We have searched for low-mass pre-main-sequence stars in the Upper
Sco-Cen star-forming region using ROSAT observations
analyzed with an innovative source detection method based on wavelet
transforms. In a 5.2 ksec ROSAT PSPC image we find
32 sources down to a limiting flux of 3.9 10-14 erg
cm-2 s-1. 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 1.4 10-14 erg cm-2
s-1.
Half of the HRI sources where not detected either in the PSPC pointed
observation nor are 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 9.1 10-14 erg cm-2 s-1.
Considering all the available X-ray data, 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 we have surveyed.
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 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.