Gravitationally Lensed Orphan Afterglows of Gamma-Ray Bursts

TL;DR

This paper explores the potential detection of gravitationally lensed orphan afterglows of gamma-ray bursts in future wide-field surveys, analyzing models, light curves, and expected event rates.

Contribution

It introduces a comprehensive analysis of gravitational lensing effects on orphan afterglows considering different lens models and jet structures, providing estimates for detection rates in upcoming surveys.

Findings

Lensed orphan afterglows can be identified through their temporal features.

Optical band observations are optimal for detecting galaxy-lensed orphan afterglows.

Estimated event rate for galaxy-lensed orphan afterglows is up to 1.8 per year across the sky.

Abstract

The cosmological nature of gamma-ray bursts (GRBs) implies that a small portion of them could be gravitationally lensed by foreground objects during their propagation. The gravitational lensing effect on the GRB prompt emission and on-axis afterglows has been discussed, and some candidates have been found in the literature. In this work, considering the high detection rate of GRB orphan afterglows in future wide-field survey era, we investigate the gravitationally lensed orphan afterglows in view of three lens models, i.e., the point-mass model, the singular isothermal sphere model, and the Chang-Refsdal model. The structure of the GRB jet itself is also incorporated in calculating the lensed afterglow light curves. It is found that lensed optical/X-ray orphan afterglows in principle could be diagnosed through their temporal characteristics, and the optical band is the best band to observe the galaxy-lensed orphan afterglows. Moreover, the event rate for galaxy-lensed orphan afterglows is estimated to be $\lesssim$ 1.8 $\text{yr}^{-1}$ for the whole sky. If most orphan afterglows could be identified (from other transients in the survey data), the optimistic detection rates of the 2.5m Wide Field Survey Telescope of China and 8.4m Vera Rubin Observatory Legacy Survey of Space and Time for galaxy-lensed orphan afterglows in the optical band are $\lesssim$ 0.01$\sim$0.02 $\text{yr}^{-1}$ and $\lesssim$ 0.04$\sim$0.08 $\text{yr}^{-1}$, respectively.