Energizing Photospheres of Gamma-Ray Bursts

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The detection of a gravitational wave event from a neutron star merger, GW 170817, coincident with a gamma-ray burst, GRB 170817A, is a major achievement of modern astrophysics. GRBs are the most extreme explosions in the Universe and GRB research can impact fundamental physics through understanding new properties of matter under extreme conditions. In order to better understand GRB 170817A and other GRBs, a research group at Columbia University will investigate how the GRB photosphere, or outer shell from which light is radiated, is energized to become so bright. This will be accomplished using state-of-the-art numerical tools developed by the group. The simulations of shock waves in the GRB, derived from first principles, will self-consistently include the creation of electron-positron (anti-electron) pairs, and predict the spectrum of the radiation produced, which can be compared with observations. The results will be used to develop a concrete physical scenario for GRB 170817A. The group will continue with its outreach activities through both public lectures and communications with the press, and the results will be disseminated at international conferences, colloquia, and seminars. Methods and results of the research will be taught as part of astrophysics courses at Columbia University, and the project will involve training of a postdoctoral scholar and students.Photospheric emission from opaque relativistic ejecta plays a key role in cosmological gamma-ray bursts. A key physics question is why the photosphere is so bright in GRBs and what processes shape its observed nonthermal spectrum. The proposed research will focus on building physical models for two heating mechanisms: (1) internal radiation-mediated shocks and (2) damping of a turbulent cascade. An important goal is to investigate how shocks break out at the photosphere, releasing their radiation. The code will directly follow the evolution of sub-photospheric shocks, coupled with propagation of individual photons around it, and calculate their breakout, accompanied by copious electron-positron pair creation. This method is particularly applicable to studies of gamma-ray counterparts of neutron star mergers. It can give a robust physical model for GRB 170817A and lead to specific predictions for counterparts of future gravitational wave detections. The results will have applications to short and long cosmological GRBs as well as shock breakout in supernova explosions. The planned research also includes a novel physical model for turbulence damping in GRB jets and the radiative effects of free neutrons.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.