An Accretion-Modulated Internal Shock Model for Long GRBs

TL;DR

This paper proposes the Accretion-Modulated Internal Shock model (AMIS) to explain long GRB prompt emission, linking the light curve shape to the central engine's mass supply and internal shocks, with potential for future complex modeling.

Contribution

It introduces the AMIS framework connecting mass accretion history to GRB emission profiles, emphasizing the role of internal shocks and spectral evolution in long GRBs.

Findings

The prompt emission envelope follows the mass supply rate with FRED-like profiles.

Spectral evolution correlates with the envelope and pulse narrowing at high energies.

Shell Lorentz factor variations explain short-timescale features in GRB light curves.

Abstract

We introduce the Accretion-Modulated Internal Shock model (AMIS) as a possible framework for explaining the observational properties of long gamma-ray burst (GRB) prompt emission. In this scenario, the envelope of the prompt light curve follows the time-dependent mass-supply history to the central engine, associated with stellar collapse and, where applicable, fallback accretion, whose early-time onset can be approximated by $\dot{M}\propto t^{0-1/2}$ and which subsequently may decay as $\dot{M}\propto t^{-5/3}$, producing a photon count rate with a single fast-rise-exponential-decay (FRED)-like profile. In general, the prompt-emission envelope is regulated by a time-dependent mass supply to the central engine, while internal shocks produce the rapid variability. Since we only aim to introduce this framework here, we focus on the simplest single-FRED shape of the prompt emission profiles, while more complex cases involving multiple episodes and interacting shocks will be explored in forthcoming studies. The model indicates correlations between spectral evolution, FRED-pulse narrowing at high energies, and the mass-supply-controlled envelope. Stochastic Lorentz factor variations of ejected mass- or rate-driven shells, superimposed on the Accretion-Modulated envelope, explain the coexistence of smooth global trends and irregular short-timescale features, such as the widths of individual pulses in long GRB light curves, offering diagnostic tools for probing the inner engine activity.