Constraining the neutron star equation of state with gamma-ray bursts
Short gamma-ray bursts are associated with binary neutron star mergers, multimessenger events observed both in gravitational waves and throughout the electromagnetic spectrum. Numerical relativity simulations show that, depending on the masses of the stars and their equation of state, a differentially rotating remnant may be formed which survives for milliseconds before collapsing into a black hole. Due to oscillations of the remnant, the postmerger gravitational-wave signal has broad spectral features in the kilohertz range (1-5 kHz), and are thus inaccessible to current ground-based detectors. However, these oscillations may also be imprinted on the gamma-ray emission. We report quasiperiodic oscillations in archival BATSE data from the short gamma-ray bursts 910711 and 931101B, with centroid frequencies consistent with those of the quasiradial and quadrupolar oscillation modes of the postmerger remnants as predicted by the simulations. Within this assumption, we infer the source redshift, and the chirp mass and effective tidal deformability of the progenitor binary. We obtain constraints on the neutron-star mass-radius relation and determine a 68% credible interval of 12.5+/-0.4 km for the radius of a 1.4 solar-mass neutron star, which is one of the tightest constraints to date.