Abstract
Abstract On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is 5.0 × 10 − 8 . We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of ( + 1.74 ± 0.05 ) s between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between − 3 × 10 − 15 and + 7 × 10 − 16 times the speed of light, (ii) place new bounds on the violation of Lorentz invariance, (iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB 170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1–1.4 per year during the 2018–2019 observing run and 0.3–1.7 per year at design sensitivity.
Highlights
Introduction and BackgroundGW170817 and GRB 170817A mark the discovery of a binary neutron star (BNS) merger detected both as a gravitational wave (GW; LIGO Scientific Collaboration & Virgo Collaboration 2017a) and a short-duration gamma-ray burst (SGRB; Goldstein et al 2017; Savchenko et al 2017b)
The temporal proximity of GRB 170817A was immediately identified by automatic comparison of the FermiGBM Gamma-ray Coordinates Network notice to the GW trigger (Urban 2016)
Finding GRB 170817A to be subluminous, we discuss whether this dimness is an expected observational bias for joint GW–GRB detections, what insight it provides regarding the geometry of the gammaray emitting region, what we can learn about the population of SGRBs, update our joint detection estimates, and set limits on gamma-ray precursor and extended emission
Summary
GW170817 and GRB 170817A mark the discovery of a binary neutron star (BNS) merger detected both as a gravitational wave (GW; LIGO Scientific Collaboration & Virgo Collaboration 2017a) and a short-duration gamma-ray burst (SGRB; Goldstein et al 2017; Savchenko et al 2017b). Detecting GW radiation from the coalescence of BNS and neutron star (NS)– black hole (BH) binary systems has been a major goal (Abbott et al 2017a) of the LIGO (Aasi et al 2015) and Virgo (Acernese et al 2015) experiments This was at least partly motivated by their promise of being the most likely sources of simultaneously detectable GW and electromagnetic (EM) radiation from the same source. A major astrophysical implication of a joint detection of an SGRB and of GWs from a BNS merger is the confirmation that these binaries are the progenitors of at least some SGRBs. GRBs are classified as short or long depending on the duration of their prompt gamma-ray emission. Expectation rates for joint BNS–SGRB detections in the light of this discovery
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