Ding-Xiong Wang

Evolution characteristics of the central black hole of a magnetized accretion disc

Evolution characteristics of a Kerr black hole (BH) are investigated by considering the coexistence of disc accretion with the Blandford-Znajek process (the BZ process) and magnetic coupling of the BH with the surrounding disc (MC process). (i) The rate of extracting energy from the rotating BH in the BZ process and that in MC process are expressed by a unified formula, which is derived by using an improved equivalent circuit. (ii) The mapping relation between the angular coordinate on the BH horizon and the radial coordinate on the disc is given in the context of general relativity and conservation of magnetic flux. (iii) The power and torque in the BZ process are compared with those in MC process in detail. (iv) Evolution characteristics of the BH and energy extracting efficiency are discussed by using the characteristic functions of BH evolution in the corresponding parameter space. (v) Power dissipation on the BH horizon and BH entropy increase are discussed by considering the coexistence of the above energy mechanisms.

LORENTZ-FACTOR–ISOTROPIC-LUMINOSITY/ENERGY CORRELATIONS OF GAMMA-RAY BURSTS AND THEIR INTERPRETATION

The bulk Lorentz factor of the gamma-ray burst (GRB) ejecta (Γ0) is a key parameter to understanding GRB physics. Liang et al. have discovered a correlation between Γ0 and isotropic γ -ray energy: Γ0 ∝ E 0.25 γ, iso,52 . By including more GRBs with updated data and more methods to derive Γ0, we confirm this correlation and obtain Γ0 � 91E 0.29 γ, iso,52 . Evaluating the mean isotropic γ -ray luminosities L γ, iso of the GRBs in the same sample, we discover an even tighter correlation Γ0 � 249L 0.30 γ, iso,52 . We propose an interpretation to this later correlation. Invoking a neutrino-cooled hyperaccretion disk around a stellar mass black hole as the central engine of GRBs, we derive jet luminosity powered by neutrino annihilation and baryon loading from a neutrino-driven wind. Applying beaming correction, we finally derive Γ0 ∝ L 0.22 γ, iso , which is consistent with the data. This suggests that the central engine of long GRBs

CALIBRATING THE CORRELATION BETWEEN BLACK HOLE MASS AND X-RAY VARIABILITY AMPLITUDE: X-RAY ONLY BLACK HOLE MASS ESTIMATES FOR ACTIVE GALACTIC NUCLEI AND ULTRA-LUMINOUS X-RAY SOURCES

A calibration is made for the correlation between the X-ray Variability Amplitude (XVA) and black hole (BH) mass. The correlation for 21 reverberation-mapped Active Galactic Nuclei (AGNs) appears very tight, with an intrinsic dispersion of 0.20 dex. The intrinsic dispersion of 0.27 dex can be obtained if BH masses are estimated from the stellar velocity dispersions. We further test the uncertainties of mass estimates from XVAs for objects that have been observed multiple times with good enough data quality. The results show that the XVAs derived from multiple observations change by a factor of 3. This means that BH mass uncertainty from a single observation is slightly worse than either reverberation-mapping or stellar velocity dispersion measurements; however, BH mass estimates with X-ray data only can be more accurate if the mean XVA value from more observations is used. With this calibrated relation, the BH mass and accretion rate may be determined for a large sample of AGNs with the planned International X-ray Observatory mission. Proper interpretation of the first AGN X-ray quasi-periodic oscillation (QPO), seen in the Seyfert galaxy REJ1034+396, depends on its BH mass, which is not currently known very well. Applying this relation, the BH mass of REJ1034+396 is found to be 4(-2)(+3) x 10(6) M(circle dot). The high end of the mass range follows the relationship between the 2f(0) frequencies of high-frequency QPO and the BH masses derived from the Galactic X-ray binaries. We also calculate the high-frequency constant C = 2.37 M(circle dot) Hz(-1) from 21 reverberation-mapped AGNs. As suggested by Gierlinski et al., M(BH) = C/C(M), where C(M) is the high-frequency variability derived from XVA. Given the similar shape of power-law dominated X-ray spectra in ultra-luminous X-ray sources (ULXs) and AGNs, this can be applied to BH mass estimates of ULXs. We discuss the observed QPO frequencies and BH mass estimates in the ULXs M82 X-1 and NGC 5408 X-1 and favor ULXs as intermediate mass BH systems.

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.

Magnetic Coupling of a Rotating Black Hole with Its Surrounding Accretion Disk

The effects of the magnetic coupling (MC) of a rotating black hole (BH) with its surrounding accretion disk are discussed in detail in the following aspects: (1) The mapping relation between the angular coordinate on the BH horizon and the radial coordinate on the disk is modified based on a more reasonable configuration of magnetic field, and a condition for the coexistence of the Blandford-Znajek (BZ) and the MC process is derived. (2) The transfer direction of energy and angular momentum in the MC process is described equivalently by the corotation radius and by the flow of electromagnetic angular momentum and redshifted energy, where the latter is based on an assumption that the theory of the BH magnetosphere is applicable to both the BZ and MC processes. (3) The profile of the current on the BH horizon and that of the current density flowing from the magnetosphere onto the horizon are given in terms of the angular coordinate of the horizon. It is shown that the current on the BH horizon varies with the latitude of the horizon and is not continuous at the angular boundary between the open and closed magnetic field lines. (4) The MC effects on disk radiation are discussed, and a very steep emissivity is produced by the MC process, which is consistent with the recent XMM-Newton observation of the nearby bright Seyfert 1 galaxy MCG -6-30-15 in a variety of parameters of the BH-disk system.

A PHYSICAL LINK BETWEEN JET FORMATION AND HOT PLASMA IN ACTIVE GALACTIC NUCLEI

Recent observations suggest that in black hole X-ray binaries jet/outflow formation is related to the hot plasma in the vicinity of the black hole, either in the form of an advection-dominated accretion flow at low accretion rates or in a disk corona at high accretion rates. We test the viability of this scenario for supermassive black holes using two samples of active galactic nuclei distinguished by the presence (radio-strong) and absence (radio-weak) of well-collimated, relativistic jets. Each is centered on a narrow range of black hole mass but spans a very broad range of Eddington ratios, effectively simulating in a statistical manner the behavior of a single black hole evolving across a wide spread in accretion states. Unlike the relationship between the radio and optical luminosity, which shows an abrupt break between high- and low-luminosity sources at an Eddington ratio of ~1%, the radio emission?a measure of the jet power?varies continuously with the hard X-ray (2-10?keV) luminosity, roughly as . This relation, which holds for both radio-weak and radio-strong active galaxies, is similar to the one seen in X-ray binaries. Jet/outflow formation appears to be closely linked to the conditions that give rise to the hot, optically thin coronal emission associated with accretion flows, both in the regime of low and high accretion rates.

A model of the light curves of Gamma-Ray Bursts

Aims. An extreme Kerr black hole (BH) surrounded by a precessing disk is invoked to explain the light curves of gamma-ray bursts (GRBs) based on the coexistence of the Blandford-Znajek (BZ) and the magnetic coupling (MC) processes. Methods. The overall shape of the light curves and the duration of GRBs are interpreted by the evolution of the halfopening angle of the magnetic flux on the BH horizon, and the complex temporal structures are modulated by the precession and nutation of the jet powered by the BZ process. Results. The time profile of the emission exhibits a fast rise and a slow decay due to the effect of the evolution of the half-opening angle. The light curves of several GRBs are well fitted by this model with only six free parameters.

Quasi-periodic variations in x-ray emission and long-term radio observations: Evidence for a two-component jet in Sw J1644+57

The continued observations of Sw J1644+57 in X-ray and radio bands accumulated a rich data set to study the relativistic jet launched in this tidal disruption event. The X-ray light curve of Sw J1644+57 from 5-30 days presents two kinds of quasi=periodic variations: a 200 s quasi=periodic oscillation (QPO) and a 2.7 day quasi=periodic variation. The latter has been interpreted by a precessing jet launched near the Bardeen-Petterson radius of a warped disk. Here we suggest that the similar to 200 s QPO could be associated with a second, narrower jet sweeping the observer line-of-sight periodically, which is launched from a spinning black hole in the misaligned direction with respect to the black holes angular momentum. In addition, we show that this two-component jet model can interpret the radio light curve of the event, especially the re-brightening feature starting similar to 100 days after the trigger. From the data we infer that inner jet may have a Lorentz factor of Gamma(j) similar to 5.5 and a kinetic energy of E-k,E-iso similar to 3.0 x 10(52) erg, while the outer jet may have a Lorentz factor of Gamma(j) similar to 2.5 and a kinetic energy of E-k,E-iso similar to 3.0 x 10(53) erg.

A Toy Model for Gamma-Ray Bursts in Type Ib/c Supernovae

A toy model for gamma-ray burst supernovae (GRB SNe) is discussed by considering the coexistence of baryon-poor outflows from black holes (BHs) and a powerful spin connection to the surrounding disk, giving rise to consistent calorimetry as described by van Putten in a variant of the Blandford-Znajek (BZ) process. In this model the half-opening angle of the magnetic flux tube on the horizon is determined by the mapping relation between the angular coordinate on the BH horizon and the radial coordinate on the surrounding accretion disk. The GRB is powered by the baryon-poor outflows in the BZ process, and the associated SN is powered by a very small fraction of the spin energy transferred from the BH to the disk in the magnetic coupling process. The timescale of the 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.

Screw Instability of the Magnetic Field Connecting a Rotating Black Hole with Its Surrounding Disk

We discuss the screw instability of the magnetic field connecting a rotating black hole (BH) with its surrounding disk based on the model of the coexistence of the Blandford-Znajek (BZ) process and the magnetic coupling (MC) process (CEBZMC). We derive criteria for the screw instability with the state of CEBZMC based on the calculations of the poloidal and toroidal components of the magnetic field on the disk. By these criteria the screw instability will occur if the BH spin and the power-law index for the variation of the magnetic field on the disk are greater than some critical values. It turns out that the instability occurs outside some critical radii on the disk. We argue that the state of CEBZMC always accompanies the screw instability. In addition, we show that the screw instability contributes only a small fraction of magnetic extraction of energy from a rotating BH.