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Relativistic envelopes and gamma-rays from neutron star mergers

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Beloborodov,  Andrei M.
Galaxy Formation, Cosmology, MPI for Astrophysics, Max Planck Society;

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Citation

Beloborodov, A. M., Lundman, C., & Levin, Y. (2020). Relativistic envelopes and gamma-rays from neutron star mergers. The Astrophysical Journal, 897(2): 141. doi:10.3847/1538-4357/ab86a0.


Cite as: http://hdl.handle.net/21.11116/0000-0007-18DF-4
Abstract
We suggest that neutron star mergers eject an ultra-relativistic envelope of mass m∼10<sup>−7</sup>M<sub>⊙</sub>, 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 30μs of the merger. Another, more efficient, mechanism for inflating the ultra-relativistic 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∼10sup>9</sup>−10<sup>10</sup>cm, launching an ultra-relativistic opaque envelope filled with ∼10<sup>4</sup> 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∼10<sup>12</sup>cm. The ultra-relativistic 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 L<sub>γ</sub>∼10<sup>47</sup>erg/s, duration Δt<sub>obs</sub>∼0.5s, and characteristic photon energy ∼100keV. The blast wave Lorentz factor at the envelope photosphere is consistent with Γ≥5 that we derive from the observed light curve of the burst. We suggest future tests of the model.