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
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:
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
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
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.