Study Of Newborn Magnetars As The Central Engines Of High-energy Transients

A newborn magnetar is a rapidly spinning,strongly magnetized neutron star formed either by the collapse of massive stars or the merger of compact stars.Studies indicate that newborn magnetars are powerful central engines for some outbursts,such as gamma-ray bursts(GRBs)and fast X-ray transients(FXTs).In this thesis,we systematically review relevant research on magnetars over the past decade and focus on our studies on the early evolution of newborn magnetars.There have been a lot of theoretical studies on the energy mechanism of newborn magnetars as GRB s.In theory,the most straightforward way of tapping energy from a newborn magnetar is through the spin-down mechanism,which suggests that the newborn magnetar can not only drive an extreme relativistic jet but also power an XT through the magnetic dipole wind radiation.The features of the injected kinetic luminosity to the magnetic dipole wind from the spindown of a magnetar exhibit a plateau or a shallow decay phase,then it shows a rapid decay phase after a characteristic time scale.Such features have been found in some early X-ray afterglows observed by the Swift/X-ray Telescope(XRT).Based on these features,we conduct a comprehensive analysis of the Swift/XRT observations to study the properties of newborn magnetars and their early evolution.We first study long-duration GRBs(1GRBs),which originated from the collapse of massive stars,and build a sample of 1GRBs driven by newborn magnetars.Based on the magnetic dipole wind radiation model,we calculate the magnetic field strength(Bp),the initial spin period(P0),and the wind energy(Ewind)of magnetars,and find that the relation of the isotropic jet energy(Ejet,iso)and Ewind is Ewind ∝ Ejet,iso0.91±0.07,and the relation between P0 and Ejet,iso,is P0∝ Ejet,iso-0.36±0.03.These results show that the energy of GRBs and XTs originates from the rotation energy of newborn magnetar,and the quantitative relation is given.Within the jet&wind co-existing paradigm for GRB s driven by magnetars,non-detection of the jet emission may be due to off-axis observations of the jet.In this situation,we can detect the bright isotropic wind emission.We analyze the peculiar afterglows of GRB 101225A and propose that GRB 101225A should be an off-axis event.Its X-ray fluxes decay as F∝t-1,then rapid drop with a decay slope of-6,which indicates that the magnetar spin-down is gravitational wave(GW)emission dominated and the magnetar collapses to a black hole at a later time.Interestingly,we find quasi-periodic oscillation features in the X-ray flares of GRB 101225A at the time interval of 4900-7500 s.We successfully explain the lightcurves of GRB 101225A based on the magnetar precession model.Similarly,we find a quasi-periodic oscillation signal of～650 s with a higher confidence level in the early X-ray plateau of GRB 180620A.We adopt the same model to explain its lightcurves,and study the early evolution of parameters,such as P0 and ellipticity((?)).The remnant of compact star mergers may also be magnetars,associated with some short-duration GRBs(sGRBs).We systematically analyze the early X-ray observations of sGRB s from the Swift/XRT and build a sample of sGRB s driven by newborn magnetars.By calculating the physical parameters of these magnetars,we compare the characteristics of magnetars born in the collapse of massive stars and the merger of compact objects.We show that the typical braking index(n)of the magnetars is～3 in the sGRB sample and～4 in the 1GRB sample.The difference of n implies that the magnetar spin-down tends to be dominated by the magnetic dipole radiations(DRs)in the sGRBs sample,while it is co-operated by both the magnetic DRs and the GW emission in the 1GRBs sample.The Bp and P0 of newborn magnetars in the sGRBs sample are one order of magnitude higher than those in the 1GRBs sample,but both of them follow the same relation,i.e.,magnetars with faster rotation speed can power a more energetic jet,being independent of its formation approach.FXTs powered by newborn magnetars are the most prospective electromagnetic counterparts of the GWs.Due to the lack of a wide file-of-view(FoV),highly sensitive X-ray survey for the XTs,the understanding of them is very limited.Based on the results of the population synthesis numerical simulation,the possible XTs have been observed in the Chandra mission,as well as the early X-ray afterglow data of GRBs,we study the event rate density and luminosity function of XTs powered by the magnetars.And we also calculate the detectable XTs with the up-coming Einstein Probe(EP)mission.The EP survey mission will detect more events driven by newborn magnetars.The observations by EP and the Chinese-French Space Variable Objects Monitor(SVOM)over the next five years will refresh our understanding of newborn magnetars and their transients.