Cosmic String Detection via Microlensing of Stars

Cosmic superstrings are produced towards the end of the brane inflation. If the string tension is low enough, loops tend to be relatively long-lived. The resultant string network is expected to contain many loops which are smaller than typical Galactic scales. Cosmic expansion damps the center of mass motion of the loops which then cluster like cold dark matter. Loops will lens stars within the galaxy and local group. We explore microlensing of stars as a tool to detect and to characterize some of the fundamental string and string network properties, including the dimensionless string tension $G \mu/c^2$ and the density of string loops within the Galaxy. As $G \mu \to 0$ the intrinsic microlensing rate diverges as $1/\sqrt{G \mu}$ but experimental detection will be limited by shortness of the lensing timescale and/or smallness of the bending angle which each vary $\propto G \mu$. We find that detection is feasible for a range of tensions. As an illustration, the planned optical astrometric survey mission, Gaia, should be able to detect numerous microlensing events for string networks with tensions $10^{-10} \simless G \mu \simless 10^{-6}$. A null result for optical microlensing implies $G \mu \simless 10^{-10}$. If lensing of a given source is observed it will repeat because the internal motions of the loop are relativistic but the center of mass motion may be much smaller, of order the halo velocity. This distinctive hallmark $\sim 1000$ repetitions, suggests a useful method for confirmation of a potential lensing detection.