GRB 110818A

C. B. Markwardt (NASA/GSFC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
N. P. M. Kuin (UCL-MSSL), O. M. Littlejohns (U Leicester),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
T. A. Pritchard (PSU), A. Rowlinson (U Leicester),
T. Sakamoto (NASA/UMBC), C. J. Saxton (UCL-MSSL), M. H. Siegel (PSU)
and R. L. C. Starling (U Leicester) report on behalf of the Swift
Team:

At 20:37:49 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110818A (trigger=500914).  Because of an Earth-limb
observing constraint, the slew to the burst was delayed by ~4 min. 
The BAT on-board calculated location is 
RA, Dec 317.384, -63.970 which is 
   RA(J2000)  =  21h 09m 32s
   Dec(J2000) = -63d 58' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several peaks
with a total duration of about 60 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 20:44:09.7 UT, 380.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 317.3382,
-63.9814 which is equivalent to:
   RA(J2000)  = 21h 09m 21.17s
   Dec(J2000) = -63d 58' 53.1"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 83 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 (2.62 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2
(+1.14/-1.05) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the U filter starting
856 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 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt AT 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.)

C. B. Markwardt (GSFC), 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),
D. M. Palmer (LANL), 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 110818A (trigger 
#500914) (Markwardt, et al., GCN Circ. 12279
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 ).  The BAT 
ground-calculated position is RA, Dec = 317.373, -63.981 deg which is
    RA(J2000)  =  21h 09m 29.6s
    Dec(J2000) = -63d 58' 52.7"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 29%.

The mask-weighted light curve consists of a single broad peak, with 
several sharper peaks superimposed.   T90 (15-350 keV) is 103 +- 18 sec 
(estimated error including systematics).

The time-averaged spectrum from T-14.2 to T+117.4 sec is best fit by a 
simple power-law model.  The power law index of the time-averaged 
spectrum is 1.58 +- 0.11.  The fluence in the 15-150 keV band is 4.0 +- 
0.2 x 10^-6 erg/cm2.  The 1-sec peak photon flux measured from T+27.3 
sec in the 15-150 keV band is 1.6 +- 0.3 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/500914/BA/

A. Rowlinson (U. Leicester) and C. B. Markwardt (NASA/GSFC) report on
behalf of the Swift-XRT team:

We have analysed 7.8 ks of XRT data for GRB 110818A (Markwardt	et al.
GCN Circ. 12279
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 ), from 384 s to 18.3 ks after the  BAT trigger. The
data comprise 61 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The best available XRT position  (using the
promptly downlinked event data, the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 317.3378,
-63.9813 which is equivalent to:

RA (J2000): 21 09 21.07
Dec(J2000): -63 58 52.7

with an uncertainty of 2.0 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=5.8 (+0.6, -0.7). At T+461 s  the decay
flattens to an alpha of 1.57 (+0.14, -0.11) before breaking again at
T+2167 s to a final decay with index alpha=0.95 (+0.10, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+0.12, -0.11). The
best-fitting absorption column is  7.8 (+2.7, -2.4) x 10^20 cm^-2, in
excess of the Galactic value of 2.6 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 3.6 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.8 (+2.7, -2.4) x 10^20 cm^-2
Galactic foreground: 2.6 x 10^20 cm^-2
Excess significance: 5.3 sigma
Photon index:	     1.97 (+0.12, -0.11)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00500914.

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

P. D'Avanzo (INAF/Brera), M. Sparre, D. Watson, J. P. U. Fynbo, D. 
Malesani, B. Milvang-Jensen (DARK/NBI), P. Goldoni (APC/Univ. Paris 7 
and SAp/CEA), V. D'Elia (INAF/Rome), and N. R. Tanvir (U. Leicester), 
report on behalf of the X-shooter GTO GRB collaboration:

We observed the field of GRB 110818A (Markwardt et al. GCN 12279
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 ) with 
the ESO VLT equipped with X-shooter. Observations started on 2011-08-19 
at 02:47 UT (6.15 hr after the GRB).

The acquisition image shows an optical afterglow candidate inside the 
XRT error circle (Markwardt et al. GCN 12279
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 ) at the following 
coordinates (J2000):

RA = 21:09:21.04
Dec = -63:58:52.3

with an uncertainty of about 0.5". The object has a magnitude R ~ 22.3, 
calibrated assuming R = 19.2 for the USNO-B1 star 0260-0726657 (RA, Dec 
= 21:09:23.14, -63:58:33.0).

A total spectroscopic exposure of 4x1200 s was obtained, covering the 
spectral range from 300 to 2500 nm.

We report a redshift of z = 3.36 based on detection of absorption 
features from Si II, C IV, Al II, Ca H, Ca K and Mg I. Emission from the 
[O III] doublet is also observed.

We would like to thank the staff at the VLT, in particular, Alain 
Smette, Patricia Guajardo and Dimitri Gadotti for carrying out the 
observations.

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

Using 4176 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 110818A, 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 = 317.33705, -63.98105 which is equivalent
to:

RA (J2000): 21h 09m 20.89s
Dec (J2000): -63d 58' 51.8"

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.

T. A. Pritchard (PSU) and C. B. Markwardt (NASA/GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 110818A
379 s after the BAT trigger (Markwardt et al., GCN Circ. 12279
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 ).
No optical afterglow consistent with the Enhanced Swift-XRT position
(GCN Circ. 12285
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 ) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Poole et al. 2008, MNRAS, 383, 627) for the initial exposures are:

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

v                  379        14423         1513         >20.8
b                  477        18642         2455         >22.2
u                  452         8009          813         >20.4
w1                 428         7804          646         >20.9
m2                7399         7599          197         >20.9
w2                6989        13752         1082         >21.2

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).

S. Xiong (UAH) reports on behalf of the Fermi GBM Team:

"At 20:37:54.221 UT on 18 August 2011, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 110818A (trigger 335392676 / 110818860 ),
which was also detected by the Swift (Markwardt et al. 2011, GCN 12279
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 ).
The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight is 96 degrees.
This burst was also independently detected by INTEGRAL SPI-ACS.

The GBM light curve consists of multiple peaks
with a duration (T90) of about 75 s (50-300 keV).
The time-averaged spectrum from T0-16.4 s to T0+57.3 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.33 +/- 0.08 and
the cutoff energy, parameterized as Epeak, is 256.3 +/- 55.3 keV.

The event fluence (10-1000 keV) in this time interval is
(8.2 +/- 0.6)E-6 erg/cm^2. The 64 ms peak photon flux measured
starting from T0+22.66 s in the 10-1000 keV band
is 5.0 +/- 1.4 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."