Role of stellar microlensing in generating cluster gravitationally-lensed transients

Abstract

Under gravitational lensing, dim individual stars are made possible to be detected temporarily upon an extra boost of brightness over a short period of time. These objects are known as lensed transients where their temporary increase in brightness comes from either extrinsic phenomena such as stellar microlensing, or the intrinsic variability of the lensed sources themselves. In my thesis, I investigated the nature of transients that comprise lensed star clusters in the galaxy cluster Abell 370 as detected by the Flashlights project. In particular, I made use of one of these transients which is multiply-lensed itself, as a case study to understand what might have caused all these transients – Is stellar microlensing capable of explaining all these detections? Or intrinsic stellar outbursts could explain some of the transients?

To tackle the scientific question, I deployed a variety of techniques to collect evidence. To begin with, I constructed a reliable lens model that allows me to construct light curves of the lensed star clusters and a simulation to address the role of stellar microlensing in generating these lensed star cluster transients. The various light curves of the experimented multiply-lensed star cluster did not rule out nor confirm any of the hypotheses, but implied that Luminous Blue Variables could be a possible explanation behind some of the detected transients; The results from my microlensing simulation are consistent with what observed for the transients, qualitatively and quantitatively. In particular, the detection rate of lensed star cluster transients is consistent with the predicted rate from my simulation within 1σ uncertainty, which indicates that microlensing is capable of explaining all the detected lensed star cluster transients. Last but not least, the consistency between the detection rate of observed and simulated lensed star cluster transients allowed me to place constraints on the abundance of Primordial Blackholes in Dark Matter, down to ≲ 1% with 3σ confidence.