GCN Circular 4949

Please note: This is courtesy copy of Astronomer's Telegram #789 
forwarded to the GCN mailing list due to its specific interest to the 
Gamma-Ray community.  The URL for this Telegram is
http://www.astronomerstelegram.org/?read=789
Please cite the ATEL, rather than the GCN.

Swift-BAT observations of SGR 1900+14

D. Palmer (LANL), T. Sakamoto (GSFC/ORAU), S. Barthelmy (GSFC),
J. Cummings (GSFC/ORAU), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), J. Tueller (GSFC)
report on behalf of the Swift BAT team:


The recent re-activation of the Soft Gamma-ray Repeater SGR 1900+14 
(Vetere et al. GCN# 4922) has been intensively observed by the Swift 
Burst Alert Telescope (BAT) as well as by the Swift UVOT and XRT 
instruments.  Here we report on the BAT results.

The first detection by BAT was at 2006-03-25 10:12:04 UTC, and marked 
the first detection of a burst from this source since 2003 (C. 
Kouveliotou, priv. comm.)  The most recent burst was 2006-03-29 
12:19:51 (as of 2006-04-03 11:00).  A total of 22 bursts or burst 
sequences have been identified by the onboard software (where a burst 
sequence is defined as a set of bursts separated from other detected 
bursts by at least 5 minutes, including gaps due to observational and 
instrumental effects).

In the 5 months before this latest round of activity, the upper limit 
to steady-state emission on individual days with good observations of 
this source is 30 mCrab (3 sigma).  In the 9-month interval 
2004-12-15 to 2005-09-15, the long-term average steady-state emission 
is less than 1.4 mCrab (5 sigma).

The greatest intensity of activity was during an interval starting 
before 2006-03-29 02:38:10 (when the SGR entered the BAT field of 
view) to 02:59:43 (beyond which there is no detected activity through 
the end of the pointing at 03:13:11).  Within this time period there 
is a considerably more intense interval, 2:53:10 - 2:53:42, which we 
will call the storm.  Event-by-event data is available for most of 
this storm period.

The storm contains both wide and narrow peaks.  The narrow peaks have 
typical widths of 10-100 ms and peak rates up to 400 counts/ms.  The 
wide peaks have peak rates falling in the narrow range of 400-850 
counts/ms and durations of up to 1.5 seconds.  (All rates are ~12-100 
keV measured on 1 ms timescales with approximately 2300 cm^2 
effective area).

The wide peaks typically show a rapid rise (<10 ms) to near the 
maximum value, a somewhat steady decline (at rates of ~1 
count/ms/ms), sometimes with instances of re-energization, to an 
inflection point typically at a rate around 200-400 counts/ms, 
followed by a precipitous drop over the next ~10 ms.

For spectroscopy, data from the narrow peaks were accumulated into a 
single spectrum for analysis, while the wide peaks were analyzed with 
time-resolved spectra.

The accumulated spectrum of short peaks is well fitted with an 
optically thin thermal bremsstrahlung spectrum (OTTB), a power-law 
times an exponential cutoff model (CPL), and a sum of two blackbodies 
model (2BB). A single blackbody model shows a bad fit. kT of OTTB is 
21 keV. In 2BB model, the temperatures are 4.6 keV and 11.2 keV.

The time-resolved spectra of the bright peaks are well fitted in CPL 
and 2BB. But they show a bad fit in OTTB model, especially at the 
bright part.  However, if we ignore below spectral bins 30 keV, OTTB 
model shows an acceptable fit with  temperatures consistent with the 
Konus-Wind, RXTE and BATSE observations (Aptekar et al, ApJS, 137, 
227; Ibrahim et al. ApJ, 558, 237). The temperatures and the 
blackbody radii in 2BB model are consistent with the HETE-2 
parameters (2001 activity; Olive et al., ApJ, 616, 1148). There 
appears to be an excess above 70 keV during the brightest parts of 
the peaks.  However, since these spectra reach an intensity of 200 
Crab, they are dominated by systematics, and the apparent excess may 
be an artifact of our calibration.

This behavior is qualitatively consistent with confined fireball 
models.  The narrow peaks are optically thin fireballs, and the wide 
peaks are fireballs that are originally optically thick with 
contracting photospheres that reach transparency at the inflection 
point.  Quantitative comparison to this model awaits more detailed 
calculation.

Light curves and lists of burst times for this activity may be viewed 
at http://gcn.gsfc.nasa.gov/gcn/other/SGR1900+14_swift_bat.html

-- 
David Palmer  palmer@lanl.gov   (505)665-6863 (voice)   (505)665-4414 (fax)