GCN Circular 934

G. Ricker, D. Lamb, and S. Woosley on behalf of the HETE Science Team;

R. Vanderspek, G. Crew, J. Doty, G. Monnelly, J. Villasenor; N. 
Butler, T. Cline, J.G. Jernigan, A. Levine, F. Martel, G. Pizzichini, 
and G. Prigozhin, on behalf of the HETE Operations and HETE 
Optical-SXC Teams;

N. Kawai, M. Matsuoka, Y. Shirasaki, T. Tamagawa, A. Yoshida, E. 
Fenimore, M. Galassi, and  C. Graziani, on behalf of the HETE WXM 
Team;

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

write:

On 13 February 2001 at 12:35:35 UTC, a soft spectrum, high energy 
transient at high galactic latitude was detected and localized by 
HETE.  Both the WXM and the FREGATE instruments detected the event. 
Although the WXM and FREGATE cover extremely broad energy bands (WXM: 
2-25 keV; FREGATE: 6-500 keV), the burst event was only detectable in 
the 2-18 keV band by WXM, and in the 6-10 keV band by FREGATE. 
Because of the unusual spectrum of the transient, neither instrument 
triggered.

The preliminary coordinates of the burst are RA= 10h 31m 36s, Dec= 
+5d 30' 39" (J2000), derived from combining data from the WXM and 
Boresighted Optical Cameras.  The statistical error radius in the WXM 
localization is 3.5 arcmin (95% confidence).  In addition, we estimate 
a systematic error radius at present of 30 arcmin about this 
location.  The spacecraft aspect was known to an accuracy of +/- 30 
arcsec (95% confidence) from the optical cameras, and will be 
improved.

The burst exhibited a double-peaked structure and lasted about 30 
seconds.  The incident flux measured with the WXM (2-18 keV) at the 
first peak is about 0.7 Crab, and at the second peak is about 2.4 
Crab.  The spectrum in the 2-18 keV range was harder than that of the 
Crab nebula at the first peak, and softer than that of the Crab at 
the second peak.  The peak flux seen with FREGATE (6-10 keV) was ~2
Crab.  Assuming a Crab-like spectrum, the peak energy flux was ~1.5
E-8 erg/cm^2/sec in the 6-10 keV range.

The high galactic latitude of the source, well away from the Galactic 
Bulge, and the shape of its light curve suggest that it is a 
gamma-ray burst with a very unusual spectrum, perhaps similar to 
those reported for "X-ray rich" GRBs by Heise et al (2001) from 
BeppoSAX observations.  Conceivably, it could instead be a nearby 
X-ray burst source.  A preliminary catalog search of the WXM error 
circle revealed no correspondence with known globular sources, 
cataclysmic variables, low mass X-ray binaries, or flare stars.

Follow-up observations of this unusual transient are encouraged.

Additional information on the 13 February high energy transient 
(including light curves, the WXM error box and results from the 
preliminary catalog searches), as well as the HETE mission, is 
available at:

http://space.mit.edu/HETE/

Acronyms:
HETE=High Energy Transient Explorer
FREGATE=French Gamma Ray Telescope
WXM=Wide Field X-ray Monitor
SXC=Soft X-ray Camera

This message may be cited.