Abstract
Abstract Shocks that occur below a gamma-ray burst (GRB) jet photosphere are mediated by radiation. Such radiation-mediated shocks (RMSs) could be responsible for shaping the prompt GRB emission. Although well studied theoretically, RMS models have not yet been fitted to data owing to the computational cost of simulating RMSs from first principles. Here we bridge the gap between theory and observations by developing an approximate method capable of accurately reproducing radiation spectra from mildly relativistic (in the shock frame) or slower RMSs, called the Kompaneets RMS approximation (KRA). The approximation is based on the similarities between thermal Comptonization of radiation and the bulk Comptonization that occurs inside an RMS. We validate the method by comparing simulated KRA radiation spectra to first-principle radiation hydrodynamics simulations, finding excellent agreement both inside the RMS and in the RMS downstream. The KRA is then applied to a shock scenario inside a GRB jet, allowing for fast and efficient fitting to GRB data. We illustrate the capabilities of the developed method by performing a fit to a nonthermal spectrum in GRB 150314A. The fit allows us to uncover the physical properties of the RMS responsible for the prompt emission, such as the shock speed and the upstream plasma temperature.
Highlights
The launching, propagation and collimation of a highly supersonic jet unavoidably leads to immense shock formation inside the jet and its surroundings
In around one quarter of gamma-ray burst (GRB) pulses, the narrowest, time resolved spectrum is consistent with a thermal spectrum, which strongly suggests that the whole pulse is of a photospheric origin (Yu et al 2019; Acuner et al 2020; Dereli-Bégué et al 2020; Li et al 2020)
Dissipation in the optically thick regions of a GRB jet has the potential to generate a wide variety of released photospheric spectral shapes, and it is, 8 The details of how Γ is related to the fitted parameters will be described in an upcoming paper, which focuses on GRB data analysis using the Kompaneets RMS approximation (KRA) model
Summary
The launching, propagation and collimation of a highly supersonic jet unavoidably leads to immense shock formation inside the jet and its surroundings (see e.g., López-Cámara et al 2013, 2014; Gottlieb et al 2019). Shocks that occur deep inside gamma-ray burst (GRB) jets are mediated by radiation Such radiationmediated shocks (RMSs) fill the jet with hot, nonthermal radiation, which is advected toward the jet photosphere where it is released. In around one quarter of GRB pulses, the narrowest, time resolved spectrum is consistent with a thermal spectrum, which strongly suggests that the whole pulse is of a photospheric origin (Yu et al 2019; Acuner et al 2020; Dereli-Bégué et al 2020; Li et al 2020) It is plausible, that the wider, nonthermal spectra in such pulses have undergone subphotospheric dissipation (Rees & Mészáros 2005; Ryde et al 2011).
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