GRB 081127

J. Mao (INAF-OAB), W. H. Baumgartner (GSFC/UMBC),
P. A. Evans (U Leicester), C. Guidorzi (INAF-OAB),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
R. Margutti (Univ Bicocca&OAB), C. Pagani (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL),
T. Sakamoto (NASA/UMBC) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 07:05:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081127 (trigger=335715).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 332.093, +6.859 which is 
   RA(J2000) = 22h 08m 22s
   Dec(J2000) = +06d 51' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peak of 2
seconds on a broader plateau with a total duration of about 30 sec. 
The peak count rate was ~900 counts/sec (15-350 keV), at ~7 sec 
after the trigger. 

The XRT began observing the field at 07:07:10.8 UT, 121.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
332.0646, 6.8504 which is equivalent to:
   RA(J2000)  = 22h 08m 15.49s
   Dec(J2000) = +06d 51' 01.6"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 106 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
6.86e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 9.56e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 130 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
XRT error circle. 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
0.09.  The XRT position is 9.8 arcseconds from the B = 13 USNO-B1.0 star 0968-0627697. 

Burst Advocate for this burst is J. Mao (jirong.mao AT brera.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.)

M. Stamatikos (GSFC/ORAU), 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), J. Mao (INAF-OAB), C. B. Markwardt (GSFC/UMD),
D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-119 to T+183 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 081127 (trigger #335715)
(Mao, et al., GCN Circ. 8560
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 ).  The BAT ground-calculated position is
RA, Dec = 332.075, 6.858 deg, which is 
   RA(J2000)  =  22h 08m 18.0s 
   Dec(J2000) = +06d 51' 30.2" 
with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 51%.
 
The mask-weighted light curve shows a single peak starting at ~T-30 sec,
peaking around T+0 sec, and ending at ~T+55 sec.
T90 (15-350 keV) is 37 +- 10 sec (estimated error including systematics).
 
The time-averaged spectrum from T-31.2 to T+8.8 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.10 +- 0.35.  The fluence in the 15-150 keV band is 4.9 +- 1.0 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-1.19 sec in the 15-150 keV band
is 0.6 +- 0.2 ph/cm2/sec.  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/335715/BA/

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

Using 3559 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 081127, 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 = 332.0642, +6.8505 which is equivalent
to:

RA (J2000): 22h 08m 15.40s
Dec (J2000): +06d 51' 01.7"

with an uncertainty of 1.7 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
http://www.swift.ac.uk/xrt_positions/Goad.pdf), the current algorithm is an
extension of this method.

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

J. Mao, C. Guidorzi (INAF-OAB) and R. Margutti (Univ. Bicocca & 
INAF-OAB) report on behalf of the Swift XRT team:

We have analysed the first 4 orbits of Swift-XRT data obtained for GRB 
081127 (Mao et al., GCN Circ. 8560
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 ), covering from 128 to 219 s in 
Windowed Timing (WT) and from 220 s to 19.5 ks in Photon Counting (PC) 
mode data, respectively. The net exposures are 90 s and 9.4 ks for the 
WT and PC modes data, respectively.  The UVOT-enhanced XRT position was 
given by Beardmore et al. in GCN Circ. 8565
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 .

The light curve shows an inital steep decay of index 5.6 until 420 s, 
then it shows a shallow decay with index of 0.6. After 13.3 ks the 
lightcurve shows a decay with an index of about 2.0. The WT spectrum can 
be fit with an absorbed power-law model with a photon index of 4.2 +- 
0.2 and an absorbing
equivalent hydrogen column density is (2.8 +- 0.3)e21 cm^-2, which is 
larger than the Galactic one, 6.7e20 cm^-2 (Kalberla et al.  2005). The 
observed (unabsorbed) 0.3-10 keV flux is 3.7(4.5)e-10 erg cm^-2  s^-1. 
The PC spectrum can be fit with an absorbed power-law model with a 
photon index of 2.3 +- 0.3 with an absorbing equivalent hydrogen column 
density compatible with the Galactic value.

If the light curve continues to decay with an index of 2.0, the count 
rate 24 hours after the burst will be 2.9e-4 c/s, corresponding to an 
absorbed flux of 9.9e-15 erg cm^-2 s^-1.

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

S. T. Holland (CRESST/GSFC/USRA) and J. Mao (INAF-OAB) report on the
behalf of the Swift UVOT team:

       The Swift/UVOT observed the field of GRB 081127 starting 113 s
after the BAT trigger (Mao, et al. 2008, GCN Circ. 8560
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 ).  Settled
exposures started at T+131 s.  We do not find any new source, relative
to the DSS or 2MASS, or any variable source inside the UVOT-enhanced
XRT error circle (Beardmore, et al. 2008, GCN Circ. 8565
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 ).
Preliminary 3-sigma upper limits for detecting a source in the white
finding chart, and the co-added images, using a 2.5 arcsecond radius
circular aperture, are

Filter   T_start   T_stop   Exp(s)     Mag
------------------------------------------
  white       130      380      147   >21.3

     v        365   18,799     1473   >21.0
     b        466     7263      568   >21.4
     u        440   13,713     1246   >21.4
   uvw1       415   13,011     1276   >21.6
   uvm2       389   19,496     2004   >21.8
   uvw2       341   17,885     1473   >21.8
  white       490     7468      714   >22.2
------------------------------------------

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight corresponding to a
reddening of E_{B-V} = 0.09 mag (Schlegel, et al., 1998, ApJS, 500,
525).  All photometry is on the UVOT photometry system described in
Poole et al. (2008, MNRAS, 383, 627).

      The B = 13.15 mag USNO-B1.0 star 0968-0627697 is located
approximately 4 arcsec north of the XRT position.  UVOT photometry at
the location of the X-ray source is contaminated by this star.