Microlensing by Cosmic Strings

TL;DR

This paper analyzes the potential for detecting cosmic strings through gravitational microlensing of distant quasars, focusing on characteristic light curves and the likelihood of observation given current limits.

Contribution

It provides a detailed calculation of the microlensing optical depth and event rate for cosmic strings, considering observational constraints and parameter space for detection.

Findings

Optical depth for microlensing by cosmic strings is very low, less than 10^-8.

Event rates are fewer than one per billion sources per year under current limits.

A dedicated long-term survey could potentially detect cosmic strings within a specific parameter space.

Abstract

We consider the signature and detectability of gravitational microlensing of distant quasars by cosmic strings. Because of the simple image configuration such events will have a characteristic light curve, in which a source would appear to brighten by exactly a factor of two, before reverting to its original apparent brightness. We calculate the optical depth and event rate, and conclude that current predictions and limits on the total length of strings on the sky imply optical depths of $\la 10^{-8}$ and event rates of fewer than one event per $10^9$ sources per year. Disregarding those predictions but replacing them with limits on the density of cosmic strings from the CMB fluctuation spectrum, leaves only a small region of parameter space (in which the sky contains about $3\times10^5$ strings with deficit angle of order 0.3 milli-arcseconds) for which a microlensing survey of exposure $10^7$ source-years, spanning a 20--40-year period, might reveal the presence of cosmic strings.