GRB 110808A

G. Stratta (ASDC), W. H. Baumgartner (GSFC/UMBC), M. M. Chester (PSU),
V. D'Elia (ASDC), N. Gehrels (NASA/GSFC), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
A. Melandri (INAF-OAB), K. L. Page (U Leicester) and
C. J. Saxton (UCL-MSSL) report on behalf of the Swift Team:

At 06:18:54 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110808A (trigger=458918).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 57.264, -44.179 which is 
   RA(J2000) = 03h 49m 03s
   Dec(J2000) = -44d 10' 42"
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 10 sec.  The peak count rate
was ~400 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 06:20:45.1 UT, 110.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
57.2666, -44.1944 which is equivalent to:
   RA(J2000)  = 03h 49m 3.97s
   Dec(J2000) = -44d 11' 40.0"
with an uncertainty of 2.6 arcseconds (radius, 90% containment). This
location is 55 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 in excess of the Galactic value (1.04 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 3
(+3.41/-2.82) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.42e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 120 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	03:49:04.28 =  57.26782
  DEC(J2000) = -44:11:40.3  = -44.19452
with a 90%-confidence error radius of about 0.83 arc sec. This position is 5.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.93 with a 1-sigma error of about  0.19. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is G. Stratta (giulia.stratta AT asdc.asi.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.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 304 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 110808A, 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 = 57.26664, -44.19536 which is equivalent
to:

RA (J2000): 03h 49m 3.99s
Dec (J2000): -44d 11' 43.3"

with an uncertainty of 3.2 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. de Ugarte Postigo, J. P. U. Fynbo (DARK/NBI), P. Jakobsson (U.
Iceland), B. Milvang-Jensen (ESO, DARK/NBI), A. Melandri (INAF-OABr), D.
Malesani (DARK/NBI), S. Schulze (U. Iceland), P. Goldoni (APC/Univ. Paris
7 and SAp/CEA), N. R. Tanvir (U. Leicester), H. Flores (Paris Obs.), K.
Wiersema (U. Leicester), S. Covino (INAF-OABr), L. Lovisi, B. Lanzoni (U.
Bologna) report on behalf of the X-shooter GRB collaboration:

We observed the afterglow of GRB 110808A (Stratta et al. GCN 12256
Loading...
 
 ) with
the ESO VLT equipped with the X-shooter spectrograph on 8 August 2011,
beginning 09:19 UT (3 hr after the GRB). The acquisition image shows the
UVOT optical afterglow candidate to have R ~ 21.2 mag.

We detect several emission lines, including [O II], [O III] and H-alpha at
a  common redshift of z = 1.348 (based on archival calibration frames). At
the same redshift we detect the Mg II doublet and Fe transitions in
absorption.

We acknowledge the support of the VLT staff, in particular Claudio Melo.

G. Stratta and V. D'Elia (ASDC) report on behalf of the Swift-XRT team:


We have analysed 3.8 ks of XRT data for GRB 110808A (Stratta  et al.
GCN Circ. 12256
Loading...
 
 ), from 100 s to 18.0 ks after the  BAT trigger. The
data comprise 91 s in Windowed Timing (WT) mode (the first 9 s were
taken while Swift was slewing) with the remainder in Photon Counting
(PC) mode. The enhanced XRT position for this burst was given by Goad
et al. (GCN. Circ 12257).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.79 (+0.22, -0.20), followed by a break at T+459 s to
an alpha of 0.57 (+0.18, -0.23).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.07 (+0.29, -0.25). The
best-fitting absorption column is  2.9 (+1.4, -1.3) x 10^21 cm^-2, at a
redshift of 1.348 (de Ugarte Postigo et al. GCN Circ. 12258
Loading...
 
 ), in
addition to the Galactic value of 1.0 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 2.2 x 10^-11 (3.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Galactic foreground: 1.0 x 10^20 cm^-2
Intrinsic column:    2.9 (+1.4, -1.3) x 10^21 cm^-2 at z=1.348
Photon index:	     3.07 (+0.29, -0.25)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00458918.

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

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), D. M. Palmer (LANL),
G. Sato (ISAS), M. Stamatikos (OSU), G. Stratta (ASDC),
J. Tueller (GSFC), T. N. Ukwatta (MSU)
(i.e. the Swift-BAT team):
 
Using the data set from T-471 to T+639 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 110808A (trigger #458918)
(Stratta, et al., GCN Circ. 12256
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 57.322, -44.177 deg which is 
   RA(J2000)  =  03h 49m 17.2s 
   Dec(J2000) = -44d 10' 35.4" 
with an uncertainty of 2.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 76%.
 
The mask-weighted light curve shows a FRED pulse shape starting at ~T+0 sec,
peaking at ~T+1 sec, and returning the baseline at ~T+50 sec.
T90 (15-350 keV) is 48 +- 23 sec (estimated error including systematics).
 
The time-averaged spectrum from T-7.4 to T+40.6 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.32 +- 0.43.  The fluence in the 15-150 keV band is 3.3 +- 0.8 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.06 sec in the 15-150 keV band
is 0.4 +- 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/458918/BA/

T. A. Pritchard (PSU) and G. Stratta (ASDC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 110808A
120 s after the BAT trigger (Stratta et al., GCN Circ. 12256
Loading...
 
 ).
A source consistent with the Enhanced Swift-XRT position
(Goad et al., GCN Circ. 12257
Loading...
 
 ) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
     RA  (J2000) =  03:49:04.27 =  57.26780 (deg.)
     Dec (J2000) = -44:11:40.1  = -44.19448 (deg.)
with an estimated uncertainty of 0.64 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT 
photometric system
(Poole et al. 2008, MNRAS, 383, 627) for the early exposures are:

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

white              120          270          147         19.88 � 0.14
white              57           559           19         18.48 � 0.23
white              3996         4196         200         20.17 � 0.16
v                  609          629           19 >18.7
b                  535          555           20         18.64 � 0.32
u                  279          528          246         18.28 � 0.09
u                  683          703           19         17.48 � 0.25
u                  6062         6262         196 >20.62
w1                 658          678           19 >17.8
w2                 585          605           19 >18.3

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).

S. Golenetskii, R.Aptekar, D. Frederiks, E. Mazets, V. Pal'shin,
P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf
of the Konus-Wind team, report:

The short-duration extremely hard GRB 110808A
(localized by IPN: Hurley et al., GCN 12269
Loading...
 
 )
triggered Konus-Wind at T0=56695.237s UT (15:44:55.237)

The burst light curve shows a single bright hard pulse,
with a duration of ~0.5 s and much weaker tail of
the emission which is detectable till T0+8 s.
The emission is seen up to 13 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB110808_T56695/

As observed by Konus-Wind the burst
had a fluence of (1.6 � 0.2)x10-5 erg/cm2,
and a 16-ms peak flux, measured from T0+0.112 s,
of (1.1 � 0.2)x10-4 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The spectrum of the burst (measured from T0 to T0+0.256 s)
is best fitted in the 20 keV - 10 MeV range
by a power law with exponential cutoff model, for which
alpha = -1.13 (-0.08, +0.08),
and Ep = 2960(-796, +1217) keV,
chi2 = 53.2/45 dof.
Fitting this spectrum by the GRB (Band) function yields
the following model parameters:
the low-energy photon index alpha = -1.13 (-0.08, +0.07),
the high energy photon index beta = -2.5 (<-1.9),
the peak energy Ep = 2630(-744, +836) keV,
chi2 = 54.3/44 dof.

The spectrum at the maximum count rate
(measured from T0+0.064 to T0+0.128 s)
is best fitted in the 20 keV - 10 MeV range
by a power law with exponential cutoff model, for which
alpha = -1.07 (-0.11, +0.12),
and Ep = 4238(-1530, +3270) keV,
chi2 = 12.5/16 dof.

All the quoted results are preliminary.
All the quoted errors are at the 90% confidence level.