Extremely Soft X-ray Flash as the indicator of off-axis orphan GRB afterglow

TL;DR

This paper supports the off-axis jet model for X-ray flashes by analyzing XRF020903, showing that off-axis orphan GRB afterglows can be identified through achromatic brightening, suggesting a new way to discover such events.

Contribution

It provides observational evidence for off-axis orphan GRB afterglows and demonstrates how large telescopes can detect these events using wide field imaging.

Findings

Achromatic brightening observed in XRF020903's afterglow.

Off-axis jet model explains the brightening and spectral properties.

Potential for discovering more orphan GRB afterglows with large telescopes.

Abstract

We verified the off-axis jet model of X-ray flashes (XRFs) and examined a discovery of off-axis orphan gamma-ray burst (GRBs) afterglows. The XRF sample was selected on the basis of the following three factors: (1) a constraint on the lower peak energy of the prompt spectrum $E^{src}_{obs}$, (2) redshift measurements, and (3) multi-color observations of an earlier (or brightening) phase. XRF020903 was the only sample selected basis of these criteria. A complete optical multi-color afterglow light curve of XRF020903 obtained from archived data and photometric results in literature showed an achromatic brightening around 0.7 days. An off-axis jet model with a large observing angle (0.21 rad, which is twice the jet opening half-angle, $\theta_{jet}$) can naturally describe the achromatic brightening and the prompt X-ray spectral properties. This result indicates the existence of off-axis orphan GRB afterglow light curves. Events with a larger viewing angle ($>\sim2\theta_{jet}$) could be discovered using an 8-m class telescope with wide field imagers such as Subaru Hyper-Suprime-Cam and the Large Synoptic Survey Telescope.