Study Of Microlensing Effects On Fast Radio Bursts

Gravitational lensing refers to the phenomenon that the ray emitted by the background source is deflected in the gravitational field of the foreground object,which causes multiple images,shape distortion and flux change.The term "microlensing"was firstly introduced in 1986,which refers to the gravitational lensing effect caused by stellar-mass compact objects.Microlensing has played an important role in measuring accretion disk size of quasar,constraining the fraction of compact dark matter in galaxies,detecting exoplanets,etc and has become a powerful tool in astrophysical research.Fast radio bursts(FRBs)are high energy radio signals with durations on milli-second to micro-second,which were discovered in recent years.They are transient point sources on cosmological distance and can be an excellent probe in cosmological research.In this thesis,we mainly discuss the microlensing effects induced by point mass objects on fast radio bursts.This thesis is organized as follows:In Chapter 1,we briefly introduce the theoretical background and basic concepts in modern cosmology and gravitational lensing.In Chapter 2,we briefly introduce the properties and observation facts of fast radio bursts.In Chapter 3,we describe the basic theory of the point mass lens model and the point mass plus external shear lens model.We take the observations including time delay and the leading to trailing flux ratio between the two peaks of two double-peaked FRBs as examples and show what constrains on the lens mass we can get if the double peaks are gravitational lensing induced.In Chapter 4,we describe the basic theory of cosmological microlensing.We extend the isolated point mass model to the point mass population model.We develop a microimage searching method and do some relevant tests.With the microimage searching method,we simulate the microlensing effects on fast radio bursts from the star population in a galaxy.We show that microlensing from star population can be a feasible mechanism to produce a multi-peaked FRB profile.In Chapter 5,we introduce the time consuming challenges in the classical method——inverse ray shooting,which is widely used in cosmological microlensing.We develop a GPU-based interpolation inverse ray shooting algorithm.Based on a parallelization with GPU,we calculate the deflection angle contributed by all the distant stars by an interpolation approximation.This reduces the number of stars involved in the cumulative calculation.Compared with the direct inverse ray shooting method,we find that the interpolation inverse ray shooting method can increase the calculation speed by more than one order of magnitude while maintaining the accuracy.In Chapter 6,we summarize the past research and briefly introduce the next work plan.