Relativistic Envelopes and Gamma-Rays from Neutron Star Mergers

• Published in: The Astrophysical journal 2020-07, Vol.897 (2), p.141
• Main Authors: Beloborodov, Andrei M., Lundman, Christoffer, Levin, Yuri
• Format: Article
• Language: English
• Subjects: Ablation; Astrophysics; Differential rotation; Ejecta; Gamma ray bursts; Gamma rays; Gravitational wave sources; Homology; Inflating; Light curve; Lorentz factor; Luminosity; Magnetars; Neutrinos; Neutron stars; Non-thermal radiation sources; Photons; Photosphere; Relativistic fluid dynamics; Shocks; Star mergers; Stars & galaxies; Stellar winds
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/ab86a0

We suggest that neutron star mergers eject an ultrarelativistic envelope of mass m ∼ 10−7 M , which helps explain the gamma-ray burst from GW170817. One ejection mechanism is the ablation of the neutron star surface by the burst of neutrinos in the first 30 s of the merger. Another, more efficient mechanism for inflating the ultrarelativistic envelope is an internal shock in the massive ejecta from the merger. A strong shock is expected if the merger product is a magnetar, which emits a centrifugally accelerated wind. The shock propagates outward through the ejecta and accelerates in its outer layers at radii r ∼ 109-1010 cm, launching an ultrarelativistic opaque envelope filled with ∼104 photons per nucleon. The Lorentz factor profile of the envelope rises outward and determines its homologous expansion, which adiabatically cools the trapped photons. Once the magnetar loses its differential rotation and collapses into a black hole, a powerful jet forms. It drives a blast wave into the envelope, chasing its outer layers, and eventually catching up with the envelope photosphere at r ∼ 1012 cm. The ultrarelativistic photospheric breakout of the delayed blast wave emits a gamma-ray burst in a broad solid angle around the merger axis. This model explains the gamma-ray pulse from merger GW170817 with luminosity Lγ ∼ 1047 erg s−1, duration , and characteristic photon energy ∼100 keV. The blast-wave Lorentz factor at the envelope photosphere is consistent with Γ 5, which we derive from the observed light curve of the burst. We suggest future tests of the model.