[vsnet-grb-info 10405] GRB 110328A / Swift J164449.3+573451: BAT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  11842
SUBJECT: GRB 110328A / Swift J164449.3+573451: BAT refined analysis
DATE:    11/03/30 17:06:00 GMT
FROM:    Jay R. Cummings at NASA/GSFC/Swift  <james.r.cummings at nasa.gov>

T. Sakamoto (GSFC/UMBC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), M. Stamatikos (OSU), J. Tueller (GSFC),
T. N. Ukwatta (MSU)

The source originally identified as GRB 110328A (Cummings et al.
GCN Circ. #11823 and Barthelmy et al. GCN Circ. #11824) has triggered BAT
several times.  It is a highly unusual object.  It is coincident with an
optical source at redshift z = 0.35 (Levan et al., GCN Circ #11833), as
well as a radio source (Zauderer et al., GCN Circ #11836).  The source
continues to be detectable and variable in BAT more than 40 hours after
the initial trigger, with peak brightness on the order of 200 mCrab.  The
source is at high galactic latitude, 39.4 degrees.

The summary of BAT triggers so far is:
TrigNum   Date Time         TrigDur  Intensity
                 UT             [sec]    [c/sec]
450158    28 Mar 12:57:45   1208       6.1
450161    28 Mar 13:40:41     64      19.4
450257    29 Mar 18:26:25    320      15.6
450258    29 Mar 19:57:45     64      38.2

This table does not represent all the possible triggers, because the on-
board triggering algorithm requires the intensity of the any following
event to be slightly more than twice the previous intensity.  Also note
that the on-board threshold was commanded to zero between the 2nd and 3rd
triggers in order to enhance future triggers, and that it was later
commanded to be 4 times the 4th trigger's intensity after the 4th trigger
to suppress further triggers.

The observation of the source including the discovery image was T-4 sec
to T+1211 sec.  A fit to a power law spectrum from BAT survey data on this
interval has a photon index of 1.72 ± 0.18.  The fluence from 15 to
150 keV using this model was (3.0 ± 0.3) x 10^-6 ergs/cm^2.  The first
followup observation, during which the second trigger occurred, was
T+1465 sec to T+2348 sec.  The photon index from that observation was
1.37 ± 0.11.  The fluence was (2.7 ± 0.2) x 10^-6 ergs/cm^2.

The light curve from the BAT transient monitor (15-50 keV):
http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/SwiftJ164449.3p573451/
shows variability, but that the source was first detectable on 25-March-2011
and has, on average, continued to brighten since the trigger.

Please see also Kennea et al., ATel#3242 for more information:
http://www.astronomerstelegram.org/?read=3242