GRB 090531

M. Stamatikos (NASA/ORAU), S. D. Barthelmy (GSFC), C. Gronwall (PSU),
S. T. Holland (CRESST/USRA/GSFC), V. Mangano (INAF-IASFPA) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 01:45:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090531 (trigger=353627).  Swift could not immediately slew
to the burst location.  The BAT on-board calculated location is 
RA, Dec 178.687, +7.799 which is 
   RA(J2000) = 11h 54m 45s
   Dec(J2000) = +07d 47' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 50 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~10 sec after the trigger. 

Because of a Moon constraint, Swift could not slew, and there
will be no XRT or UVOT observations until June 3. 

Burst Advocate for this burst is M. Stamatikos (michael AT milkyway.gsfc.nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

S. B. Cenko (UC Berkeley) reports on behalf of a larger collaboration:

We have imaged the field of GRB090531 (Stamatikos et al., GCN 9454
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 ) with
the automated Palomar 60 inch telescope.  Observations began at 4:47 UT on
2009 May 31 (~ 4.03 hr after the burst trigger) and were taken in the
Sloan r', i', and z' filters.  In coadded images, we find no new sources
inside the BAT error circle to the following limits (calculated
with reference to the SDSS DR7):

Filter		Magnitude		Time Since Burst (hours)
----------------------------------------------------------------------
r'		> 21.5			4.22
i'		> 21.3			4.25
z'		> 20.5			4.30

A. Rossi (Tautenburg), F. Olivares, and J. Greiner (both MPE Garching)
report on behalf of the GROND team:

We observed the field of GRB 090531 (Swift trigger 353627, Stamatikos et al.,
GCN #9454
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 ) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m
MPI/ESO telescope at La Silla Observatory (Chile). Observations started on
31 May 2009 at 01:47 UT, 2 minutes after the burst and continued until
03:29 UT.

We do not find any new source within the BAT error circle. Image subtraction
between the first and last images of each band does not reveal any variable
source. The first image , consisting of 4.4 min exposure in g'r'i'z' and
4 min in JHK, yields the following upper limits (all AB):

g' > 22.6
r' > 22.8
i' > 22.4
z' > 22.0
J  > 21.5
H  > 20.9
K  > 20.2

which were obtained using the SDSS and 2MASS field stars as reference.

T. Sakamoto (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL),
G. Sato (ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC),
T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-60 to T+243 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 090531 (trigger #353627)
(Stamatikos, et al., GCN Circ. 9454
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 ).  The BAT ground-calculated position is
RA, Dec = 178.672, 7.824 deg, which is 
   RA(J2000)  =  11h 54m 41.4s 
   Dec(J2000) = +07d 49' 27.0" 
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 73%.
 
The mask-weighted light curve shows a single peak starting at ~T-25 sec,
peaking at T+9 sec, and ending at ~T+60 sec.  There is a possible (2.5 sigma CL)
episode of emission from T+80 to ~T+140 sec.  T90 (15-350 keV) is 32 +- 6 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-12.6 to T+29.4 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.86 +- 0.42, 
and Epeak of 64.7 +- 14.3 keV (chi squared 51.91 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.3 +- 0.1 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+9.29 sec in the 15-150 keV band is
1.3 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.68 +- 0.09 (chi squared 65.50 for 57 d.o.f.).  All the quoted errors
are at the 90% confidence level. 
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/353627/BA/