TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3554 SUBJECT: H11603: An X-ray flare from V4641 Sgr, or a possible XRF DATE: 05/06/26 21:06:09 GMT FROM: Geoffrey B. Crew at MIT/HETE H11603: An X-ray flare from V4641 Sgr, or a possible XRF J. Kotoku, G. Ricker, J-L. Atteia, N. Kawai, D. Lamb, and S. Woosley, on behalf of the HETE Science Team; M. Arimoto, T. Donaghy, E. Fenimore, M. Galassi, C. Graziani, M. Maetou, M. Matsuoka, Y. Nakagawa, T. Sakamoto, R. Sato, Y. Shirasaki, M. Suzuki, T. Tamagawa, K. Tanaka, Y. Yamamoto, and A. Yoshida, on behalf of the HETE WXM Team; N. Butler, G. Crew, J. Doty, G. Prigozhin, R. Vanderspek, J. Villasenor, J. G. Jernigan, A. Levine, G. Azzibrouck, J. Braga, R. Manchanda, and G. Pizzichini, on behalf of the HETE Operations and HETE Optical-SXC Teams; M. Boer, J-F Olive, J-P Dezalay, and K. Hurley, on behalf of the HETE FREGATE Team; report: The HETE WXM instruments detected an untriggered burst H11603 at 04:05 UT on 26 June 2005. The burst is extremely soft (most photons < 5 keV), has a spiky profile with timescale of 1 second, and the duration is about 100 sec. The peak flux (2-30 keV over 5 s) was ~7x10^-8 erg cm^-2 s^-1. H11603 is localized by the WXM to a circle of radius of 6' centered at RA = 18h 19m 38s, Dec = -25d 26' 30" (J2000). A GCN Ground Analysis Notice was issued. Its position is marginally consistent with that of V4641 Sgr = SAX J1819.3-2525 = XTE J1819-254. Examination of earlier data revealed another event at 03:01 UT on 26 June 2005, consistent with these locations. This source was recently reported to be X-ray active by Swank, Smith and Markwardt (ATEL #536). The spiky profile of the burst is similar to that observed for the X-ray flares from V4641 Sgr (e.g. Wijiand and van der Klis 2000, ApJ 528, L93). With a preliminary analysis we find the burst has a spectrum consistent with a blackbody with kT=1 keV with a hard tail, which is also similar to the X-ray spectrum reported for V4641 Sgr by earlier observations (e.g. in't Zand et al. 2000, A&A 357, 526). Thus H11603 is very likely to be a X-ray flare from V4641 Sgr, and not an XRF of an extragalactic origin, although we cannot absolutely exclude the possiblity. Optical and X-ray follow-up observations of the counterpart are encouraged. This message may be cited.