Gravitational lensing in gamma-ray bursts.

Gravitationally lensed gamma-ray bursts (GRBs) offer critical advantages over other lensed sources. They can be detected via continuously operating detectors covering most of the sky. They offer extremely high time resolution to determine lensing delays and find short time delays accurately. They are detectable across most of the visible Universe. However, they are also rare and frequently poorly localized. In this article, we review searches for gravitational lensing in GRBs and comment on promising avenues for the future. We note that the highly structured jets in GRBs can show variations on sufficiently small scales that, unlike lensing of most transient sources, gravitational lensing in GRBs may not be achromatic. Such behaviour would weaken the stringent requirements for identifying lensed bursts but would also make robust identification of lensing more challenging. A continuously running search that could identify candidate lensed events in near real-time would enable afterglow searches with current and near-future wide-field optical/infrared surveys that could yield the first unambiguous detection of a lensed GRB. The new generation of sensitive X-ray and γ-ray detectors, such as the Einstein Probe and SVOM, will complement Swift and significantly enhance the number of well-localized γ-ray and X-ray transients. Tuned strategies could dramatically improve the probability of observing a lensed GRB.This article is part of the Theo Murphy meeting issue 'Multi-messenger gravitational lensing (Part 1)'.