GRB 151120A

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

a gamma ray burst lasting about 50 s has been detected by IBAS in the 
IBIS/ISGRI data at 8:22:50 UT of 2015 November 20.

The refined coordinates (J2000) are:

R.A.=  157.2502 deg
DEC.=  -32.5237 deg

with an uncertainty of   1.5   arcmin (90% c.l.).

The burst saturated the available ISGRI telemetry, therefore we can only 
provide a lower limit for the fluence in the 20-200 keV energy range of 
2e-6 erg/cmq.

A plot of the light curve will  be posted at
http://ibas.iasf-milano.inaf.it/IBAS_Results.html

T. Kruehler, J. Greiner, J. Ridl (all MPE) and A. Hempel (Universidad 
Andres Bello) report on behalf of a larger collaboration:

We observed the field of GRB 151120A (INTEGRAL trigger 7295; Mereghetti
et al., GCN #18623
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 ) simultaneously in grizJHK with GROND (Greiner et
al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La
Silla Observatory (Chile).

Observations started at November 20, 2015, 08:27 UTC, 4 minutes after
the GRB trigger. They were performed at an average seeing of 1.3" and
at an average airmass of 1.2.

Inside the INTEGRAL error circle (Mereghetti et al., GCN #18623
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 ), we 
detect an uncatalogued and variable source, which we consider the 
optical/NIR afterglow of GRB 151120A. The object is at:

RA (J2000): 10:28:57.94
Dec (J2000): -32:31:35.1

with an estimated uncertainty of 0.5" in each coordinate.

Based on 4.4 min of total exposures in griz and 4 min in JHK obtained
at a midtime of 15 minutes after the trigger, we derive preliminary
magnitudes of:

g = 20.0 +- 0.1
r = 19.4 +- 0.1
i = 19.0 +- 0.1
z = 18.8 +- 0.1
J = 18.2 +- 0.1
H = 18.0 +- 0.2
K = 18.0 +- 0.3

These measurements are in the AB system and calibrated against GROND
zero-points in griz as well as 2MASS field stars in JHK. They are not
corrected for the expected Galactic foreground extinction corresponding
to a reddening of E_(B-V)=0.07 mag in the direction of the burst
(Schlafly & Finkbeiner et al. 2011).

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the INTEGRAL GRB 151120A. 
Automated analysis of the XRT data will be presented online at
http://www.swift.ac.uk/ToO_GRBs/00020559

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the INTEGRAL event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a 
GCN Circular after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

A. von Kienlin (MPE)reports on behalf of the Fermi GBM Team:

"At 08:22:53.70 UT on 20 November 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 151120A (trigger 469700577 / 151120349),
which was also detected by the INTEGRAL/ISGRI (Mereghetti et al. 2015, GCN
18623
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 )
The GBM on-ground location is consistent with the INTEGRAL position.

The angle from the Fermi LAT boresight at the GBM trigger time is 65.6
degrees.

The GBM light curve consists of multiple peaks 
with a duration (T90) of about 20 s (50-300 keV).
The time-averaged spectrum from T0+0.003 s to T0+21.760 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.29 +/- 0.03 
and the cutoff energy, parameterized as Epeak, is 243 (+23/-19)keV.

The event fluence (10-1000 keV) in this time interval is
(9.4 +/- 0.3)E-06 erg/cm^2. The 1.024-sec peak photon flux measured
starting from T0+11.9682 s in the 10-1000 keV band
is 7.0 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well with 
Epeak = 217 (+27/-25) keV, alpha = -1.26 +/- 0.04  
and beta = -2.30 (+0.20/-0.40). 

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

A.P. Beardmore (U. Leicester), A. Maselli  (INAF-IASFPA), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N.
Burrows (PSU), T.G.R. Roegiers (PSU), S.L. Gibson (U. Leicester) and
P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the INTEGRAL-detected
burst GRB 151120A (Mereghetti et al. GCN Circ. 18623
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 ), collecting 3.9
ks of Photon Counting (PC) mode data between T0+16.4 ks and T0+23.6 ks.


An uncatalogued X-ray source is detected inside or close to the
INTEGRAL error region and is above the RASS limit, and is therefore
likely the GRB afterglow.  The position of this source is RA,
Dec=157.2410, -32.5262 which is equivalent to:

RA (J2000): 10:28:57.83
Dec(J2000): -32:31:34.2

with an uncertainty of 3.6 arcsec (radius, 90% confidence).  This
position is 29 arcsec from the INTEGRAL position and 1.6 arcsec from
the afterglow detected by GROND (GCN 18624
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 ). 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.3 (+1.0, -0.9).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.00 (+0.31, -0.28). The
best-fitting absorption column is  2.7 (+1.3, -1.1) x 10^21 cm^-2, in
excess of the Galactic value of 9.0 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.7 x 10^-11 (5.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.7 (+1.3, -1.1) x 10^21 cm^-2
Galactic foreground: 9.0 x 10^20 cm^-2
Excess significance: 2.8 sigma
Photon index:	     2.00 (+0.31, -0.28)

If the light curve continues to decay with a power-law decay index of
2.3, 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 7.5 x
10^-14 (1.1 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020559/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020559.

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

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

The long-duration GRB 151120A
(INTEGRAL ISGRI detection: S. Mereghetti et al., GCN Circ. 18623
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 ;
Fermi GBM detection: A. von Kienlin, GCN Circ. 18628
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 )
triggered Konus-Wind at T0=30171.357 s UT (08:22:51.357).

The burst light curve shows a multipeaked structure
started at ~T0-1.3 s with a total duration of ~20 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB151120_T30171/

As observed by Konus-Wind, the burst
had a fluence of 7.20(-1.25,+1.79)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0+0.752 s,
of 2.09(-0.88,+1.15)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+24.832 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.26 (-0.48,+0.77),
the high energy photon index beta = -2.70 (-0.88,+0.35),
the peak energy 106 (-19,+20) keV
(chi2 = 84/85 dof)

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = 0.81 (-0.75,+3.91),
the high energy photon index beta = -2.42 (-0.38,+0.35),
the peak energy 113 (-40,+22) keV
(chi2 = 81/82 dof)

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