Fast radio bursts and the radio perspective on multi-messenger gravitational lensing

Abstract

Fast radio bursts (FRBs) are extragalactic millisecond-duration radio transients whose nature remains unknown. The advent of numerous facilities conducting dedicated FRB searches has dramatically revolutionized the field: hundreds of new bursts have been detected, and some are now known to repeat. Using interferometry, it is now possible to localize FRBs to their host galaxies, opening up new avenues for using FRBs as astrophysical probes. One promising application is studying gravitationally lensed FRBs. This review outlines the requirements for identifying a lensed FRB, taking into account their propagation effects and the importance of capturing the amplitude and phase of the signal. It also explores the different lens masses that could be probed with FRBs throughout the duration of an FRB survey, from stellar masses to individual galaxies. This highlights the unique cosmological applications of gravitationally lensed FRBs, including measurements of the Hubble constant and the compact object content of dark matter. Finally, we discuss future radio interferometers and the prospects for finding gravitationally lensed FRBs.This article is part of the Theo Murphy meeting issue 'Multi-messenger gravitational lensing (Part 1)'.

Keywords: fast radio bursts; gravitational lensing; transients.

Figure 1.

Diagram of a gravitationally lensed FRB.

Figure 2.

Observable lens masses through gravitationally lensed FRBs. The top panel shows the fiducial timescale $\mathrm{\Delta }{t}_{\text{fid}}$ as a function of lens masses for lenses at redshifts $z_l=(0.1,1,5)$; the bottom panel shows the Einstein radius ${\theta }_{\text{E}}$ for the same lens masses, $z_l=(0.1,1,5)$ and $z_s=(0.2,2,10)$. The coloured shaded regions indicate different lensing regimes; from left to right, green shows microlensing (MACHOs, PBHs, stars, free-floating planets), orange shows millilensing (intermediate mass black holes, dark matter halos), purple shows lensing by galaxies and pink shows lensing by clusters. The grey shaded areas show the regions that cannot be identified with current or upcoming surveys. Adapted from ([49], fig. 2).

Figure 3.

Left: redshift distribution of CHIME one-offs (green) and repeater FRBs (orange), estimated from the Macquart relation. The dashed and dotted lines represent lensing from compact objects if they contribute to a fraction of ${10}^{-3}$ and ${10}^{-4}$ of dark matter, respectively. The grey solid line shows the probability of lensing by galaxies as a function of redshift. Adapted from [49]. Right: observing properties of current and upcoming surveys. The field of view (FoV) of each instrument is plotted as a function of their sensitivity. Circular markers indicate current instruments, while squares show future surveys. Filled markers indicate surveys that can achieve (sub-)arcsecond localization, while empty markers show those that cannot. Grey dashed lines show the detection rate per 24 h of observations.