Prospects of Cosmic Superstring Detection through Microlensing of Extragalactic Point-Like Sources

Abstract

The existence of cosmic superstrings may be probed by astronomical time domain surveys. When crossing the line of sight to point-like sources, strings produce a distinctive microlensing signature. We consider two avenues to hunt for a relic population of superstring loops: frequent monitoring of (1) stars in Andromeda, lensed by loops in the haloes of the Milky-Way and Andromeda and (2) supernovae at cosmological distances, lensed by loops in the intergalactic medium. We assess the potential of such experiments to detect and/or constrain strings with a range of tensions, $10^{-15} \lesssim G \mu/c^2 \lesssim 10^{-6}$. The practical sensitivity is tied to cadence of observations which we explore in detail. We forecast that high-cadence monitoring of $\sim 10^5$ stars on the far side of Andromeda over a year-long period will detect microlensing events if $G\mu/c^2 \sim 10^{-13}$, while $\sim 10^6$ stars will detect events if $10^{-13.5} <G\mu/c^2 < 10^{-11.5}$; the upper and lower bounds of the accessible tension range continue to expand as the number of stars rises. We also analyse the ability to reject models in the absence of fluctuations. While challenging, these studies are within reach of forthcoming time-domain surveys. Supernova observations can hypothetically constrain models with $10^{-12} < G\mu/c^2 < 10^{-6}$ without any optimisation of the survey cadence. However, the event rate forecast suggests it will be difficult to reject models of interest. As a demonstration, we use observations from the Pantheon Type Ia supernova cosmology data-set to place modest constraints on the number density of cosmic superstrings in a poorly tested region of the parameter space.