TITLE:   GCN CIRCULAR
NUMBER:  20496
SUBJECT: LIGO/Virgo G270580: INTEGRAL search of temporally coincident prompt hard X-ray 
DATE:    17/01/21 16:03:28 GMT
FROM:    Carlo Ferrigno at ISDC/INTEGRAL  <carlo.ferrigno@unige.ch>

V. Savchenko and C. Ferrigno (ISDC, University of Geneva, CH)
on behalf of the INTEGRAL group: S. Mereghetti (IASF-Mi, Italy), E. Kuulkers (ESTEC/ESA, The Netherlands), A. Bazzano (IAPS-Roma, Italy), E. Bozzo, T. J.-L. Courvoisier (ISDC, University of Geneva, CH) S. Brandt (DTU - Denmark) R. Diehl (MPE-Garching, Germany) L. Hanlon (UCD, Ireland) P. Laurent (APC-CeA, France) A. Lutovinov (IKI, Russia) J.P. Roques (CESR, France) R. Sunyaev (IKI, Russia) P. Ubertini (IAPS-Roma, Italy)

We investigated serendipitous INTEGRAL observations carried out at the time of the LIGO/Virgo burst candidate G270580. The satellite was pointing at RA = 10:41:53 Dec=-56:35:09, away from the high-probability region, derived from the LIGO LALInference pipeline. The anti-coincidence shield of  spectrometer on board of INTEGRAL (SPI/ACS) covered the full LIGO 90% confidence region and provided  stringent constraints on the flux of possible electromagnetic counterparts in the energy range covered by INTEGRAL instruments.

We investigated the SPI-ACS light curve between -500 and +500 s from the trigger time (2017-01-20 12:30:59.35 UTC) on temporal scales from 0.1 to 100 s. We find a low-significance excess at the time of the trigger with a duration about 30s and an SNR of 2.8 sigma. If this excess were a gamma-ray transient, we need to assume a spectral shape  to derive a fluence, since SPI-ACS has no spectral capability. For such a long transient, we use a typical Band model with indexes  −1, −2.5, and E_peak ~ 300 keV: we estimate a fluence  of 7 +/- 3e-7 erg/cm2 in the optimal direction for SPI-ACS sensitivity. The fluence estimate changes widely in the localisation region, reaching 4.7+/-1.9e-6 erg/cm2 in the least favourable direction; detailed maps will be made available in future publications.

In the preferred assumption that the above excess is a background fluctuation, we estimate typical 3-sigma upper limits of  4.4e-7 erg/cm2 for 10s duration assuming Band model parameters −1, −2.5 with E_ peak ~ 300 keV. To derive a limit for a typical short burst with 1 s duration, we use a harder cutoff power law spectrum with slope of −0.5 and Epeak ~ 500 keV: we find a limiting fluence of 1.5e-7 erg/cm2 at 3 sigma c.l.. These limits assume an optimal perpendicular direction of the burst to the INTEGRAL pointing direction. However the extent of the region with optimal response depends on the possible source spectrum: we perform a detailed calculation only for a cuttoff powerlaw spectrum with slope of −0.5 and Epeak ~ 500 keV: we estimate that 25% of the LIGO localisation probability region is covered with a range of sensitivity from optimal to 50% worse , while 10% of the area is covered with a substantially reduced SPI-ACS sensitivity, more than factor 3 lower than optimal.

Due to temporary technical problems at the time of the trigger, only SPI/ACS data could be used to search for gamma-ray counterparts with INTEGRAL. 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 substantially as a function of the source incident angle with an optimal effective area of about 6000 cm2 at 1 MeV.