GCN Circular 4938

E. Fenimore, G. Ricker, J-L. Atteia, N. Kawai, D. Lamb, and S. Woosley,
on behalf of the HETE Science Team;

M. Arimoto, T. Donaghy, M. Galassi, C. Graziani, N. Ishikawa,
A. Kobayashi, J. Kotoku, M. Maetou, M. Matsuoka, Y. Nakagawa,
T. Sakamoto, R. Sato, T. Shimokawabe, Y. Shirasaki, S. Sugita,
M. Suzuki, T. Tamagawa, K. Tanaka, 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, G. Pizzichini, and S. Gunasekera, on behalf of the HETE
Operations and HETE Optical-SXC Teams;

M. Boer, J-F Olive, A. Pelangeon, J-P Dezalay, and K. Hurley, on behalf
of the HETE FREGATE Team;

report:

At 06:41:42 UTC on March 31, 2006 HETE-2 detected a long, bright
and soft event at the extreme edge of the WXM field of view.

Ground analysis yields an error box centered at (J2000):
(RA,DEC) (deg) = 233.444d  -15.235d (15h 33m 47s  -15d 14' 04")

with the following corners (J2000):
(RA,DEC) (deg) =   233.792   -15.396
(RA,DEC) (deg) =   233.081   -15.456
(RA,DEC) (deg) =   233.096   -15.072
(RA,DEC) (deg) =   233.806   -15.015

The error box area is 944 square arc minutes.

The time profile shows a rise time of ~10 seconds and a fall time
of >100s, with little obvious spectral evolution. Neither a power-law
nor a blackbody yields an acceptable spectral fit.

This event is unusual. It resembles an X-ray burst from a galactic binary,
yet its location at high galactic latitude, its long duration without the
expected spectral evolution from photospheric expansion, and the failure
of a blackbody spectral fit all argue against this interpretation.

On the other hand, it is unusually bright for an X-ray flash, and the
failure of a power law fit argues against this interpretation.

There is  a known x-ray source (1RXS J153521.7-150420) in the error box,
but this is not unlikely for a box this large.

A light-curve of the event is given at http://space.mit.edu/HETE/Bursts/GRB060331/

We urge further observations of this unusual event at all wavelengths.

[GCN OPS NOTE(31mar06): Per author's request, the URL was corected; see 
GCN Circ 4939
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