GCN Circular 12159

Swift J1822.3-1606: A Probable New SGR in Ground Analysis of BAT Data

J. R. Cummings (GSFC/UMBC), D. Burrows (PSU), S. Campana (INAF-OAB),
J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA), D. M. Palmer (LANL),
T. Sakamoto (NASA/UMBC), S. Zane (MSSL-UCL)
on behalf of the Swift Team

At 2011-07-14 at 12:47:47.1 UTC, Swift-BAT triggered (#457261) on
a previously unknown source, Swift J1822.3-1606.  This was at the
same time as Fermi-GBM trigger #332340476.  Only a subthreshold source
was detected onboard.  There were two subsequent rate increases of
similar size, probably from the same source at about T+26 sec and
T+308 sec, the latter also causing a rate trigger with no
significant source found onboard (#457263).  There were also 2 other
rate triggers within the same pointing period (about 1650 seconds
long), but no significant flux was observed from the source at these
times.

Each pulse was about 0.005 +- 0.001 sec long.  All the emission
was < 100 keV.  Because of the repeated soft and short rate
increases, and the location of the source near the Galactic plane,
we tentatively classify this object as a new Soft Gamma-ray
Repeater.

Combining BAT pulse-height events from the two rate triggers
yields a highly significant source detection at
RA, Dec 275.595, -16.100, which is:

RA (J2000)  18h 22m 22.8s
Dec (J2000) -16d 05' 59"

with an estimated error of 2.3 arcmin (estimated radius, 90%
containment).

There was also a rate trigger (#457253) on a longer (~0.1 sec),
larger, and harder event that was out of the BAT field of view
at T-10754 sec.  SWIFT J1822.3-1606 was above the horizon and
out of the BAT FOV, and we cannot determine if the event was
associated with the source or not.

The Swift/BAT hard X-ray transient archive was examined to search
for previous emission from SWIFT J1822.3-1606 in daily averages.
No significant excess was found back to 2011 June 15 with an
average 1-sigma error in the 15-50 keV band of 0.0016 ct/s/cm2
(~7 mCrab).  The history of Swift/BAT on-board source detections
was examined and we found no previous rate-triggered or image-
triggered detections of the source.

We have detailed data for two rate increases.  A powerlaw fit to
the combined spectrum has a photon index of 3.7 +- 0.5.  A
blackbody fit to the combined spectrum has a marginally better
chisquared with a temperature of 4.9 +- 0.5 keV.  The combined
fluence of the two rate increases for which we have data, using
the blackbody model, was (2.0 +- 0.4) x 10^-7 ergs/cm^2

A Swift TOO has been approved, but the source is Moon constrained
until at least 17:34 UT on July 15.  An observation is planned
soon after the source comes out of constraint.

This circular has also been posted as ATel #3488:
http://www.astronomerstelegram.org/?read=3488