Modeling the Multi-Wavelength Afterglow of Short Gamma-Ray Bursts with a Plateau Phase

TL;DR

This study models the broadband afterglow of short gamma-ray bursts with a plateau phase using a magnetar energy injection framework, revealing key physical parameters and differences from long GRBs.

Contribution

It introduces a comprehensive broadband modeling approach combining X-ray, optical, and radio data to better constrain magnetar parameters in short GRBs.

Findings

Broadband modeling reduces parameter degeneracy.

Short GRBs have lower magnetic fields and shorter spin periods than previously estimated.

Differences in Lorentz factor and gamma-ray energy distinguish short from long GRBs.

Abstract

Short gamma-ray bursts (GRBs) exhibiting a plateau phase provide valuable insights into the post-merger activity of their central engines. Although the physical origin of the plateau remains uncertain, the magnetar energy injection model offers a compelling explanation that reproduces the observed temporal and luminosity features. However, previous studies relying solely on X-ray data have suffered from strong parameter degeneracies when constraining the magnetar parameters. Here we perform broadband afterglow modeling on seven short GRBs with plateau features by combining X-ray, optical, and radio observations within the framework of the magnetar energy injection model. Key model parameters are derived by using the Markov Chain Monte Carlo method. It is found that the energy injection substantially modifies the afterglow dynamics in most events. Compared with X-ray-only analyses, our broadband modeling yields systematically a lower magnetic field strength and a shorter spin period for the central magnetar, corresponding to a higher injection luminosity. The study clearly shows that incorporating multi-wavelength data effectively alleviates the degeneracy between the magnetar parameters and X-ray radiative efficiency. In addition, the distribution of our short GRBs differs markedly from long GRBs when they are plotted on the initial Lorentz factor versus gamma-ray energy plane. This offset, consistent with the observed harder spectrum of short GRBs, may serve as a useful diagnostic for investigating the progenitor as larger samples are available.