Observations of short-duration X-ray transients by WATCH on Granat

During 1990-92, the WATCH all-sky X-ray monitor on Granat discovered six sh ort-duration X-ray transients. In this paper we discuss their possible rela tionship to peculiar stars. Only one of the fast (few hours) X-ray transien ts (GRS 1100-771) might be tentatively ascribed to a superflare arising fro m a young stellar object in the Chamaeleon I star-forming cloud. At the dis tance of similar to 150 pc, L-x = 1.35 x 10(34) erg s(-1). (8-15 keV), or 2 .6 x 10(34) erg s(-1) (0.1-2.4 keV) assuming a thermal spectrum with kT sim ilar to 10 keV, a temperature higher than those previously seen in T Tauri stars (Tsuboi et al. 1998). The peak X-ray luminosity is at least 2 times h igher than that derived for the protostar IRS 43 (Grosso et al. 1997) which would make - to our knowledge- the strongest flare ever seen in a YSO. How ever, the possibility of GRS 1100-771 being an isolated neutron star unrela ted to the cloud cannot be excluded, given the relatively large error box p rovided by WATCH. Regarding the longer duration (similar to 1 day) X-ray tr ansients, none of them seem to be related to known objects. We suggest that the latter are likely to have originated from compact objects in low-mass or high-mass X-ray binaries, similarly to XTE J0421+560.