[vsnet-grb-info 18341] INTEGRAL/SPI-ACS and IBIS_VETO search for prompt gamma-ray counterpart of IceCube-160806A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19789
SUBJECT: INTEGRAL/SPI-ACS and IBIS_VETO search for prompt gamma-ray counterpart of IceCube-160806A
DATE:    16/08/10 16:40:10 GMT
FROM:    Volodymyr Savchenko at ISDC,U of Geneve  <savchenk at in2p3.fr>

V. Savchenko (APC, Paris, France) , C. Ferrigno (ISDC, University of 
Geneva, CH), P. Ubertini,  A. Bazzano, L. Natalucci (IAPS-Roma, Italy), 
S. Mereghetti (IASF-Milano, Italy),  P. Laurent (CEA, Saclay, France),  
E. Kuulkers (ESAC/ESA, Madrid, Spain)

Using INTEGRAL/SPI-ACS and INTEGRAL/IBIS VETO we have performed a search 
for prompt gamma-ray counterpart of cosmic neutrino candidate 
IceCube-160806A (Cowen 2016, GCN 19787).

At the time of the event (2016-08-06T12:21:33 UTC, hereafter T0), 
INTEGRAL IBIS was operating in the nominal mode. INTEGRAL/SPI was 
finishing the annealing cycle: at this stage of the procedure it has no 
effect on SPI-ACS.
The spacecraft was pointing in the direction of Norma Arm Region, and 
the neutrino localization was at 110 deg angle with respect to the 
pointing axis.
This orientation enables high response of both SPI-ACS and IBIS_VETO.

The nearest confirmed GRB (GRB160806A), was triggered by 
INTEGRAL/SPI-ACS at 2016-08-06 14:00:57, 1h 40m after the event. This 
GRB is also detected in the IBIS_VETO light curve.

SPI-ACS and  IBIS_VETO (bottom) light-curves around the time of the 
neutrino candidate event can be found at:

http://isdc.unige.ch/~savchenk/IceCube-160806A/

The count rate data around the event reveals variability, including 
smooth background instabilities as well as burst-like events.

Several burst-like excesses can be seen, most prominent ones at T0+50s, 
T-150s, T0-2000s, and T0-10000s.
Ratio of SPI-ACS to IBIS_VETO rates for these events is compatible with 
a gamma-ray source from the direction of IceCube-160806A.
However, we note that these flares are coincident with GOES proton 
background flares.

Although we can not exclude cosmic origin of some of these bursts, 
judging from long-lasting background instability near the region of 
interest, as well as the activity observed by GOES, we are inclined to 
conclude that these bursts are associated with background variability 
introduced by solar activity.

We estimate a 3-sigma upper limit on fluence in 75-2000 keV energy range 
in the interval ±1000 s around the IceCube trigger of 3.3x10^-7 erg/cm^2 
for a burst with duration less than 1 s with a characteristic short GRB 
spectrum (an exponentially cut off power law with alpha=-0.5 and Ep=500 
keV).
For a typical long GRB spectrum (the Band function with alpha=-1, 
beta=-2.5, and Ep=250 keV), the corresponding limiting peak flux is 
~2.2x10^-7 erg/cm^2/s at 1 s time scale in 75-2000 keV energy range.

The SPI-ACS light curves, binned at 50 ms, are derived from 91 
independent detectors with different lower energy thresholds (mainly 
between 50 keV and 150 keV) and an upper threshold at about 100 MeV.

The IBIS_VETO light curves, binned at 8 s, are derived from 16 
independent detectors with lower energy thresholds near 80 keV and an 
upper threshold at about 50 MeV. Two groups of detector rates are 
recorded independently: bottom and lateral IBIS VETO. The response of 
the bottom IBIS VETO peaks in the direction opposite to satellite 
pointing axis, where the response of SPI-ACS is suppressed.

Due to solar angle constrains, INTEGRAL can not perform pointed 
observation of the location of IceCube-160806A until September 22, so no 
pointed follow-up will be scheduled.