TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4938 SUBJECT: H4043, an unusual high-energy transient observed by HETE DATE: 06/03/31 16:00:17 GMT FROM: Jean-Luc Atteia at Lab d Astrophys.,OMP,Toulouse 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.]