GRB 120927A

M. Andreev,  A. Sergeev (Terskol Branch of Institute of Astronomy), A. 
Pozanenko (IKI) on behalf of larger GRB follow up collaboration report:

We observed field of the Swift  GRB 120927A  (Maselli et al. GCN  13822
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 ) 
with Zeiss-600 telescope of Mt.Terskol observatory starting on Sep. 28 (UT) 
01:31:05. Several unfiltered images were taken under favorable weather 
conditions. We do not detect any sources in the enhanced XRT error circle 
(Beardmore et al. GCN  13824
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 ). Photometry is based on nearby USNO-B1.0 (R2) 
stars:

  T0+,     Filter      Exposure,  OT,     UL (3 sigma)
  mid, d                (s)

0.1292    none      30x60         n/d     22.0

S. Immler (NASA/CRESST/GSFC) and A. Maselli (INAF-IASFPA)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 120927A
158 s after the BAT trigger (Maselli et al., GCN Circ. 13822
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 ).
No optical afterglow consistent with the enhanced XRT position
(Beardmore et al. GCN Circ. 13824
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 ) is detected in the initial UVOT
exposures. Preliminary 3-sigma upper limits using the UVOT
photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373)
for the first finding chart (FC) and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           158          308          147         >20.3
u_FC               317          567          246         >19.5
white              158         6331          580         >21.0
v                  648         6625          317         >19.1
b                  572         6127          432         >20.1
u                  317         5921          659         >20.2
w1                 697         5717          413         >19.6
m2                 672         5511          225         >19.6
w2                 623         6537          432         >20.3

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).

GRB 120927A, Swift-BAT refined analysis
  
D. M. Palmer (LANL), 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), A. Maselli (INAF-IASFPA),
T. Sakamoto (GSFC/UMBC), G. Sato (ISAS), M. Stamatikos (OSU), J. Tueller (GSFC),
T. N. Ukwatta (MSU)
(i.e. the Swift-BAT team):
  
Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 120927A (trigger #534815)
(Maselli, et al., GCN Circ. 13822
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 ).  The BAT ground-calculated position is
RA, Dec = 136.606, 0.420 deg which is
    RA(J2000)  =  09h 06m 25.3s
    Dec(J2000) = +00d 25' 13.5"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).  The
partial coding was 64%.
  
The mask-weighted light curve shows multiple overlapping peaks with an overall
FRED profile starting about T-15 seconds and significant until about
T+32 seconds.  T90 (15-350 keV) is 43 +- 18 sec (estimated error including
systematics).
  
The time-averaged spectrum from T-37.94 to T+41.04 sec is best fit by a power
lawwith an exponential cutoff.  This fit gives a photon index 0.63 +- 0.38,
and Epeak of 66.1 +- 9.6 keV (chi squared 50.98 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 2.6 +- 0.3 x 10^-06 erg/cm2
and the 1-sec peak flux measured from T-3.93 sec in the 15-150 keV band is
1.6 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.64 +- 0.08 (chi squared 77.30 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/534815/BA/

A. Rossi (TLS Tautenburg), P. Schady and  J. Greiner (MPE Garching)  
report on behalf of the GROND team:

We observed the field of GRB 120927A (Swift trigger 534815; Maselli et  
al., GCN 13822
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et  
al. 2008, PASP 120, 405) mounted at the 2.2m MPG/ESO telescope at La  
Silla Observatory (Chile).

Observations started at  09:05 UT on Sep 28, 10.4 hrs after the GRB  
trigger, as soon the target became visible from La Silla. They were  
performed at an average seeing of 2.6 arcsec and at an average airmass  
of 2.3. It was only possible to observe for a total exposure time of  
7.2 min in the g'r'i'z' bands during morning twilight, whereas in the  
JHK bands we were able to observe for a total of 32 min.

Using our full data set, we do not detect any source within the  
refined XRT error circle (Beardmore et al., GCN 13824
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 ) in our stacked  
GROND images down to the following 3 sigma limiting magnitudes (AB)  
mags:

g' > 22.4
r' > 22.5
i' > 21.9
z' > 21.9
J  > 21.3
H  > 21.0
K  > 19.4

The g'r'i'z and JHK bands are calibrated against SDSS and 2MASS,  
respectively, and the above limits are not corrected for the Galactic  
foreground reddening of E_(B-V)= 0.04 mag in the direction of the  
burst (Schlegel et al. 1998).

We note the presence of a source south of the enhanced XRT position,  
which is already present in DSS2 images.

M.C. Stroh (PSU), D.N. Burrows (PSU), P.A. Evans (U. Leicester), O.M.
Littlejohns (U. Leicester), J.P. Osborne (U. Leicester), G. Stratta
(ASDC), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), J.A. Kennea (PSU) and
A. Maselli report on behalf of the Swift-XRT team:

We have analysed 7.9 ks of XRT data for GRB 120927A (Maselli  et al.
GCN Circ. 13822
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 ), from 138 s to 29.1 ks after the  BAT trigger. The
data comprise 8 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. The enhanced
XRT position for this burst was given by Beardmore et al. (GCN. Circ
13824).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.94 (+0.04, -0.06), followed by a break at T+11.5 ks to
an alpha of 2.6 (+1.7, -0.7).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.75 (+0.18, -0.17). The
best-fitting absorption column is  2.8 (+/-0.7) x 10^21 cm^-2, in
excess of the Galactic value of 3.2 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.7 x 10^-11 (6.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.8 (+/-0.7) x 10^21 cm^-2
Galactic foreground: 3.2 x 10^20 cm^-2
Excess significance: 6.2 sigma
Photon index:	     1.75 (+0.18, -0.17)

If the light curve continues to decay with a power-law decay index of
2.6, the count rate at T+24 hours will be 2.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.8 x
10^-14 (1.3 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00534815.

This circular is an official product of the Swift-XRT team.

A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 4566 s of XRT Photon Counting mode data and 10 UVOT
images for GRB 120927A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 136.61376, +0.41620 which is equivalent
to:

RA (J2000): 09h 06m 27.30s
Dec (J2000): +00d 24' 58.3"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

A. Maselli (INAF-IASFPA), W. H. Baumgartner (GSFC/UMBC),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC),
S. T. Holland (STScI), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
V. Mangano (INAF-IASFPA), C. B. Markwardt (NASA/GSFC) and
F. E. Marshall (NASA/GSFC) report on behalf of the Swift Team:

At 22:40:46 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120927A (trigger=534815).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 136.600, +0.417 which is 
   RA(J2000) = 09h 06m 24s
   Dec(J2000) = +00d 25' 02"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a broad single-peaked
structure with a duration of about 50 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 22:43:20.0 UT, 154 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 136.614125, 0.415722 which is equivalent
to:
   RA(J2000)  = 09h 06m 27.20s
   Dec(J2000) = +00d 24' 56.6"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 52 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 158 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
BAT and XRT error circles. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.04. 

Burst Advocate for this burst is A. Maselli (maselli AT ifc.inaf.it). 
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.)