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

TL;DR

GW170817/GRB 170817A provides the first joint detection of a binary neutron star merger in gravitational waves and a short gamma-ray burst, confirming BNS mergers as SGRB progenitors. The authors combine GW and EM observations to constrain fundamental physics (speed of gravity, Lorentz invariance, equivalence principle), probe GRB central-engine timescales, and derive NS EOS implications from the measured masses. They analyze GRB energetics, jet geometry, and potential cocoon contributions, highlighting the importance of viewing geometry and detector sensitivity for nearby, subluminous events. The work updates joint-detection rate projections and advocates for continued subthreshold searches to map the populations of mergers and their EM counterparts, advancing multi-messenger astronomy and compact-object physics.

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.

Figures (6)

• Figure 1: Final localizations. The$90 \%$ contour for the final sky-localization map from LIGO-Virgo is shown in green (LIGO Scientific Collaboration & Virgo Collaboration 2017a, 2017b, 2017c). The $90 \%$ GBM targeted search localization is overlaid in purple (Goldstein et al. 2017). The $90 \%$ annulus determined with Fermi and INTEGRAL timing information is shaded in gray (Svinkin et al. 2017). The zoomed inset also shows the position of the optical transient marked as a yellow star (Abbott et al. 2017f; Coulter et al. 2017a, 2017b). The axes are R.A. and decl. in the Equatorial coordinate system.
• Figure 2: Joint, multi-messenger detection of GW170817 and GRB 170817A. Top: the summed GBM lightcurve for sodium iodide ( NaI ) detectors 1,2 , and 5 for GRB 170817A between 10 and 50 keV , matching the 100 ms time bins of the SPI-ACS data. The background estimate from Goldstein et al. (2016) is overlaid in red. Second: the same as the top panel but in the$50-300 \mathrm{keV}$ energy range. Third: the SPI-ACS lightcurve with the energy range starting approximately at 100 keV and with a high energy limit of least 80 MeV . Bottom: the time-frequency map of GW170817 was obtained by coherently combining LIGO-Hanford and LIGOLivingston data. All times here are referenced to the GW170817 trigger time $T_{0}^{\mathrm{GW}}$.
• Figure 3: Critical mass boundaries for different EOSs in comparison with the$90 \%$ credible region of the gravitational masses inferred from GW170817 (prior limits on the spin magnitude, $\left|\chi_{z}\right|$, given in the legend). The slanted curves in the left panel and middle panel correspond to the maximum baryonic mass allowed for a single non-rotating NS (left) and for a uniformly rotating NS (middle). Arrows indicate for each EOS the region in the parameter space where the total initial baryonic mass exceeds the maximum mass for a single non-rotating or uniformly rotating NS, respectively. The right panel illustrates EOS-dependent cuts on the gravitational mass $m_{1}$ of the heavier star, with arrows indicating regions in which $m_{1}$ exceeds the maximum possible gravitational mass $M_{\mathrm{G}}^{\text{Static }}$ for non-rotating NSs. In all three panels the black solid line marks the $m_{1}=m_{2}$ boundary, and we work in the $m_{1}>m_{2}$ convention.
• Figure 4: GRB 170817A is a dim outlier in the distributions of$E_{\text{iso }}$ and $L_{\text{iso }}$, shown as a function of redshift for all GBM-detected GRBs with measured redshifts. Redshifts are taken from GRBOX (http://www.astro.caltech.edu/grbox/grbox.php) and Fong et al. (2015). Short- and long-duration GRBs are separated by the standard $T_{90}=2 \mathrm{~s}$ threshold. For GRBs with spectra best modeled by a power law, we take this value as an upper limit, marking them with downward pointing arrows. The power law spectra lack a constraint on the curvature, which must exist, and therefore, will overestimate the total value in the extrapolated energy range. The green curve demonstrates how the (approximate) GBM detection threshold varies as a function of redshift. All quantities are calculated in the standard 1 keV 10 MeV energy band.
• Figure 5: Three potential jet viewing geometries and jet profiles that could explain the observed properties of GRB 170817A, as described by scenarios (i)-(iii) in Section 6.2.