INTEGRAL IBIS, SPI, and JEM-X observations of LVT151012

Abstract

During the first observing run of LIGO, two gravitational wave events and one lower-significance trigger (LVT151012) were reported by the LIGO/Virgo collaboration. At the time of LVT151012, the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) was pointing at a region of the sky coincident with the high localization probability area of the event and thus permitted us to search for its electromagnetic counterpart (both prompt and afterglow emission). The imaging instruments on board INTEGRAL (IBIS/ISGRI, IBIS/PICsIT, SPI, and the two JEM-X modules) have been exploited to attempt the detection of any electromagnetic emission associated with LVT151012 over three decades in energy (from 3 keV to 8 MeV). The omni-directional instruments on board the satellite, i.e., the SPI-ACS and the IBIS/Veto, complemented the capabilities of the IBIS/ISGRI and IBIS/PICsIT for detections outside their imaging field of view in order to provide an efficient monitoring of the entire LVT151012 localization region at energies above 75 keV. We did not find any significant transient source that was spatially and/or temporally coincident with LVT151012, obtaining tight upper limits on the associated hard X-ray and γ-ray radiation. For typical spectral models, the upper limits on the fluence of the emission from any 1 s counterpart of LVT151012 ranges from F γ = 3.5 × 10 −8 erg cm −2 (20-200 keV), within the field of view of the imaging instruments, to F γ = 7.1 × 10 −7 erg cm −2 (75-2000 keV), considering the least favorable location of the counterpart for a detection by the omni-directional instruments. These results can be interpreted as a tight constraint on the ratio of the isotropic equivalent energy released in the electromagnetic emission to the total energy of the gravitational waves: E 75−2000 keV /E GW < 4.4 × 10 −5 . Finally, we provide an exhaustive summary of the capabilities of all instruments on board INTEGRAL to hunt for γ-ray counterparts of gravitational wave events, exploiting both serendipitous and pointed follow-up observations. This will serve as a reference for all future searches.

INTEGRAL Observations of GW170104

“…This allows us to generate an instrumental response function for any position in the sky, which can then be used to compute the expected number of counts for a given source spectral energy distribution. As shown in our previous paper (Savchenko et al 2017), this response produces results for the bursts detected simultaneously by the SPI-ACS and other detectors (Fermi/GBM and Konus-Wind) that are consistent to an accuracy better than 20%.…”

“…As extensively described by Savchenko et al (2017), INTEGRAL provides unique instantaneous coverage of the entire high-energy sky by taking advantage of the synergy between its four all-sky detectors: IBIS/ISGRI, IBIS/PICsIT, IBIS/Veto, and SPI-ACS. These provide complementary capabilities for the detection of transient events characterized by different durations, locations on the sky, and spectral energy distributions.…”

“…Following the approach in Savchenko et al (2016Savchenko et al ( , 2017 and the nondetection of any significant electromagnetic counterpart to GW170104 in the INTEGRAL data, we derived the corresponding upper limits assuming the cases of (i) a shorthard burst, i.e., a 1 s long event characterized by a cutoff power-law spectral energy distribution with parameters α=−0.5, E peak =600keV; (ii) a long-soft burst, i.e., an 8 s long event whose spectral energy distribution is described by the Band model (Band et al 1993) with parameters α=−1, β=−2.5, and E peak =300keV. While the reference short GRB duration of 1s is close to the peak of the short GRB duration distribution, the 8s timescale for the long-soft GRB is motivated by the sampling rate of IBIS/Veto, in analogy with the approach presented previously by Savchenko et al (2017).…”

INTEGRAL Detection of the First Prompt Gamma-Ray Signal Coincident with the Gravitational-wave Event GW170817

“…This can be seen in Fig. 1, where we show the complete IN-TEGRAL sensitivity map combining all instruments as described in Savchenko et al (2017a). We note that with this orientation, the sensitivity of IBIS (ISGRI, Lebrun et al 2003;PICsIT, Labanti et al 2003 andVeto, Quadrini et al 2003 detectors) to a signal from the direction of AT2017gfo was much lower if compared to SPI-ACS for any plausible type of event spectrum.…”

“…Search for a Soft Gamma-Ray Afterglow INTEGRAL allows us to search for an afterglow emission in a broad energy range from 3 keV to 8 MeV. This was covered in detail in Savchenko et al (2017a), where we exploited the serendipitous coverage of part of the LVT151012 localization region within the field of view of the INTEGRAL pointed instruments.…”