A luminous precursor in the extremely bright GRB 230307A

TL;DR

GRB 230307A, an extremely bright gamma-ray burst, exhibits a luminous precursor phase that suggests a magnetar or rapidly rotating neutron star origin, challenging existing models of GRB progenitors.

Contribution

This paper identifies and analyzes a luminous precursor in GRB 230307A, proposing new insights into its progenitor's magnetic field and post-merger neutron star formation.

Findings

The precursor is bright and soft, preceding the main burst.

A magnetar-like magnetic field is needed to explain the brightness.

The precursor may indicate formation of a rapidly rotating neutron star.

Abstract

GRB 230307A is an extremely bright long duration GRB with an observed gamma-ray fluence of $\gtrsim$3$\times$10$^{-3}$ erg cm$^{-2}$ (10-1000 keV), second only to GRB 221009A. Despite its long duration, it is possibly associated with a kilonova, thus resembling the case of GRB 211211A. In analogy with GRB 211211A, we distinguish three phases in the prompt gamma-ray emission of GRB 230307A: an initial short duration, spectrally soft emission; a main long duration, spectrally hard burst; a temporally extended and spectrally soft tail. We intepret the initial soft pulse as a bright precursor to the main burst and compare its properties with models of precursors from compact binary mergers. We find that to explain the brightness of GRB 230307A, a magnetar-like ($\gtrsim 10^{15}$ G) magnetic field should be retained by the progenitor neutron star. Alternatively, in the post-merger scenario, the luminous precursor could point to the formation of a rapidly rotating massive neutron star.