[vsnet-alert 8477] V4641 Sgr: HETE detection of an X-ray flare (GCN 3554)

[Taichi Kato]

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  <gbc at space.mit.edu>

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. 

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