Zhao-Ming Gan

Magnetically Torqued Neutrino-dominated Accretion Flows for Gamma-ray Bursts

Recent observations and theoretical work on gamma-ray bursts favor the central engine model of a Kerr black hole (BH) surrounded by a magnetized neutrino-dominated accretion flow (NDAF). The magnetic coupling between the BH and the disk through a large-scale closed magnetic field exerts a torque on the disk and transports the rotational energy from the BH to the disk. We investigate the properties of the NDAF with this magnetic torque. For a rapid spinning BH, the magnetic torque transfers enormous rotational energy from BH into the inner disk. There are two consequences: (1) the luminosity of neutrino annihilation is greatly augmented; (2) the disk becomes thermally and viscously unstable in the inner region, displaying s-shaped curves of the surface density versus accretion rate. It turns out that the magnetically torqued NDAF can be invoked to interpret the variability of gamma-ray luminosity. In addition, we discuss the possibility of restarting the central engine to produce the X-ray flares with required energy.

A model of magnetically induced disc–corona for black hole binaries

We propose a model of magnetic connection (MC) of a black hole with its surrounding accretion disc based on large-scale magnetic field. The MC gives rise to transport of energy and angular momentum between the black hole and the disc, and the closed field lines pipe the hot matter evaporated from the disc, and shape it in the corona above the disc to form a magnetically induced disc–corona system, in which the corona has the same configuration as the large-scale magnetic field. We numerically solve the dynamic equations in the context of the Kerr metric, in which the large-scale magnetic field is determined by dynamo process and equipartition between magnetic pressure and gas pressure. Thus we can obtain a global solution rather than assuming the distribution of large-scale magnetic field beforehand. The main MC effects lie in three aspects. (1) The rotational energy of a fast-spinning black hole can be extracted, enhancing the dissipation in the accretion disc, (2) the closed field lines provide a natural channel for corona matter escaping from disc and finally falling into black hole and (3) the scope of the corona can be bounded by the conservation of magnetic flux. We simulate the high-energy spectra of this system by using Monte Carlo method, and find that the relative hardness of the spectra decreases as accretion rate or black hole spin a∗ increases. We fit the typical X-ray spectra of three black hole binaries (GRO J1655−40, XTE 1118+480 and GX 339−4) in the low/hard or very high state.

A resonance model with magnetic connection for 3:2 HFQPO pairs in black hole binaries

We apply epicyclic resonances to the magnetic connection (MC) of a black hole (BH) with a relativistic accretion disc, interpreting the high frequ ency quasi-periodic oscillations (HFQPOs) with 3:2 pairs observed in three BH X-ray binaries. It turns out that the 3:2 HFQPO pairs are associated with the steep power-law states, and the severe damping can be overcome by transferring energy and angular momentum from a spinning BH to the inner disc in the MC process.

A simplified model of jet power from active galactic nuclei

Aims. A simplified model of jet power from active galactic nuclei is proposed in which the relationship between jet power and disk luminosity is discussed by combining disk accretion with two mechanisms of extracting energy magnetically from a black hole accretion disk, i.e., the Blandford-Payne (BP) and the Blandford-Znajek (BZ) processes. Methods. By including the BP process into the conservation laws of mass, angular momentum and energy, we derive the expressions of the BP power and disk luminosity, and the jet power is regarded as the sum of the BZ and BP powers. Results. We find that the disk radiation flux and luminosity decrease because a fraction of the accretion energy is channelled into the outflow/jet in the BP process. It is found that the dominant cooling mode of the accretion disk is determined mainly by how the poloidal magnetic field decreases with the cylindrical radius of the jet. By using the parameter space we found, which consists of the black hole spin and the self-similar index of the configuration of the poloidal magnetic field frozen in the disk, we were able to compare the relative importance of the following quantities related to the jet production: (1) the BP power versus the disk luminosity, (2) the BP power versus the BZ power, and (3) the jet power versus the disk luminosity. In addition, we fit the jet power and broad-line region luminosity of 11 flat-spectrum radio quasars (FSRQs) and 17 steep-spectrum radio quasars (SSRQs) based on our model.

A toy model for magnetized neutrino-dominated accretion flows

In this paper, we present a simplified model for a magnetized neutrino-dominated accretion flow (NDAF) in which the effect of black hole (BH) spin is taken into account by adopting a set of relativistic correction factors, and the magnetic field is parameterized as β, the ratio of the magnetic pressure to the total pressure. It is found that the disc properties are sensitive to the values of the BH spin and β, and more energy can be extracted from NDAFs by using a faster spin and lower β.

Effects of magnetic coupling on radiation from accretion disc around a Kerr black hole

ABSTRACT The effects of magnetic coupling (MC) process on the inner edge of the disc are dis-cussed in detail. It is shown that the inner edge can deviate from the innermost stablecircular orbit (ISCO) due to the magnetic transfer of energy and angular momentumbetween a Kerr black hole (BH) and its surrounding accretion disc. It turns out thatthe inner edge could move inward and outward for the BH spin a ∗ being greater andless than 0.3594, respectively. The MC effects on disc radiation are discussed based onthe displaced inner edge. A very steep emissivity can be provided by the MC process,which is consistent with the observation of MCG-6-30-15. In addition, the BH spinsof GRO J1655-40 and GRS 1915+105 are detected by X-ray continuum fitting basedon this model.Key words: accretion, accretion disc— black hole physics — magnetic field 1 INTRODUCTIONAs is well shown, disc accretion is a very efficient energysource in astrophysics, which is widely used to explain thehigh-energy radiation of AGNs, X-ray binaries and so on. Inthe model of standard accretion disc (SAD), the motion ofthe accreting matter is assumed to be Keplerian with a smallradial velocity, and the inner edge of the disc lies at the in-nermost stable circular orbit (ISCO) of radius r

Magnetic connection and current distribution in black hole accretion discs

We discuss one of the possible origins of large-scale magnetic fields based on a continuous distribution of toroidal electric current flowing in the inner region of the disc around a Kerr black hole (BH) in the framework of general relativity. It turns out that four types of configuration of the magnetic connection (MC) are generated, i.e. MC of the BH with the remote astrophysical load (MCHL), MC of the BH with the disc (MCHD), MC of the plunging region with the disc (MCPD) and MC of the inner and outer disc regions (MCDD). It turns out that the Blandford-Znajek process can be regarded as one type of MC, i.e. MCHL. In addition, we propose a scenario for fitting the quasi-periodic oscillations in BH binaries based on MCDD associated with the magnetic reconnection.

Energy dissipation and angular momentum transfer within a magnetically torqued accretion disc

We discuss transportation and redistribution of energy and angular momentum in the magnetic connection (MC) process and Blandford-Payne (BP) process. MC results in readjusting the interior viscous torque, and its effects are operative not only in but also beyond the MC region. The BP process is invoked to transfer the “excessive” angular momentum from an accretion disc. In addition, we derive a criterion for the interior viscous torque to resolve the puzzle of the overall equilibrium of angular momentum in disc accretion. It turns out that the efficiency of BP at extracting angular momentum and the intensity of the outflow are required to be greater than some critical values.

Association of the 3:2 HFQPO Pairs with the Broad Fe K Line in XTE J1550-564 and GRO J1655-40

Association of the high-frequency quasi-periodic oscillation (HFQPO) pairs with the broad Fe K line in XTE J1550-564 and GRO J1655-40 is discussed based on the magnetic coupling (MC) of a rotating black hole (BH) with its surrounding disc. The 3:2 HFQPO pairs are interpreted by virtue of the inner and outer hotspots arising from non-axisymmetric magnetic field, where the inner hotspot is produced by a torque exerted at the inner edge of the disc, and the outer hotspot is created by the screw instability of the large-scale magnetic field. The very steep emissivity index is created predominantly by the torque exerted at the inner edge of the disc. It turns out that the 3:2 HFQPO pairs observed in the two sources can be fitted by tuning several model parameters, such as the BH spin, and the main features of this model lie in three aspects. (1) The condition for only one HFQPO is discussed based on the two mechanisms for producing the 3:2 HFQPO pairs, (2) an explanation is given for a systematic shift away from disc dominated flux with the increasing power-law flux as the HFQPO pairs shift from the higher to lower frequencies, which is consistent with the analysis given by Remillard et al. (2002), and (3) the BH spin in XTE J1550-564 and GRO J1655-40 can be estimated by combining the 3:2 HFQPO pairs with the very steep emissivity index required for fitting the broad Fe K emission line.

Correlations of Astrophysical Phenomena in Black Hole Systems of Different Scales

Correlation of a puzzling 3:2 ratio of High frequency quasi‐periodic oscillations with the jets in black hole X‐ray binaries are discussed based on the model of the coexistence of the Blandford‐Znajek mechanism and the Magnetic Coupling mechanism. Similar astrophysical phenomena in black hole systems of different scales are also discussed.