Gamma Ray Bursts Are Observed Off-Axis

TL;DR

This study uses advanced modeling to analyze a large sample of gamma-ray burst afterglows, revealing that many are observed off-axis, which impacts our understanding of their energies and observed properties.

Contribution

The paper introduces a novel method to infer off-axis observer angles directly from light curves using the ScaleFit package and a large Swift-XRT dataset.

Findings

Median off-axis observer angle is 0.57 of the jet opening angle.

Jet half opening angles are mostly less than 0.1 radians.

The electron spectral index has a broad distribution with median 2.23.

Abstract

We use the ScaleFit package to perform Markov chain Monte Carlo light curve fitting on a large sample of Swift-XRT gamma-ray burst afterglows. The ScaleFit model uses scaling relations in the hydrodynamic and radiation equations to compute synthetic light curves directly from a set of high resolution two-dimensional relativistic blast wave simulations. The data sample consists of all Swift-XRT afterglows from 2005 to 2012 with sufficient coverage and a known redshift, 188 bursts in total. We find the jet half opening angle varies widely but is commonly less than 0.1 radians. The distribution of the electron spectral index is also broad, with a median at 2.23. This approach allows, for the first time, for the off-axis observer angle to be inferred directly from the light curve. We find the observer angle to have a median value of 0.57 of the jet opening angle over our sample, which has profound consequences for the predicted rate of observed jet breaks and effects the beaming corrected total energies of gamma-ray bursts.