Research On Millisecond Magnetars And Their Gravitational Wave Radiation

A newly born millisecond magnetar with a strong dipole magnetic field(B～1014-1015G)and a very short spinning period(P～1-10 ms)can convert its rotational energy into electromagnetic radiation and/or gravitational waves,providing energy for various high-energy phenomena,such as gamma-ray bursts,kilonovae/merger novae,superluminous supernovae.The formation of millisecond magnetars includes instantaneous channel and delay channel.The collapse of massive stars is thought to be an instantaneous channel for magnetar formation,thus newly born magnetars are associated with long gamma-ray bursts and core-collapse supernovae.The delay channel includes double neutron star merger、neutron star and white dwarf merger、double white dwarf merger and white dwarf accretion induced collapse.Therefore,millisecond magnetars are considered as possible central engines for short gamma-ray bursts and kilonovae.The transient sources with millisecond magnetars as the central energy source are a popular direction of time-domain astronomy research.In Chapter 1,we give a brief overview of the connections between millisecond magnetars and gamma-ray bursts,kilonovae and superluminous supernovae,which include the basic radiation theory of magnetars,the most popular model of magnetars driving these transient sources,and indirect evidence for magnetars as central engines for these transient sources.In addition,we introduce the mechanisms that trigger continuous gravitational radiation from magnetar,e.g.,elastic and/or magnetically driven deformation,stellar surface mountain supported by elastic strain or magnetic field,and free precession or unstable oscillation modes.Gravitational wave astronomy is crucial to verifying the most mysterious and extreme matter in the universe-neutron stars.In Chapter 2,we searched for the signatures of gravitational wave in the electromagnetic observation data of short gamma-ray bursts.The joint detection of gravitational wave event GW170817 and its electromagnetic counterpart GRB 170817A confirms that at least some of the short gamma-ray bursts are associated with binary neutron star mergers.The remnants of a binary neutron star merger may be a long-lived,rapidly rotating and highly magnetized magnetar that can emit extended gravitational waves due to its non-axisymmetric deformation or fluid oscillations.We use the magnetar model to simulate the light curve of the short gamma-ray burst sample,and the results show that there are gravitational wave radiation signatures in the light curves of GRB 090426 and GRB 150424A.By comparing the characteristic time scale of magnetic dipole radiation and the characteristic time scale of gravitational wave radiation,we can constrain the e1lipticity of newborn magnetars:ε<1.58 ×10-3(B/1015 G)(P/1 ms).We found that the the gravitational-wave signals from magnetars may not be detectable by the aLIGO and ET detectors.For a rapidly spinning magnetar(P 1 ms),the detection horizons for aLIGO 03,aLIGO 05 and ET detectors are 60 Mpc,210 Mpc and 900 Mpc,respectively.Detection of the GW emission from new-born millisecond magnetar may reveal the interior composition of magnetar in the near future.In Chapter 3,We use observations of gamma-ray burst to constrain the ellipticity of millisecond magnetars.Observations of Long GRBs and their afterglows show that a number of LGRBs are accompanied by an X-ray plateau,which is usually interpreted as the spin-down luminosity of millisecond magnetars.Millisecond magnetars may undergo non-axisymmetric deformations or various stellar oscillations,emitting continuous gravitational wave radiation associated with the X-ray plateau.Under the assumption of the magnetar model,we analyzed the X-ray light curves of 30 long gamma-ray bursts.We utilize the observations of the LGRB plateau to constrain the properties of the new-born magnetar,including the initial spin period P0,diploe magnetic field strength Bp and the ellipticity ε.We find that there are some tight relations between magnetar parameters,e.g.,ε ∝ Bp1.29 and Bp∝P01.14.The correlation between ε-Bp suggests that magnetars with stronger magnetic fields correspond to larger ellipticities.The relationship of log Bp-log P0 indicates that the magnetic dipole radiation and the gravitational wave radiation may occur in the magnetic propeller stage.The ellipticity of most long GRBs are constrained to be less than about 10-3,implying that the magnetar would lose a large amount of spin-down energy via gravitational wave radiation if the ellipticity ε>10-3.Furthermore,we deduce the gravitational wave strain through the spin-down process of the magnetar.The detection of such GW signal associated with the X-ray plateau would be a smoking gun that the central engine of GRB is a magnetar.In Chapter 4,we extend the existing magnetar-driven supernova model to consider the effect of gravitational wave radiation on the luminosity evolution of supernovae.Focusing on the effect of gravitational wave radiation on the loss of the magnetar rotational energy,we study the characteristics of the electromagnetic radiation produced by this supernova.There is a competition between the magnetic dipole dominated spin down and the gravitational wave dominated spin down.The loss of gravitational wave energy results in a reduction in the magnetic dipole radiation energy available for injection into the supernova,which will affect the luminosity evolution of the supernova,such as the peak luminosity and its decay rate during decline phase.The luminosity of supernovae SN 1997ef,SN 2009bb,and SN 2017ens show slow decay behavior,which is inconsistent with the 56Ni decay model.However,our magnetar model can reproduce the luminosity evolution of SN 1997ef,SN 2009bb,SN 2017ens.By using of Markov Chain Monte Carlo(MCMC)method can obtain the best fitting model and posterior parameters.Based on the model,we find that the ellipticity value of the magnetar is constrained to about 10-4,if the magnetar deformation is induced by the magnetic pressure of the internal magnetic field,which means that the internal magnetic field strength needs to reach 1016 G.In Chapter 5,we briefly summarize the above work,and make an outlook on the research on magnetars and their gravitational waves.