The Soft X-ray Aspect of Gamma-ray Bursts in the Einstein Probe Era

TL;DR

This paper discusses the detection and analysis of gamma-ray bursts by the Einstein Probe satellite, highlighting the potential of off-axis viewing effects and the unique properties of long-duration X-ray GRBs in the soft X-ray band.

Contribution

It introduces the first statistical analysis of soft X-ray properties of GRBs detected by Einstein Probe, proposing off-axis viewing as a key factor in their extended durations and spectral features.

Findings

EP detects longer-duration GRBs than Swift, suggesting off-axis viewing effects.

Statistical properties of off-axis GRBs are consistent with observed X-ray bursts.

Predicted off-axis GRB characteristics can be tested with future EP data.

Abstract

The Einstein Probe (EP) satellite, dedicated at time-domain high-energy astrophysics and multi-messenger astronomy, was recently launched and successfully put into operation. The wide-field X-ray telescope (WXT, 0.5-4 keV) onboard has identified multiple gamma-ray burst (GRB) events, with an average duration of several hundred seconds. This duration is several times longer than the average duration of long gamma-ray bursts (LGRBs) detected by the Neil Gehrels Swift Observatory, which typically stands at several tens of seconds. Additionally, EP has detected some unknown X-ray transients whose connection to GRBs is uncertain, due to the absence of gamma-ray counterparts and efficient follow-up observation at multi-wavelengths. Several main factors could account for the longer time, including the Doppler effect of off-axis viewing, the spectral lag effect of the synchrotron spectrum of cooling electrons, and some unknown prolonged intrinsic X-ray activities. Our studies indicate that EP GRBs may primarily consist of off-axis viewed bursts, forming a unique population among the GRB zoo, yet the intrinsic origin for the specific bursts could not be excluded. By analyzing the statistical properties of the historical LGRB samples, we explored observable properties of on-axis and off-axis LGRBs in the soft X-ray band. The predicted characteristics of off-axis viewed GRBs, including the duration, energy fluence, low-energy spectral index, and the slopes of Amati and Yonetoku relations, could be tested with a larger sample of GRB events detected by EP in the future.