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Astron. Astrophys. 347, 927-931 (1999)

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

A.J. Castro-Tirado 1,2, S. Brandt 3, N. Lund 4 and R. Sunyaev 5

1 Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), P.O. Box 50727, E-28080 Madrid, Spain
2 Instituto de Astrofísica de Andalucía (IAA-CSIC), P.O. Box 03004, E-18080 Granada, Spain
3 Los Alamos National Laboratory, Los Alamos, NM 87545, USA
4 Danish Space Research Institute, Marianne Julies Vej 30, DK-2100, Copenhagen O, Denmark
5 Space Research Institute, Russian Academy of Sciences, Profsouznaya 84/32, 117810 Moscow, Russia

Received 1 October 1998 / Accepted 3 May 1999

Abstract

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 [FORMULA] 150 pc, Lx = 1.35 [FORMULA] 1034 erg s-1 (8-15 keV), or 2.6 [FORMULA] 1034 erg s-1 (0.1-2.4 keV) assuming a thermal spectrum with kT [FORMULA] 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 higher 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. 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 ([FORMULA] 1 day) 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.

Key words: stars: flare – stars: pre-main sequence – ISM: individual objects: Chamaeleon I cloud – X-rays: bursts – X-rays: stars

Send offprint requests to: A.J. Castro-Tirado (ajct@laeff.esa.es)

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999
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