Studying The Electromagnetic Signals From The Merger Of Compact Stars

Compact binary coalescences(e.g.,binary black hole(BH-BH),black holeneutron star(BH-NS),and binary neutron star(NS-NS)mergers)are the main sources of gravitational wave radiation in the Universe.For binary neutron star mergers,they can produce multi-band electromagnetic(EM)radiations,e.g.,short y-ray bursts and kilonovae,except gravitational wave(GW)radiations.F or binary black hole mergers,they can not produce any EM radiations generally.Black holeneutron star mergers are special.NSs can be tidal disrupted if the tidal disruption radius of the NS is larger than the innermost stable circular orbit(ISCO)radius of the BH and the massive debris can be expelled or accreted onto the BH to power the EM radiations.If the tidal disruption radius of the NS is less than the innermost stable circular orbit radius of the BH,there is not any EM radiation.We study two research work associated with the EM radiations of compact binary coalescences.The first research work concentrates on GRB 201221D which is an event that is short duration(T90～0.1s),high redshift(z=1.046)and likely originated from compact binary coalescences.By analyzing data observed with the Swift/BAT and Fermi/GBM,we find that a cutoff power-law model can adequately fit the spectrum with a peak energy Ep=113-7+9 keV and isotropic energy Eγ,iso=1.36-0.14+0.17×1051 erg.In order to reveal the physical origin of GRB 201221D,we adopted multi-wavelength criteria,e.g.,Amati relation,εparameter,amplitude parameter,local event rate density,luminosity function,and the properties of host galaxy.The result suggests that binary neutron star mergers can interpret mostly observational characters.Besides,we find the emission region is possible undergoing acceleration during prompt emission phase,and the distance of this region from the central engine is about 1016 cm.This distance is not consistent with the prediction of both photosphere and internal shock models,but is favored with the Internal Collision-induced Magnetic Reconnection and Turbulence(ICMART),therefore the jet is Poynting-flux-dominated,which is consistent with the result of interpreting through binary neutron star mergers.The second work is constraining the charge of the BH within a charged BH and NS system.Recently,LIGO and Virgo detected two GW events(GW200105 and GW200115)which are confirmed from the merger of NS-BH systems.Unfortunately,no conclusive EM counterparts have been detected yet within follow-up observations,except several possible optical candidates associated with these two GW events.We assume black holes are charged and try to constrain the charge of the BH using the possible associated EM emission from the charged BH and NS system.The charged BH and NS system can give to rise electric dipole and magnetic dipole radiation during inspiral.Radiated energy propagates outward with a Poynting-flux-dominated outflow,then the magnetic energy will accelerate electrons through magnetic reconnection.The accelerated electrons can produce observable EM radiation via synchrotron radiation finally.We study firstly the conversion efficiency of electric dipole and magnetic dipole radiation within different energy bands,and the result suggest that the conversion efficiency in the X-ray band is much higher than that of the ultraviolet(UV),near-infrared,and radio bands.Then we constrain the charge of BHs,the maximum charge of the BH in GW200105 and GW200115 events is 1.16 × 1029 esu and 5.36 ×1028 esu,respectively,when the surface magnetic field strength Bp(?)1015G and period P>1 ms of the NS.