The Study Of The Central Engines Of Gamma-ray Bursts With X-ray Flares

Gamma-ray bursts(GRBs)are known as the most violent explosion event in the Universe.GRBs are generally divided into two classes according to the duration and spectral hardness:long-soft GRB(from the collapse of massive stars)and short-hard GRB(from the merger of compact stars).A hyper-accreting stellar-mass black hole(BH)or a millisecond magnetar is usually invoked as a possible GRB central engine.Since both of the two types of central engine can satisfy most observational constraints,it is quite difficult to identify the right one base on the observational data.Swift/X-ray Telescope(XRT)has opened a new window to the study of GRB physics.The central engine-powered flare can reveal the physics of the central engine.This article is divided into seven chapters to launch the elaboration.Chapter 2 to Chapter 5 mainly expound the study of GRB central engine.Chapter 6 introduces the method to search for BH candidates.The scope of the dissertation is as follows.In Section 1,we introduce the involved background knowledge:the open question-s in GRB physics,neutrino-dominated accretion flows,X-ray flares and BH binaries.In Section 2,we focus on a sample of seven extremely late-time X-ray flares with peak time tP>104s,among which two flares can be confirmed as the late-time ac-tivity of central engine.The main purpose is to investigate the mechanism of such late-time flares based on the internal origin assumption.In the hyper-accreting black hole scenario,we study the possibility of two well-known mechanisms as the central engine to power such X-ray flares,i.e.,the neutrino-antineutrino annihilation and the Blandford-Znajek process.Our results show that the annihilation luminosity is far be-low the observational data.Thus,the annihilation mechanism cannot account for such late-time flares.For the BZ process,if the role of outflows is taken into consideration,the inflow mass rate near the horizon will be quite low such that the magnetic field will probably be too weak to power the observed X-ray flares.We therefore argue that,for the late-time flares with internal origin,the central engine is unlikely to be associated with BHs.On the contrary,a fast rotating neutron star with strong bipolar magnetic fields may be responsible for such flares.In Section 3,we propose that the bright X-ray flare related to the central engine reactivity may hint a BH-NS merger,since such a merger can provide more fall-back materials and therefore a more massive accretion disk than the NS-NS merger.Based on the observed 49 short bursts with Swift/XRT follow-up observations,we find that three bursts have bright X-ray flares,among which three flares from two bursts are probably related to the central engine reactivity.We argue that these two bursts may originate from the BH-NS merger rather than the NS-NS merger.Our suggested link between the central engine-powered bright X-ray flare and the BH-NS merger event can be checked by the future gravitational wave detections from advanced LIGO and Virgo.In Section 4,the occurrence rates of bright X-ray flares in z<1 GRBs with or without observed supernovae(SNe)association were compared.Our Sample I:the z<1 long GRBs(LGRBs)with SNe association(SN-GRBs)and with early Swift/X-Ray Telescope(XRT)observations,consists of 18 GRBs,among which only two GRBs have bright X-ray flares.Our Sample II:for comparison,all the z<1 LGRBs without observed SNe association and with early Swift/XRT observations,consists of 45 GRBs,among which 16 GRBs present bright X-ray flares.Thus,the study indicates a lower occurrence rate of bright X-ray flares in Sample I(11.1%)than in Sample II(35.6%).In addition,if dim X-ray fluctuations are included as flares,then 16.7%of Sample I and 55.6%of Sample II are found to have flares,again showing the discrepancy be-tween these two samples.We examined the physical origin of these bright X-ray flares and found that most of them are probably related to the central engine reactivity.To understand the discrepancy,we propose that such a lower occurrence rate of flares in the SN-GRB sample may hint at an energy partition among the GRB,SNe,and X-ray flares under a saturated energy budget of massive star explosion.In Section 5,we present time-resolved spectral analysis of the steep decay seg-ments of 29 bright X-ray flares of GRBs observed with the Swift/XRT,and model their lightcurves and spectral index evolution behaviors with the curvature effect model.Our results show that the observed rapid flux decay and strong spectral index evolution with time can be well fit with this model,and the derived characteristic timescales(tc)are in the range of 33?264 seconds.Using an empirical relation between the peak lu-minosity and the Lorentz factor derived from the prompt gamma-rays,we estimate the Lorentz factors of the flares(?x).We obtain ?x=17?87 with a median value of 52,which is smaller than the initial Lorentz factors of prompt gamma-ray fireballs.With the derived tc and ?x,we constrain the radiating regions of 13 X-ray flares,yielding Rx=(0.2?1.1)× 10¹⁶ cm,which are smaller than the radii of the afterglow fireballs at the peak times of the flares.A long evolution feature from prompt gamma-ray phase to the X-ray epoch is found by incorporating our results with a sample of GRBs whose initial Lorentz factors are available in literatures,i.e.,? ?[tp/(1+z)]-^{0.69±0 06}.These results may shed lights on the long term evolution of GRB central engines.In the end,the analytical formulae of flux evolution and spectral evolution are also shown.In Section 6,we propose a method to search for stellar-mass BH candidates with giant companions from spectroscopic observations.Based on the stellar spectra of LAMOST Data Release 6,we obtain a sample of seven giants in binaries with large radial velocity variation ?_VR>80 km s^-1.With the effective temperature,surface gravity,and metallicity provided by LAMOST,and the parallax given by Gaia,we can estimate the mass and radius of the giant,and therefore evaluate the possible mass of the optically invisible star in the binary.We show that the sources in our sample are potential BH candidates,and are worthy of dynamical measurement by further spec-troscopic observations.Our method may be particularly valid for the selection of BH candidates in binaries with unknown orbital periods.Finally,we make a prospect to the research topics in this field.