INTEGRAL Picture Of the Month
November 2017

INTEGRAL POM
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First detection of Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB170817A

On 17 August, a burst of gamma rays lit up in space for almost two seconds. It was promptly recorded by INTEGRAL and NASA's Fermi satellite. Such short gamma-ray bursts are not uncommon: INTEGRAL catches about 20 every year. But this one was special: just seconds before the two satellites saw the blast, an entirely different instrument was triggered on Earth: the detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) experiment, in the USA, recorded the passage of gravitational waves – fluctuations in the fabric of spacetime caused by powerful cosmic events.

This is a ground-breaking discovery, revealing for the first time gravitational waves and highly energetic light released by the same cosmic source.

The Figure shows the joint, multimessenger detection of GW170817 and GRB170817A. Top: The summed Fermi/GBM lightcurve for sodium iodide (NaI) detectors 1, 2, and 5 for GRB170817A between 50 and 300 keV, matching the 100 ms time bins of SPI-ACS data shown at the bottom. Middle: The time-frequency map of GW170817 was obtained by coherently combining LIGO-Hanford and LIGO-Livingston data. Bottom: The SPI-ACS lightcurve with the energy range starting approximately at 100 keV and with a high energy limit of least 80 MeV. All times here are referred to the GW170817 trigger time, T0GW.

This is the closest (i.e., about 40 Mpc) short gamma-ray burst detected among the ones for which we've measured the distance, and by far the dimmest one - nearly a million times less bright than average. The unusual properties of this source suggest that the powerful jets that arise in the cosmic clash of the neutron stars are not pointing straight towards us, as happens in the majority of gamma-ray bursts detected.

When the LIGO/Virgo experiments start their observations again, with improved sensitivity, in late 2018, it is crucial that as many gamma-ray satellites as possible are active to check on the electro-magnetic gamma-ray radiation coincident with the gravitational wave detections. With the high sensitivity to gamma-rays and almost full-sky coverage for brief events, INTEGRAL is among the best astronomical facilities for keeping an eye on these gamma-ray bursts.

Credits:     LIGO/Virgo; Fermi; INTEGRAL; NASA/DOE; NSF; EGO; ESA.

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