Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A (1710.05834v2)

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 Anticoincidence 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 $5.0\times 10^{-8}$. 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 gamma-ray bursts. We use the observed time delay of $(+1.74 \pm 0.05)\,$s between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between $-3\times 10^{-15}$ and $+7\times 10^{-16}$ 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.

• The paper presents the first joint detection of gravitational waves and gamma-rays from a binary neutron star merger, establishing a concrete link to short gamma-ray bursts.
• Utilizing a 1.74 ± 0.05 s timing delay, the study constrains the speed difference between gravity and light and tests Lorentz invariance.
• Observations of a nearby, less energetic SGRB with hard and soft components provide new insights into jet propagation and neutron star physics.

Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

This paper presents the landmark detection of gravitational waves (GWs) from a binary neutron star (BNS) merger, GW170817, accompanied by a short gamma-ray burst (SGRB), GRB 170817A. This joint detection marks a pivotal moment in astrophysics, affirming that BNS mergers are indeed progenitors of at least some SGRBs. The Advanced LIGO and Virgo detectors registered GW170817 on August 17, 2017, while the gamma-ray counterpart, GRB 170817A, was observed independently by the Fermi Gamma-ray Burst Monitor and the INTEGRAL satellite. The extraordinarily low probability of such a temporal and spatial coincidence arising by chance was calculated to be $5.0 \times 10^{-8}$.

The joint detection of GWs and gamma-rays from the same astrophysical event allowed the researchers to derive significant constraints on fundamental physics. Utilizing the timing difference of $+1.74 \pm 0.05$ seconds between the gamma-ray and gravitational wave signals, the authors assessed the relative speeds of gravity and light, yielding constraints on the Lorentz invariance violation and testing the equivalence principle. Specifically, the difference in speed of gravity relative to electromagnetic waves was limited to between $-3 \times 10^{-15}$ and $+7 \times 10^{-16}$ times the speed of light.

From an astrophysical perspective, GRB 170817A stands out due to its proximity at 40 Mpc, making it the closest SGRB with a known distance, yet remarkably, it is several orders of magnitude less energetic than any other burst with a measured redshift. The gamma-ray burst exhibited two components: a hard peak and a softer tail, with all observed emissions below 511 keV, implying that the gamma-ray emission did not require a high bulk Lorentz factor ($\Gamma$) to avoid photon-photon pair production at the source.

These constraints have strong implications for the understanding of the BNS merger aftermath and the SGRB mechanism. A potential scenario could involve jet propagation from the accreting mass around the remnant compact object. The paper's predictions suggest a joint detection rate for future operations between 0.1 and 1.7 per year at design sensitivity, highlighting the potential for further multimessenger observations if the capabilities of gamma-ray detectors and subthreshold analyses improve as predicted.

Theoretically, the findings from this observation suggest several implications for SGRB engines and the neutron star equation of state (EOS). The connection between SGRBs and BNS mergers was previously supported only by indirect evidence; thus, GW170817 and GRB 170817A provide incontrovertible observational support. This paper sets the stage for future investigations into the physics of extreme states of matter and high-energy astrophysical phenomena, especially as technology progresses to allow observations at greater distances and with finer sensitivities.

In summary, this paper not only corroborates the link between BNS mergers and SGRBs but also advances our understanding of various aspects of fundamental physics through the unique capability of multimessenger astronomy. It paves the way for deeper exploration into the dynamics governing neutron star mergers and their resultant high-energy emissions, offering a glimpse into the dense world of degenerate astrophysical objects and the rich tapestry of high-energy processes in the universe.