Long GRB 250916A: an Off-axis Powerlaw Jet with Thermal Cocoon

TL;DR

This paper analyzes GRB 250916A, revealing it as an off-axis structured jet with a thermal cocoon, and suggests the precursor emission results from shock breakout, providing insights into jet structure and central engine activity.

Contribution

It introduces a detailed model of a structured jet with a thermal cocoon for GRB 250916A, linking precursor emission to shock breakout and off-axis viewing geometry.

Findings

The jet has a narrow core with a 0.8° opening angle.

Precursor emission is thermal with a 13.2 keV temperature.

A long 150 s quiescent interval suggests central engine turn-off.

Abstract

Some gamma-ray bursts (GRBs) exhibit precursor emission episodes preceding the main emission, with a quiescent period in between. The properties of the precursor emission and the duration of the quiescent interval are related to the central engine activity and jet formation processes, thus providing insights into the physics of GRBs. We present a comprehensive analysis of the prompt emission and multi-wavelength afterglow of GRB 250916A. Using detailed afterglow modeling, we find that the broadband data are best described by a powerlaw structured jet with a relatively narrow core ($\theta_c \approx 0.8^\circ$), viewed moderately off-axis at a viewing angle $\theta_v \approx 2.7^\circ$. The isotropic-equivalent kinetic energy of the jet ($E_{k,iso} \approx 2.4 \times 10^{54}$ erg) is on the higher side for typical GRBs. The precursor emission is well described by a blackbody spectrum with a temperature of kT $\approx$ 13.2 keV and is separated from the main emission by a long quiescent interval of 150 s. Put together, our results indicate that the precursor is likely to be a shock breakout from a cocoon formed by the interaction of the relativistic jet with the progenitor star. The resulting cocoon pressure and shock collimation naturally lead to the launch of a narrowly collimated jet, consistent with the jet geometry inferred from afterglow observations. The long quiescent interval may imply the central engine turn-off in addition to the effect of the off-axis geometry.