Towards an understanding of GRB prompt emission mechanism: I. The origin of spectral lags

TL;DR

This paper proposes a new physical model for GRB prompt emission that explains spectral lags through synchrotron radiation in an expanding, accelerating outflow, challenging the traditional curvature effect explanation.

Contribution

It introduces a synchrotron-based model with a decreasing magnetic field and rapid bulk acceleration to explain spectral lags in GRBs, advancing understanding of emission mechanisms.

Findings

Spectral lags require a spectral peak sweep across energy bands.

The emission region is optically thin and far from the central engine.

Magnetic field decreases with radius and the outflow accelerates rapidly.

Abstract

Despite decades of investigations, the physical mechanism that powers the bright prompt $\gamma$-ray emission from gamma-ray bursts (GRBs) is still not identified. One important observational clue that remains not properly interpreted so far is the existence of time lags of broad light curve pulses in different energy bands, named "spectral lags". Here we show that the traditional view invoking the high-latitude emission "curvature effect" of a relativistic jet cannot account for spectral lags. Rather, the observed spectral lags demand the sweep of a spectral peak across the observing energy band in a specific manner. The duration of the broad pulses and inferred typical Lorentz factor of GRBs require that the emission region is in an optically thin emission region far from the GRB central engine. We construct a simple physical model invoking synchrotron radiation from a rapidly expanding outflow. We show that the observed spectral lags appear naturally in our model light-curves given that (1) the gamma-ray photon spectrum is curved (as observed), (2) the magnetic field strength in the emitting region decreases with radius as the region expands in space, and (3) the emission region itself undergoes rapid bulk acceleration as the prompt $\gamma$-rays are produced. These requirements are consistent with a Poynting-flux-dominated jet abruptly dissipating magnetic energy at a large distance from the engine.