Study On The Electromagnetic Counterparts Of Binary Compact Star Merger

This thesis mainly focuses on the kilonova and gamma-ray burst(GRB)associated with a compact binary star merger.Our primary interest is on the AT 2017gfo(kilonova)and the afterglow of GRB 170817 A,which is the electromagnetic counterparts to the first detected gravitational-wave event GW170817 from binary neutron star(NS)merger.We detailedly studied the evolution and radiation of the kilonova-PWN(i.e.,pulsar wind nebula)system.Here,the PWN is embedded in the kilonova and is powered by a long-lived magnetized NS,which is assumed as the remnant compact object of binary NS merger.We find that the evolution of PWN associated with a kilonova is much faster than that associated with a supernova.The radiation of PWN may be absorbed by or escapes from the system,and thus would significantly affect the observed light curves and spectral energy distribution(SED).Different from the classical kilonova,the radiation of the kilonova-PWN system has following characteristics: the light curve may be different for different bands and may have a bimodal morphology in the ultraviolet-optical bands;the SED is time-dependent and evolved from thermal-dominated to nonthermal-dominated.If the ejecta is optically thin,the SED will dominated by the PWN emission.Based on the kilonova-PWN system,we fitted the multiband light curves of AT 2017 gfo by the Markov chain Monte Carlo(MCMC)method.Our fitting indicates that the model can describe the multiband light curves of AT 2017 gfo very well.Moreover,the fitting could infer the physical parameters of the merger remnant.An off-axis observed structured jet is required to explain the afterglow of GRB 170817 A.Different from previous works,we adopt the angle-dependent energy and baryon loading to describe the jet structure.With our structured jet,we fit the multi-band afterglow of GRB 170817 A with MCMC method to infer the parameters of jet structure,circum-burst particle number density,and microphysical parameters of external shock.We also estimate the total baryon loading of the jet in GRB 170817 A.By comparing it with the mass loss rate of outflow in the numerical simulations for binary NS mergers,we suggest that the jet should be launched within <0.1 seconds after the merger.The jet launch time is much less than the delay time,?1.74 seconds,of GRB 170817 A relative to GW170817.This result disfavors the delayed jet launch scenario that attributes the prompt emission of GRB 170817 A to a shock breakout.It reveals that the prompt emission may origins from other mechanisms.