[vsnet-grb-info 2475] An extraordinary long burst on December 3 from SGR1806-20

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4312
SUBJECT: An extraordinary long burst on December 3 from SGR1806-20
DATE:    05/12/06 15:48:11 GMT
FROM:    Valentin Pal'shin at Ioffe Inst  <val at mail.ioffe.ru>

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks,
and T. Cline on behalf of the Konus-Wind team,

S. Barthelmy, J. Cummings, N. Gehrels, H. Krimm
on behalf of the Swift team,

A bright extraordinary long SGR-like event triggered the Konus-Wind
at T0=42203.684 s UT (11:43:23.684) on December 3.

This burst was also observed by Swift-BAT
out of the BAT field of view.
The measured time delay between the burst arrival times
on the two s/c is 5.245-5.276 sec (3 sigma confidence range).
For the SGR 1806-20 position the expected time delay is 5.274 sec.
As the measured time delay is consistent with the expected for the
position of SGR 1806-20, we conclude that this burst originated from it.
But we should notice that the measured time delay also consistent
with the SGR 1801-23 position.

The shape of the burst light curve was typical for SGR bursts,
but with an unusually long duration. There was a short weak
precursor ~0.3 sec long at T0-0.4 sec, followed by a 0.1 sec
rise and a 22 sec decay. There was a weaker short soft burst
at T-T0 ~110 sec.

As observed by Konus-Wind the burst had a fluence
(1.53+/-0.02)x10^-4 erg/cm2, and a peak flux on 16-ms time scale
(3.50 ± 0.16)x10^-5 erg/cm2/sec (both in the 20-200 keV range).

The time-integrated spectrum of the burst can be fit by the OTTB
spectral model:
dN/dE ~ E^{-1} exp(-E/kT)
with kT = 19.9 + 0.5 keV, which is typical for SGR 1806-20.

All the quoted errors are at 90% c.l.

This is the longest SGR burst ever triggered the Konus-Wind.

Despite its long duration there is apparently no
periodicity in the light curve.

The Konus-Wind light curve of this GRB can be seen
at http://www.ioffe.rssi.ru/LEA/SGRs/051203_T42203/

Likely, the two previous unusually long SGR-like events
detected by the Konus-Wind (Golenetskii et al., GCN 4310)
originated from this SGR
(both these events were not observed by the Swift-BAT because
the SGR 1806-20 was below the horizon).

BAT has since observed two short (~0.1 sec) bursts from
SGR 1806-20, similar to many bursts seen earlier this year.

Note, that the ~40-s long burst from SGR 1900+14
was detected on April 18, 2001 by BeppoSAX, and the Konus-Wind
in the waiting mode
(Guidorzi et al., 2004, A&A, 416, 297 and references therein).
That event was strongly modulated with a 5-s period,
but had the comparable fluence and peak flux.