Jet precession in neutrino-cooled disks for gamma-ray bursts:The effects of the mass and spin of a black hole

We present a model of jet precession driven by a neutrino-cooled disk around a spinning black hole to explain the quasi-periodic features observed in some gamma-ray burst light curves. The different orientations of the rotational axes between the outer part of a neutrino-cooled disk and a black hole result in precessions of the central black hole and the inner part of the disk. Hence, the jet arising from the neutrino annihilation above the inner disk is driven to precession. We find that the period of precession is positively correlated with the mass as well as the spin of a black hole.

Gamma-ray Bursts: a Probe of Black Holes

There is strong evidence for the existence of black holes (BHs) in some X-ray binaries and in most galactic nuclei based on different types of measurement, but black holes have not been definitely identified for the lack of very firm observational evidence up to now. Because direct evidence for BHs should come from determination of strong gravitational redshift, we expect an object can fall into the region near the BH horizon where radiation can be detected. Therefore the object must be a compact star such as a neutron star (NS), and intense astrophysical processes will release highly energetic radiation that is transient and fast-varying. These characteristics may point to the observed gamma-ray bursts (GRBs). Recent observations of iron lines suggest that afterglows of GRBs show properties similar to those observed in active galactic nuclei (AGNs), implying that the GRBs may originate from intense events related to black holes. A model for GRBs and after-glows is proposed here to obtain the range of gravitational redshifts (zg) of GRBs with known cosmological redshifts. Here, we provide a new method that, with a search for high-energy emission lines (X- or -γ-rays) in GRBs, one can determine the gravitational redshift. We expect zg to be 0.5 or even larger, so we can rule out the possibility of other compact objects such as NSs, and identify the central progenitors of GRBs as black holes.

Hyperaccretion after the Blandford-Znajek Process:A New Model for GRBs with X-Ray Flares Observed in Early Afterglows

We propose a three-stage model with Blandford-Znajek （BZ） and hyperaccretion process to interpret the recent observations of early afterglows of Gamma-Ray Bursts （GRBs）. In the first stage, the prompt GRB is powered by a rotating black hole （BH） invoking the BZ process. The second stage is a quiet stage, in which the BZ process is shut off, and the accretion onto the BH is depressed by the torque exerted by the magnetic coupling （MC） process. Part of the rotational energy transported by the MC process from the BH is stored in the disk as magnetic energy. In the third stage, the MC process is shut off when the magnetic energy in the disk accumulates and triggers magnetic instability. At this moment, the hyperaccretion process may set in, and the jet launched in this restarted central engine generates the observed X-ray flares. This model can account for the energies and timescales of GRBs with X-ray flares observed in early afterglows.

Dynamics and collisions of episodic jets from black holes and accretion disk systems

In many astrophysical black hole systems, episodic jets of plasma blobs have been observed, which are much faster and more powerful than continuous jets. A magnetohydrodynamical model was proposed by Yuan et al. to study the formation of episodic jets in Sgr A＊. By taking Sgr A＊ and a stellar mass black hole as examples, we modify the model of Yuan et al. by including the effects of relativity, and further study the relativistic motion and expansion of episodic jets of plasma blobs. Then we study the collision between two consecutive ejections in the modified model, and calculate the magnetic energy released in the collision. Our results show two consecutive blobs can collide with each other, and the released magnetic energy is more than 1050 erg, which supports the idea that a gamma-ray burst is powered by the collision of episodic jets, as suggested by Yuan ＆ Zhang.

Black Hole in Binary System is the Source of Cosmic Gamma-Ray Bursts and Their Counterparts

Discovery of GRB clusters allows us to determine coordinates and characteristics of their sources. The objects radiating GRBs are reliably identified with black hole binaries, including the Galactic binaries. One of the unusual GRB properties, which are determined by black hole, is revealed in that the measured arrival direction of GRB does not coincide with the real location of its source. Just this fact allows us to find the objects radiating GRBs. On the basis of the general relativity theory's effects, observed in the GRB clusters, the technique of the black hole masses measurement is developed. The calculated black hole masses for the majority of known Galactic BH binaries are presented. It is briefly shown how the incorrect interpretations of observational facts result in an erroneous idea of the GRB cosmological origin. In fact, two problems are solved in the paper: the GRB origin and the reality of BH existence.

Nucleosynthesis in the accretion disks of Type Ⅱ collapsars

We investigate nucleosynthesis inside the gamma-ray burst （GRB） accre- tion disks formed by the Type II collapsars. In these collapsars, the core collapse of massive stars first leads to the formation of a proto-neutron star. After that, an out- ward moving shock triggers a successful supernova. However, the supernova ejecta lacks momentum and within a few seconds the newly formed neutron star gets trans- formed to a stellar mass black hole via massive fallback. The hydrodynamics of such an accretion disk formed from the fallback material of the supernova ejecta has been studied extensively in the past. We use these well-established hydrodynamic models for our accretion disk in order to understand nucleosynthesis, which is mainly ad- vection dominated in the outer regions. Neutrino cooling becomes important in the inner disk where the temperature and density are higher. The higher the accretion rate （M） is, the higher the density and temperature are in the disks. We deal with accre- tion disks with relatively low accretion rates： 0.001 Mo s-1 ~ 3）/~ 0.01 Mo S--1 and hence these disks are predominantly advection dominated. We use He-rich and Si- rich abundances as the initial condition of nucleosynthesis at the outer disk, and being equipped with the disk hydrodynamics and the nuclear network code, we study the abundance evolution as matter inflows and falls into the central object. We investigate the variation in the nucleosynthesis products in the disk with the change in the initial abundance at the outer disk and also with the change in the mass accretion rate. We report the synthesis of several unusual nuclei like 31p, 39K, 43Sc＇ 35C1 and various isotopes of titanium, vanadium, chromium, manganese and copper. We also confirm that isotopes of iron, cobalt, nickel, argon, calcium, sulphur and silicon get synthe- sized in the disk, as shown by previous authors. Much of these heavy elements thus synthesized are ejected from the disk via outflows and hence they should leave their signature in observed data.

A model of low-frequency quasi-periodic oscillations in black hole X-ray binaries

A model of low-frequency quasi-periodic oscillations （LFQPOs） of black hole X-ray binaries （BHXBs） is proposed based on the perturbed magnetohydrody- namic equations of an accretion disk. It turns out that the LFQPO frequencies of some BHXBs can be fitted by the frequencies of the toroidal Alfv6n wave oscillation cor- responding to the maximal radiation flux. In addition, the positive correlation of the LFQPO frequencies with the radiation flux from an accretion disk is well interpreted.

Jet-dominated quiescent state in black hole X-ray binaries: the cases of A0620–00 and XTE J1118+480

The radiative mechanism of black hole X-ray transients （BHXTs） in their quiescent states （defined as the 2-10 keV X-ray luminosity ≤ 10^34 erg s-1） remains unclear. In this work, we investigate the quasi-simultaneous quiescent state spectrum （including radio, infrared, optical, ultraviolet and X-ray） of two BHXTs, A0620-00 and XTE J1118＋480. We find that these two sources can be well described by a coupled accretion - jet model. More specifically, most of the emission （radio up to infrared, and the X-ray waveband） comes from the collimated relativistic jet. Emission from hot accretion flow is totally insignificant, and it can only be observed in mid-infrared （the synchrotron peak）. Emission from the outer cold disk is only evident in the UV band. These results are consistent with our previous investigation on the quiescent state of V404 Cyg and confirm that the quiescent state is jet-dominated.

Annihilation luminosity of a neutrino-cooled accretion disk in a gamma-ray burst

We discuss how the annihilation luminosity of a neutrino-cooled accretion disk in a gamma-ray burst, Lνν, is determined by the disk’s fundamental parameters, namely, the mass of the central black hole M, the mass accretion rate M, and the viscosity parameter α. It is shown that Lνν depends mainly on M in evidence, and decreases with increasing M, but is almost independent of α. This result argues additionally that the central black hole in a gamma-ray burst must be with a stellar mass.

A magnetic reconnection model for quasi-periodic oscillations in black hole systems

The quasi-periodic oscillations （QPOs） in black hole （BH） systems with different scales are interpreted based on the magnetic reconnection of large-scale mag- netic fields generated by toroidal electric currents flowing in the inner region of the accretion disk, where the current density is assumed to be proportional to the mass density of the accreting plasma. The magnetic connection （MC） is taken into account in resolving dynamic equations describing the accretion disk, in which the MC be- tween the inner and outer disk regions, between the plunging region and the disk, and between the BH horizon and the disk are involved. It turns out that a single QPO frequency associated with several BH systems with different scales can be fitted by in- voking the magnetic reconnection due to the MC between the inner and outer regions of the disk, including the BH binaries XTE J1859＋226, XTE J1650-500 and GRS 1915＋105 and the massive BHs in NGC 5408 X-1 and RE J1034＋396. In addition, the X-ray spectra corresponding to the QPOs for these sources are fitted based on the typical disk-corona model.

Ignition of neutrino-dominated accretion disks

The vertical structure of neutrino-dominated accretion flows (NDAFs) in spherical coordinates is revisited.We define the ratio of the neutrino cooling rate to the viscous heating rate per unit volume,f=q v/q vis.The ignition region is presented when f >0.5.The solutions show that NDAF is significantly thick.The ignition region is determined by the mass accretion rate and the viscosity parameter,which can be considered as the neutrino radiation-dominated region.

A magnetic model for low/hard state of black hole binaries

A magnetic model for the low/hard state （LHS） of two black hole X-ray binaries （BHXBs）, H1743-322 and GX 339-4, is proposed based on transport of the magnetic field from a companion into an accretion disk around a black hole （BH）. This model consists of a truncated thin disk with an inner advection-dominated accretion flow （ADAF）. The spectral profiles of the sources are fitted in agreement with the data observed at four different dates corresponding to the rising phase of the LHS. In addition, the association of the LHS with a quasi-steady jet is modeled based on transport of magnetic field, where the Blandford-Znajek （BZ） and Blandford-Payne （BP） processes are invoked to drive the jets from BH and inner ADAE It turns out that the steep radio/X-ray correlations observed in H 1743-322 and GX 339-4 can be interpreted based on our model.

Relation between the “Double-Hump” Behavior in the Radio Band and the Broad-Line Luminosity for Blazars

The physics behind the spectral energy distribution （SED） of blazars remains open. We assembled 36 blazars to tackle the factors that control their SED. Now, many blazar spectra have the ＂double hump＂ feature in the radio and far-IR frequencies. For these a parameter, △, is created to characterize the behavior of the SED. We found a significant correlation between the broad-line luminosity （LBLR） and △. Because L BLR is an indicator of the accreting power of the source in blazars, we derived a linear correlation, △ oc M1/3.18, which suggests that the SED of blazars may depend on the accretion rate, like that of BL Lac objects. We also found a significant correlation between m and △ for a sample of 11 blazars （out of one of 36） with available black hole masses. This implies the Eddington accretion ratio may influence the shape of the SED of blazars.

The correlation timescale of the X-ray flux during the outbursts of soft X-ray transients

Recent studies of black hole and neutron star low mass X-ray binaries(LMXBs) show a positive correlation between the X-ray flux at which the low/hard(LH)-to-high/soft(HS) state transition occurs and the peak flux of the following HS state.By analyzing the data from the All Sky Monitor(ASM) onboard the Rossi X-ray Timing Explorer(RXTE),we show that the HS state flux after the source reaches its HS flux peak still correlates with the transition flux during soft X-ray transient(SXT) outbursts.By studying large outbursts or flares of GX 339-4,Aql X-1 and 4U 1705-44,we have found that the correlation holds up to 250,40,and 50 d after the LH-to-HS state transition,respectively.These time scales correspond to the viscous time scale in a standard accretion disk around a stellar mass black hole or a neutron star at a radius of-104-5 Rg,indicating that the mass accretion rates in the accretion flow either correlate over a large range of radii at a given time or correlate over a long period of time at a given radius.If the accretion geometry is a two-flow geometry composed of a sub-Keplerian inflow or outflow and a disk flow in the LH state,the disk flow with a radius up to-105 Rg would have contributed to the nearly instantaneous non-thermal radiation directly or indirectly,and therefore affects the time when the state transition occurs.

The Correlation between Optical Spectral Index and Continuum Luminosity Variation in the Seyfert 1 Galaxy NGC 5548

Using the archived optical spectra of NGC 5548 between 1989 and 2001, we derived the optical spectral index by fitting the spectra in wavelength windows unaffected by strong emission lines. We found that the index is anti-correlated with the continuum luminosity at 5100 A with a correlation coefficient of -0.8. Based on the standard thin accretion disk model, we investigated whether the correlation is related to the variations of the dimensionless accretion rate m (mass accretion rate in Eddington unit), or the inner radius of the accretion disk Rin, or both. The correlation can be modeled well using a co-variable mode of Rin/Rs = 12.5m-0.8 (Rs is Schwarzschild radius). As luminosity increases, m increases from 0.05 to 0.16 and at the same time Rin decreases from 133.9.RS to 55.5.RS, consistent with the prediction for a transition radius within which an ADAF structure exists. We concluded that the change of both inner accretion radius and the dimensionless accretion rate are key factors for the variations of spectral index and luminosity in the optical band for NGC 5548.