1999A&A...347..927C

During 1990-92, the WATCH all-sky X-ray monitor on Granat discovered six short-duration X-ray transients. In this paper we discuss their possible relationship to peculiar stars. Only one of the fast (few hours) X-ray transients (GRS 1100-771) might be tentatively ascribed to a superflare arising from a young stellar object in the Chamaeleon I star-forming cloud. At the distance of ∼150pc, L_x=1.35x10³⁴erg/s (8-15keV), or 2.6x10³⁴erg/s (0.1-2.4keV) assuming a thermal spectrum with kT∼10keV, a temperature higher than those previously seen in T Tauri stars (Tsuboi et al., 1998ApJ...503..894T). The peak X-ray luminosity is at least 2 times higher than that derived for the protostar IRS 43 (Grosso et al., 1997Natur.387...56G) which would make -to our knowledge- the strongest flare ever seen in a YSO. However, the possibility of GRS 1100-771 being an isolated neutron star unrelated to the cloud cannot be excluded, given the relatively large error box provided by WATCH. Regarding the longer duration (∼1day) X-ray transients, 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.