GRB 110903A

S.Mereghetti (IASF-Milano), D.Gotz (CEA-Saclay), J. Borkowski (CAMK, 
Torun), C.Ferrigno, E.Bozzo (ISDC, Versoix), on behalf of the IBAS 
Localization Team report:

a long gamma ray burst lasting about 400 s has been detected by IBAS in 
the IBIS/ISGRI data at 02:39:55 UT of September 3.

Its refined coordinates (J2000) are:

  RA: 197.0614 [degrees]
  DEC: 58.9852 [degrees]

with an uncertainty of 2 arcmin (90% c.l.).

The light curve consists of three main peaks starting approximately at t0, 
t0+200 s, and t0+300 s. The high count rate of the burst saturated the 
ISGRI telemetry.

A plot of the light curve will be posted at

http://ibas.iasf-milano.inaf.it/IBAS_Results.html

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 very long GRB 110903A
(localized by INTEGRAL: Mereghetti et al., GCN 12322
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 )
triggered Konus-Wind at T0=09573.115s UT (02:39:33.115)

The light curve shows two equally hard peaks started
at ~T0 and at ~T0+190s respectively, and a third,
softer peak, started at ~T0+320s.
A total duration of the burst is ~370 s.
The emission is seen up to ~7 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB110903_T09573/

As observed by Konus-Wind the burst
had a fluence of 4.2(-0.7,+2.2)x10-5 erg/cm2,
and a 256-ms peak flux, measured from T0+196.352 s,
of 1.8(-0.4,+0.6)x10-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+368.896 s)
is best fitted in the 20 keV - 10 MeV range
by a power law with exponential cutoff model, for which
alpha = -0.96 (-0.29, +0.37),
and Ep = 301(-80, +185) keV,
chi2 = 77.8/88 dof.
Fitting this spectrum by the GRB (Band) function yields
the following model parameters:
the low-energy photon index alpha = -0.93 (-0.31, +0.41),
the high energy photon index beta = -2.7 (<-1.9),
the peak energy Ep = 287(-89, +191) keV,
chi2 = 77.6/87 dof.

The spectrum near the maximum count rate
(measured from T0+196.864 to T0+205.056 s)
is best fitted in the 20 keV - 10 MeV range
by the GRB (Band) model, for which:
the low-energy photon index alpha = -0.69 (-0.20, +0.24),
the high energy photon index beta = -2.7 (<-2.2),
the peak energy Ep = 295(-57, +77) keV,
chi2 = 95.3/87 dof.

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

K.L. Page (U. Leicester)  reports on behalf of the Swift-XRT team:

We have analysed 3.9 ks of XRT data for the INTEGRAL-detected burst:
GRB 110903A (Mereghetti et al., GCN Circ. 12322
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 ), from 33.8 ks to 45.3
ks after the INTEGRAL trigger. This GRB was also detected by Konus-Wind
(Golenetskii et al., GCN Circ. 12323
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 ). The data are entirely in Photon
Counting (PC) mode.  An X-ray source is detected within the INTEGRAL
error circle. Using 2176 s of PC mode data and 2 UVOT images, we find
an enhanced XRT position (using the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue): RA, Dec = 197.06570,
+58.98150 which is equivalent to:

RA (J2000): 13h 08m 15.77s
Dec(J2000): +58d 58' 53.5"

with an uncertainty of 1.8 arcsec (radius, 90% confidence). This
position is 15.5 arcsec from the INTEGRAL position.  The light curve is
consistent with a constant source of mean count rate 6.1e-02 ct/sec. A
power-law fit gives an index of 0.9 (+/-1.9).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.4, -0.5). The
best-fitting absorption column is  3.7 (+2.2, -1.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.3 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 4.8 x 10^-11 (7.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.7 (+2.2, -1.5) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^20 cm^-2
Excess significance: 4.0 sigma
Photon index:	     1.8 (+0.4, -0.5)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020184.

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

E. A. Hoversten (PSU) and K.L. Page (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 110903A
33.8 ks after the INTEGRAL trigger (Mereghetti et al., GCN Circ. 12322
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 ).
No optical afterglow consistent with the enhanced XRT position (Page,
GCN Circ. 12324
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 ) is detected in the initial UVOT exposures.  UVOT
magnitude 3-sigma upper limits are reported in the following table:

Filter  T_start  T_stop  Exp(s)  Mag (3-sigma upper limit)
-------------------------------------------------------------
white    38935   45300    1507        > 21.94
v        39664   40380     705        > 19.86
u        33776   44483    1589        > 20.73

The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight of E_(B-V) = 0.01
(Schlegel et al. 1998). All photometry is on the UVOT photometric
system described in Poole et al. (2008, MNRAS, 383, 627).

S. McBreen (UCD/MPE) and G. Fitzpatrick (UCD)
report on behalf of the Fermi GBM Team:

"At 02:39:34.42 UT on 03 September 2011, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 110903A (trigger 336710376 / 110903111),
which was also detected by  INTEGRAL (Mereghetti et  al. 2011, GCN 12322
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 )
and Konus-Wind (Golenetskii et al. 2011, GCN 12323
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 ).
The GBM on-ground location is consistent with the INTEGRAL position.
The angle from the Fermi LAT boresight is 44 degrees.

The GBM light curve shows/consists of 3 main emission phases
with a duration (T90) of about 339.9 (+/-2.3)s (50-300 keV).
The time-averaged spectrum from T0-0 s to T0+339 s is
well fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.88 +/- 0.04 and
the cutoff energy, parameterized as Epeak, is 281.3 +/- 21.1 keV.

The event fluence (10-1000 keV) in this time interval is
(3.3E-05 +/- 0.1)E-05 erg/cm^2. The 1.024-sec peak photon flux measured
starting from T0+7.7 s in the 10-1000 keV band
is 5.24 +/- 0.22 ph/s/cm^2.

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

Y. Hanabata, T. Uehara, T. Takahashi, M. Mizuno, M. Ohno, Y. Fukazawa
(Hiroshima U.), T. Yasuda, Y. Terada, M. Tashiro, W. Iwakiri,
K. Takahara, M. Asahina, S. Kobayashi, A. Sakamoto (Saitama U.),
S. Sugita (Nagoya U.), K. Yamaoka (Aoyama Gakuin U.), M. Kokubun,
T. Takahashi (ISAS/JAXA), Y. E. Nakagawa (Waseda U.), N. Ohmori,
M. Akiyama, M. Yamauchi (Univ. of Miyazaki), Y. Urata, P. Tsai,
C-J. Chuang (NCU), K. Nakazawa, K. Makishima (Univ. of Tokyo),
on behalf of the Suzaku WAM team, report:

The very long, bright GRB110903A (Mereghetti et al. GCN 12322
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 )
was detected by the Suzaku Wide-band All-sky Monitor (WAM) which
covers an energy range of 50 keV - 5 MeV at 02:39:34.298 UT (=T0).

The observed light curve shows a multi-peaked structure starting at
T0-57 s, ending at T0+364 s with a duration (T90) of about 340 
secnds. There is a possible precursor at T0-57 s with a duration of
about 15 seconds. The fluence in 100 - 1000 keV was 2.18 (-0.47,
+0.22) x 10^-5 erg/cm^2. The 1-s peak flux measured from T0+198 s was
2.45 (-0.20, +0.17) photons/cm^2/s in the same energy range.

Preliminary result shows that the time-averaged spectrum from T0 to
T0+364 s is well fitted by a power-law with exponential cutoff model:
 dN/dE ~  E^{-alpha} * exp(-(2-alpha)*E/Epeak) with
 alpha : 1.19 (-0.44, +0.54), and
 Epeak : 281 (-78, +33) keV (chi^2/d.o.f. = 39.2/31).
All the quoted errors are at statistical 90% confidence level.

The light curves for this burst are available at:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/untrig/grb_table.html