GCN Circular 18453.5
Subject
LIGO/Virgo G194575: INTEGRAL search of temporally coincident prompt hard X-ray emission
Date
2015-10-23T18:25:44Z (9 years ago)
From
Carlo Ferrigno at ISDC/INTEGRAL <carlo.ferrigno@unige.ch>
V. Savchenko (APC, Paris, France), S. Mereghetti (IASF-Mi, Italy), C.
Ferrigno, E. Bozzo, T. J.-L. Courvoisier (ISDC, University of Geneva,
CH), E. Kuulkers (ESAC/ESA, Madrid, Spain), A. Bazzano (IAPS-Roma,
Italy)
The anti-coincidence shield of the spectrometer on board of INTEGRAL
(SPI/ACS) is sensitive to photons above ~50 keV, but without directional
information.
We investigated the SPI/ACS light curve at -1000 +1000 s from the trigger time
(2015-10-22 13:33:19 UTC) on temporal scales from 0.1 to 100 s. The
nearest excess on the constant background is at 2015-10-22 13:36:54, i.e.
215 seconds after the LIGO/Virgo trigger. The excess has a duration of
less than 0.05 seconds and a post-trial significance of 5.1 sigma. This
event is compatible with particle effects in SPI-ACS. We estimate the
chance of a particle effect to happen in the 200 seconds following the
trigger to be 20%. However, with SPI-ACS data alone we can not exclude
a cosmic origin of this excess.
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
ACS response varies as a function of the incident angle. The highest
probability sky location of the LIGO trigger is in the direction of the
satellite Z-axis (the direction pointing to the Sun, where the SPI/ACS
has reduced sensitivity). For this direction we estimate a 3-sigma
upper limits corresponding to fluences in the 75-1000 keV band of 4e-6 erg/cm2
for a 100 s duration, 1.8e-6 erg/cm2 for 10 s, 5.8e-7 erg/cm2 for 1 s,
and 2.1e-7 erg/cm2 for 0.1 s. The upper limits assume a typical GRB
spectrum with Band model parameters alpha=1, beta=2.5 and E0 ~ 500 keV.
For the other probability region, the estimated upper limits are roughly
a factor three tighter. Given the same assumptions, if the excess at
2015-10-22 13:36:54 were of cosmic origin, it would have a fluence of 3.2e-7
erg/cm2 in the 75-1000 keV energy range.
We also searched for transient events in time coincidence with the LIGO
trigger in the data of the coded-mask imager IBIS, which at the time of
the trigger was pointed at R.A.=297.1 deg, Dec.=+29.3 deg (i.e., more
than 90 degrees from both LIGO highest probability regions). No
significant events were found on timescales from 2 ms to 100 s.
Typically, the minimum peak flux on 1 s time scale of gamma-ray
bursts detected with IBIS is 0.1 ph/s/cm2 in the 20-200 keV energy band,
in the central 9x9 deg of the field of view, which has zero response at 29x29 degrees.
However, due to the presence of the bright and variable source Cyg X-1 in the
field of view, we estimate that the sensitivity in the current
observation might be at least a factor 3 worse.
[GCN OPS NOTE(26oct15): Because of a processing error, two circulars
received the same ID Number. The second was given an "a' suffix.]