The Study Of Magnetized Neutrino-cooling Dominated Accretion Flow And Its Applications In Gamma-Ray Bursts

Gamma-ray bursts(GRBs)are the cosmological transients with large amount of gamma-ray photons emitted in a very short timescale,which are known as the most violent explosions after the universe big bang.Over more than fourty years after its discovery,GRB has been one of the most interesting astrophysical objects to study.Much progress has been made in GRB research during the last several decades.For example,GRBs were verified to be events located at cosmological distances;GRBs have been classified into two types,i.e.,long GRBs and short GRBs;long bursts are likely related with the collapse of massive stars,while short bursts may be the coalescence of compact binaries;the emission of a burst is believed to come from an extremely relativistic jet;the fireball shock theory has been proposed to interpret the afterglow radiation.However,some questions are still open:Is the traditional duration-based classification valid and resonable?Are all the progenitors of the short GRBs the compact binaries?Which component(bayrons,leptons,or magnetic fields)dominates in the jet?Are the jets structured?What is radiation process(synchrotron,synchrotron self-Compton,or external inverse Compton)accounting for the prompt emission?Can GRBs be used as the standard candles?All these questions are related to the central engine of GRBs.The leading model involves a blackhole hyper-accreting system.This thesis is devoted to study the GRB central engine models,focusing on neutrino-cooling dominated accretion flow(NDAF).The thesis is organized as follows:In Chapter ?,we firstly review the main features of the prompt emissions and afterglows.Secondly,we briefly introduce the fireball theory of GRBs.Thirdly,we review the GRB progenitor models,like the collapsar model,supranova model,and compact binaries merge model.In addition,we present several central engine mechanisms such as NDAF,Blandford-Znajek process,magnetar model and so on.In Chapter ?,we study the NDAF model with a non-zero torque boundary condition(nztNDAF).In previous NDAF model,a zero torque boundary condition was assumed.This is,however,not true for a magnetized hyper-accretion disk.We explore the effects of the inner edge torque based on the modified angular momentum equation.We find that the inner edge torque will greatly enhance the neutrino annihilation luminosity of NDAF,and lead to the instability in the inner region.The enhanced luminosity enable nztNDAF as an efficient central engine model for GRBs.In addition,the instability in nztNDAF can naturally interpret the temporal variation of the GRBs.In Chapter ?,we try to understand two empirical correlations related with the minimum variability timescale(MTS).We investigate the theoretical expectations from two popular black hole(BH)central engine models,i.e.,Blandford&Znajek mechanism and neutrino-dominated accretion flow.By taking the MTS as the timescale of viscous instability of the neutrino-dominated accretion flow(NDAF),we find that the variability related correlations favor the scenario in which the jet is driven by Blandford-Znajek(BZ)mechanism.In Chapter ?,we carry out a time-dependent simulation to NDAF using ZEUS-2D code.This work was supposed to check our previous results such as the structure and the neutrino luminosity of the NDAF(or nztNDAF)and their dependences to blackhole mass,accretioin rate as well as other paramters.As the first step,we currently consider only the the URCA cooling process and the equation of state of ideal gas,and further considerations are expected to be performed in future.In Chapter ?,we summarize our works and present some prospects for my research.