Blandford-Znajek Process

Gamma-ray bursts (GRBs) are short, sudden and intense flashes of gamma rays from the cosmos distance, the study of which is a hot topic in astronomy and astrophysics.It is generally believed that GRBs may originate from the merger of a compact binaries, such as neutron star-neutron star binaries and black hole(BH)-neuron star binaries, or from the collapse of a massive star. The common feature of these scenarios is the formation or existence of a rotating BH with its surrounding disk (or tours) at the center region. If the magnetic field threading the black hole is strong enough ( B ~1015gauss ), then a rapid extraction of energy is possible with 10 53ergs , it may power a GRB.The mechanism of extracting rotational energy from a Kerr black hole magnetically is called Blandford-Znajek (BZ) process. It is originally suggested to explain the jets from active galaxy nuclei (AGNs) and quasars. In the BZ process, the BH horizon and the remote astrophysics load are connected by the open magnetic field lines, and energy and angular momentum are extracted from a rotating BH and transported to the remote load. Magnetic coupling (MC) process can be regarded as a variation of the BZ process, in which the BH horizon and the disk are connected by closed magnetic field lines, and energy and angular momentum are transported between the BH and the disc. In the MC process, the transportation direction is related intimately to the BH spin, if the BH rotates faster than the disc, energy and angular momentum are extracted from the BH and transferred to the disc; on the contrary if the black hole rotates slower than the disc, energy and angular momentum are transferred from the disc to the BH.The BZ process can provide clean energy for GRBs, and thus it can be a competent mechanism for powering GRBs. The study in my thesis focuses on the BZ mechanism as a"central engine".(1) We have studied the evolutionary characteristics of BH in the center of GRB, and the rotational energies extracted by the BZ process. The black hole may be surrounded by a transient disk. It is found that the ratio k≡ΩF /ΩH affects the evolutionary characteristics of the BH significantly. Our calculation shows that the system of a BH accretion disk with k > 0.5118 might provide a much higher output energy in a shorter timescale for gamma-ray bursts than the BH system with k=0.5.(2) We have studied the"inner engine"for GRB/supernovae (GRB/SNe). Recently, the observations and theoretical considerations have linked long-duration GRBs with Type Ib/c SNe. We proposed a toy model for GRB/SNe by considering the coexistence of the BZ and MC process. GRB is powered by the BZ process, and the associated SN is powered by the MC process. The timescale of GRB is fitted by the duration of the open magnetic flux on the horizon. It turns out that the data of several GRB/SNe are well fitted with our model.(3) We studied the effects of screw instability on extracting energy from a rotating BH. The magnetic field with both poloidal and tororidal components may be instable. We find that screw-instability in the MC process always occurs when the BZ and MC processes coexist. Our calculation shows that the screw instability affects the configuration of the magnetosphere and the evolution characteristics of the BH, moreover it augments the energy extracted in the BZ and MC processes.(4) We studied the variability in light curves of GRBs. The magnetic in the BZ process can be screw unstable, either. We suggested the variability could be fitted by a series of flares arising from the screw instability in the BZ process. The screw instability will occur repeatedly because of the twist of the poloidal magnetic field arising from the rotation of the BH. Inductors are introduced into the equivalent circuits to modulate the screw instability. It is found that the variability timescale of GRBs can be estimated by the relax time of the R-L circuit.