Effect of viewing angle in Gamma-ray Burst properties

TL;DR

This study explores how the viewing angle affects observed properties of Gamma-Ray Bursts, suggesting off-axis viewing can explain certain burst classifications and characteristics.

Contribution

It introduces a simulation-based analysis of how viewing angles influence GRB duration and spectral hardness, challenging traditional progenitor classifications.

Findings

Viewing angle significantly affects spectral hardness.

Off-axis bursts tend to be softer and less luminous.

Some X-ray transients may originate from off-axis jets.

Abstract

The empirical classification of Gamma-Ray Bursts (GRBs) is based on their distribution in the plane of burst duration and spectral hardness. Two distinct distributions, long-soft and short-hard bursts, are observed in this plane, forming the basis for the long and short classification scheme. Traditionally, this scheme was mapped to two different GRB progenitor classes. However, several recent bursts have challenged this mapping. This work investigates how an observer's viewing angle relative to the jet axis influences the duration-hardness plane. We simulate single-pulse GRBs using an optically and geometrically thin homogeneous top-hat jet model. Bursts are simulated with an isotropic viewing angle distribution, and we calculate the pulse duration and spectral hardness corresponding to \textit{FERMI} Gamma-Ray Burst Monitor (GBM) energy bands. The viewing angle significantly impacts spectral hardness for our assumed broken power-law spectra, while its effect on duration is less pronounced. Our analysis indicates that soft and low-luminous bursts are likely off-axis events. It is possible that some of the fast X-ray transients and X-ray rich GRBs observed by the Einstein Probe and the Space Variable Objects Monitor (SVOM) missions originate from off-axis jets.