GCN Circular 2308

H2768, H2770, and H2771: Three Bursts from SGR1806-20

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

C. Barraud, M. Boer, J-F Olive, and J-P Dezalay on behalf of the HETE 
FREGATE Team;

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

T. Tamagawa, M. Suzuki, C. Graziani, Y. Shirasaki, T. Donaghy, M. 
Matsuoka, K. Torii, T. Sakamoto, A. Yoshida, E. Fenimore, M. Galassi, 
Y. Nakagawa, R. Satoh, Y. Urata, T. Yamazaki and Y. Yamamoto, on 
behalf of the HETE WXM Team;

report:

At 08:02:20.59 UT on 17 July, the HETE FREGATE and WXM instruments 
detected and localized H2768, a short duration, soft spectrum burst 
event from the vicinity of the soft gamma repeater SGR1806-20.  The 
coordinates (J2000) of H2768 derived by the WXM are:

R.A. = 271.716 deg
Dec. = -20.461 deg

The one sigma (statistical) error circle for the localization is 5.2 
arcmin in radius.  SGR1806-20 lies about 24 arcmin from the HETE 
position.  However, a number of factors combined to produce 
larger-than-normal systematic errors for H2768.  They are related to 
the fact that H2768 was detected in orbit day, after star camera 
turnoff. Using reasonable assumptions about the spacecraft drift, the 
position derived for H2768 is believed to be completely compatible 
with that of SGR1806-20.

At 23:43:12.12 UT on 18 July, a second short duration event (H2770), 
was detected and localized from the vicinity of SGR1806-20. The 
duration (t50) of H2770 was 60 +/- 7 ms in the 7-40 keV band. The 
coordinates (J2000) derived by the WXM are:

R.A. =  272.176 deg
Dec. =  -20.590  deg

The localization for H2770 was determined on board HETE by the flight 
software, and disseminated 7 minutes later. The error circle (90% 
confidence) for the flight localization is 30 arcmin in radius. 
SGR1806-20 lies about 10 arcmin from the center of the HETE WXM error 
circle. Thus, the position derived for H2770 is also fully consistent 
with that of SGR1806-20.

Most recently, at 03:24:15.28 UT on 19 July, a third short duration 
event (H2771), was detected and localized from the vicinity of 
SGR1806-20. The duration (t50) of H2771 was 90 +/- 25 ms in the 7-40 
keV band. The coordinates (J2000) of H2771 derived by the WXM are:

R.A. =  272.241 deg
Dec. =  -20.626  deg

The localization for H2771 was determined on board HETE by the flight 
software, and disseminated 20 seconds later. The error circle (90% 
confidence) for the flight localization is 30 arcmin in radius. 
SGR1806-20 lies about 12 arcmin from the center of the HETE WXM error 
circle. Thus, the position derived for H2771 is also fully consistent 
with that of SGR1806-20.

Based on the accurate localization and the rapid cadence of these 
three HETE burst events, we conclude that the soft gamma repeater 
SGR1806-20 has evidently entered a heightened state of burst activity.

This message may be cited.