GCN Circular 4949
Subject
SGR 1900+14: BAT observations
Date
2006-04-03T22:34:53Z (18 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
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)