics of available X-ray spectral information.