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 possible origin of viscosity in Keplerian accretion disks due to secondary perturbation:Turbulent transport without magnetic fields

The origin of hydrodynamic turbulence in rotating shear flow is a long standing puzzle.Resolving it is especially important in astrophysics when the flow＇s angular momentum profile is Keplerian which forms an accretion disk having negligible molecular viscosity.Hence,any viscosity in such systems must be due to turbulence,arguably governed by magnetorotational instability,especially when temperature T ≥10 5.However,such disks around quiescent cataclysmic variables,protoplanetary and star-forming disks,and the outer regions of disks in active galactic nuclei are practically neutral in charge because of their low temperature,and thus are not expected to be coupled with magnetic fields enough to generate any transport due to the magnetorotational instability.This flow is similar to plane Couette flow including the Coriolis force,at least locally.What drives their turbulence and then transport,when such flows do not exhibit any unstable mode under linear hydrodynamic perturbation？ We demonstrate that the three-dimensional secondary disturbance to the primarily perturbed flow that triggers elliptical instability may generate significant turbulent viscosity in the range 0.0001 ≤νt≤ 0.1,which can explain transport in accretion flows.

Comptonization and Reprocessing Processes in Accretion Disks: Applications to the Seyfert 1 Galaxies NGC 5548 and NGC 4051

Simultaneous multi-wavelength observations have revealed complex variability in AGNs. To explain the variability we considered a theoretical model consisting of an inner hot comptonizing corona and an outer thin accretion disk, with interactions between the two components in the form of comptonization and reprocessing. We found that the variability of AGNs is strongly affected by the parameters of the model, namely, the truncated disk radius rmin, the corona radius rs, the temperature KTe and the optical depth TO of the corona. We applied this model to the two best observed Seyfert 1 galaxies, NGC 5548 and NGC 4051. Our model can reproduce satisfactory the observed SEDs. Our fits indicate that NGC 5548 may have experienced dramatic changes in physical parameters between 1989-1990 and 1998, and that NGC 4051 has a much larger truncated disk radius （700 Schwarzschild radii） than NGC 5548 （several tens of Schwarzschild radii）. Since we adopted a more refined treatment of the comptonization process rather than simply assuming a cut-off power law, our results should be more reasonable than the previous ones.

Numerical simulation of the Hall effect in magnetized accretion disks with the Pluto code

We investigate the Hall effect in a standard magnetized accretion disk which is accompanied by dissipation due to viscosity and magnetic resistivity. By consider- ing an initial magnetic field, using the PLUTO code, we perform a numerical magne- tohydrodynamic simulation in order to study the effect of Hall diffusion on the physi- cal structure of the disk. Current density and temperature of the disk are significantly modified by Hall diffusion, but the global structure of the disk is not substantially affected. The changes in the current densities and temperature of the disk lead to a modification in the disk luminosity and radiation.

Transition from radiatively inefficient to cooling dominated phase in two temperature accretion disks around black holes

We investigate the transition of a radiatively inefficient phase of a viscous two temperature accreting flow to a cooling dominated phase and vice versa around black holes. Based on a global sub-Keplerian accretion disk model in steady state, including explicit cooling processes self-consistently, we show that general advective accretion flow passes through various phases during its infall towards a black hole. Bremsstrahlung, syn- chrotron and inverse Comptonization of soft photons are considered as possible cooling mechanisms. Hence the flow governs a much lower electron temperature ～10^8 - 10^9.5 K compared to the hot protons of temperature ～10^10.2 - 10^11.8 K in the range of the accretion rate in Eddington units 0.01≤M≤ 100. Therefore, the solutions may potentially explain the hard X-rays and the γ-rays emitted from AGNs and X-ray binaries. We finally compare the solutions for two different regimes of viscosity and conclude that a weakly viscous flow is expected to be cooling dominated compared to its highly viscous counterpart which is radiatively inefficient. The flow is successfully able to reproduce the observed luminosities of the under-fed AGNs and quasars （e.g. Sgr A＊）, ultra-luminous X-ray sources （e.g. SS433）, as well as the highly luminous AGNs and ultra-luminous quasars （e.g. PKS 0743-67） at different combinations of the mass accretion rate and ratio of specific heats.

Structure of neutrino-dominated accretion disks with magnetic fields

The neutrino-dominated disk is regarded as a plausible model for the central engine of Gamma-Ray Bursts. By taking into account magnetic fields and detailed microphysics, we calculate the structure of neutrino-dominated disks in the frame of the well-known Paczy nski-Witta potential. The results show that the temperature of the disk is lower than that without magnetic fields, whereas the density is nearly the same as that without magnetic fields. There also exists a significant difference in the electron degeneracy between the above two cases. In addition, we present the variation of the electron fraction with the radius.

Launching jets from accretion belts

We propose that sub-Keplerian accretion belts around stars might launch jets. The sub-Keplerian inflow does not form a rotationally supported accretion disk, but it rather reaches the accreting object from a wide solid angle. The basic ingredients of the flow are a turbulent region where the accretion belt interacts with the accreting object via a shear layer, and two avoidance regions on the poles where the accretion rate is very low. A dynamo that is developed in the shear layer amplifies magnetic fields to high values. It is likely that the amplified magnetic fields form polar outflows from the avoidance regions. Our speculative belt-launched jets model has implications on a rich variety of astrophysical objects, from the removal of common envelopes to the explosion of core collapse supernovae by jittering jets.

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.

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.

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.

Exploring the Accretion Disk Structure and X-ray Radiation of GX 17+2 Based on kHz QPOs and Cross-correlations

Applying the timing tools of kilohertz quasi-periodic oscillations(k Hz QPOs)and cross-correlations,we study the influence of the magnetosphere-disk relation on the X-ray radiation process of GX 17+2.First,as the spectral state track of X-ray emission evolves along the horizontal branch(HB),the magnetosphere-disk radii of the source derived by k Hz QPOs shrink from r~24 km to r~18 km,while its average X-ray intensities in≤10 ke V and in≥10 ke V show the opposite evolutional trends.Moreover,this branch has been detected with the anti-correlations between the low-/high-energy(e.g.,2–5 ke V/16–30 ke V)X-rays.We suggest that in HB there may exist an X-ray radiation transfer process at the disk radii near the neutron star(NS),i.e.,~5–10 km away from the surface,which probably originates from the interaction between the corona or jet with high-energy X-rays and accretion disk with low-energy X-rays.Second,as the source evolves along the normal branch(NB)and along the flaring branch(FB),their average X-ray intensities in all~2–30 ke V show the monotonously decreasing and monotonously increasing trends,respectively.In addition,these two branches are both dominated by the positive correlations between the low-and high-energy(e.g.,2–5 ke V/16–30 ke V)X-rays.Moreover,the evolution along NB is accompanied by the shrinking of the magnetosphere-disk radii from r~18 km to r~16 km.We ascribe these phenomena to that as the shrinking of the accretion disk radius,the piled up accretion matter around the NS surface may trigger the radiation that produces both the low-and high-energy X-rays simultaneously,and then form the branches of NB and FB.

A Monte Carlo study of the spectra from inhomogeneous accretion flow

A model of an inhomogeneous accretion flow,in which cold clumps are surrounded by hot gas or corona,has been proposed to explain the spectral features of black hole X-ray binaries.In this work,we try to find possible observational features in the continuum that can indicate the existence of clumps.The spectra of an inhomogeneous accretion flow are calculated via the Monte Carlo method.Since the corresponding accretion flow is unsteady and complex,the accretion flow is described by a set of free parameters,the ranges of which can include the real cases.The influences of the parameters are investigated.It is found that the thermal component of the spectra deviates from multi-color black body spectra in the middle power-law part.On one hand,a warp appears due to the gaps between the clumps and the outer cold disk,and on the other hand,the slope of the line connecting the thermal peaks deviates from 4/3.The warp feature,as well as the correlation between the thermal peak at higher frequency and the spectral index,possibly indicate the existence of clumps,and are worthy of further investigation with more self-consistent models.

On the torque exerted by a warped, magnetically threaded accretion disk

Most astrophysical accretion disks are likely to be warped.In X-ray binaries,the spin evolution of an accreting neutron star is critically dependent on the interaction between the neutron star magnetic field and the accretion disk.There have been extensive investigations on the accretion torque exerted by a coplanar disk that is magnetically threaded by the magnetic field lines from the neutron stars,but relevant works on warped/tilted accretion disks are still lacking.In this paper we develop a simplified twocomponent model,in which the disk is comprised of an inner coplanar part and an outer,tilted part.Based on standard assumption on the formation and evolution of the toroidal magnetic field component,we derive the dimensionless torque and show that a warped/titled disk is more likely to spin up the neutron star compared with a coplanar disk.We also discuss the possible influence of various initial parameters on the torque.

Accretion properties of MAXI J1813-095 during its failed outburst in 2018

We present the results obtained from detailed timing and spectral studies of a black hole candidate MAXI J1813-095 using Swift,NICER,and NuSTAR observations during its 2018 outburst.The timing behavior of the source is mainly studied by examining NICER light curves in the 0.5−10 keV range.We did not find any signature of quasi-periodic oscillations in the power density spectra of the source.We carry out spectral analysis with a combined disk blackbody&power law model,and physical two-component advective flow(TCAF)model.From the combined disk blackbody&power-law model,we extracted thermal and non-thermal fluxes,photon index and inner disk temperature.We also find evidence for weak reflection in the spectra.We have tested the physical TCAF model on a broadband spectrum from NuSTAR and Swift/XRT.The parameters like mass accretion rates,the size of Compton clouds and the shock strength are extracted.Our result affirms that the source remained in the hard state during the entire outburst which indicates a‘failed’outburst.We estimate the mass of the black hole as 7.4±1.5M⊙from the spectral study with the TCAF model.We apply the LAOR model for the Fe K line emission.From this,the spin parameter of the black hole is ascertained as a^(∗)>0.76.The inclination angle of the system is estimated to be in the range of 28°−45°from the reflection model.We find the source distance to be∼6 kpc.

Effect of non-stationary accretion on spectral state transitions:An example of a persistent neutron star LMXB 4U 1636–536

Observations of black hole and neutron star X-ray binaries show that the luminosity of the hard-to-soft state transition is usually higher than that of the soft-to-hard state transition,indicating additional parameters other than mass accretion rate are required to interpret spectral state transitions.It has been found in some individual black hole or neutron star soft X-ray transients that the luminosity corresponding to the hard-to-soft state transition is positively correlated with the peak luminosity of the following soft state. In this work,we report the discovery of the same correlation in the single persistent neutron star low mass X-ray binary（LMXB） 4 U 1636–536 based on data from the All Sky Monitor（ASM） on board RXTE,the Gas Slit Camera（GSC） on board MAXI and the Burst Alert Telescope（BAT） on board Swift. We also found such a positive correlation holds in this persistent neutron star LMXB in a luminosity range spanning about a factor of four. Our results indicate that non-stationary accretion also plays an important role in driving X-ray spectral state transitions in persistent accreting systems with small accretion flares,which is much less dramatic compared with the bright outbursts seen in many Galactic LMXB transients.

An SPH simulation for cooling and self-gravitating protoplanetary disks

We investigate the effects of the cooling function in the formation of clumps of protoplanetary disks using two-dimensional smoothed particle hydrody- namic simulations. We use a simple prescription for the cooling rate of the flow, du/dt = -u/τcool, where u and %ool are the internal energy and cooling timeseale, respectively. We assume the ratio of local＇cooling to dynamical timescale, Ωτcool =β, to be a constant and also a function of the local temperature. We found that for the constantβ and γ = 5/3, fragmentation occurs only forβ ≤ 7. However, in the case ofβ having temperature dependence and γ = 5/3, fragmentation can also occur for larger values ofβ. By increasing the temperature dependence of the cooling timescale, the mass accretion rate decreases, the population of clumps/fragments increases, and the clumps/fragments can also form in the smaller radii. Moreover, we found that the clumps can form even in a low mass accretion rate, ≤10-7M⊙yr-1, in the case of temperature-dependentβ. However, clumps form with a larger mass accretion rate, 〉 10-7M⊙ yr-1, in the case of constantβ.

A dynamical model for radiatively inefficient accretion flows with convection

We explore the time evolution of radiatively-inefficient accretion flows. Since these types of accretion flows are convectively unstable, we also study the ef- fects of convection in the present model. The effects of convection are applied to equations describing angular momentum and energy. In analogy to the traditional c^- prescription, we introduce the convection parameter c^e to study the influences of con- vection on physical quantities. The model is studied in two cases： the transport of angular momentum due to convection inward and outward. We found the physical variables are sensitive to the parameter C^c and are also dependent on the direction of angular momentum that is transported by convection. As for angular momentum transfer inward, the accretion flow can be convectively dominated and radial infall velocity becomes zero. Moreover, we found the radial dependence of the density and radial velocity takes an intermediate place between steady state radiatively-inefficient accretion flow and steady state advection-dominated accretion flow. This property is in accord with direct numerical simulation of radiatively-inefficient accretion flows.

The density and temperature dependence of the cooling timescale for fragmentation of self-gravitating disks

The purpose of this paper is to explore the influences of cooling timescale on fragmentation of self-gravitating protoplanetary disks. We assume the cooling timescale, expressed in terms of the dynamical timescale Ω tcool, has a power-law dependence on temperature and density, Ω toool ∝∑-aT-b, where a and b are con- stants. We use this cooling timescale in a simple prescription for the cooling rate, du/dt = -u/tcoll, where u is the internal energy. We perform our simulations using the smoothed particle hydrodynamics method. The simulations demonstrate that the disk is very sensitive to the cooling timescale, which depends on density and tem- perature. Under such a cooling timescale, the disk becomes gravitationally unstable and clumps form in the disk. This property even occurs for cooling timescales which are much longer than the critical cooling timescale, Ω toool≥ 7. We show that by adding the dependence of a cooling timescale on temperature and density, the number of clumps increases and the clumps can also form at smaller radii. The simulations im- ply that the sensitivity of a cooling timescale to density is more than to temperature, because even for a small dependence of the cooling timescale on density, clumps can still form in the disk. However, when the cooling timescale has a large dependence on temperature, clumps form in the disk. We also consider the effects of artificial viscos- ity parameters on fragmentation conditions. This consideration is performed in two cases, where Ω tcool is a constant and Ω tcool is a function of density and temperature. The simulations consider both cases, and results show the artificial viscosity param- eters have rather similar effects. For example, using too small of values for linear and quadratic terms in artificial viscosity can suppress the gravitational instability and consequently the efficiency of the clump formation process decreases. This property is consistent with recent simulations of self-gravitating disks. We perform simulations with and without the Balsara form of artificial viscosity. We find that in the cooling and self-gravitating disks without the Balsara switch, the clumps can form more easily than those with the Balsara switch. Moreover, in both cases where the Balsara switch is present or absent, the simulations show that the cooling timescale strongly depends on density and temperature.

A numerical study of self-gravitating protoplanetary disks

The effect of self-gravity on protoplanetary disks is investigated.The mechanisms of angular momentum transport and energy dissipation are assumed to be the viscosity due to turbulence in the accretion disk.The energy equation is considered in a situation where the released energy by viscosity dissipation is balanced with cooling processes.The viscosity is obtained by equality of dissipation and cooling functions,and is used to derive the angular momentum equation.The cooling rate of the flow is calculated by a prescription,du/dt = u/τ cool,where u and τ cool are the internal energy and cooling timescale,respectively.The ratio of local cooling to dynamical timescales Ωτ cool is assumed to be a constant and also a function of the local temperature.The solutions for protoplanetary disks show that in the case of Ωτ cool = constant,the disk does not exhibit any gravitational instability over small radii for a typical mass accretion rate,˙ M = 10 6 M yr 1,but when choosing Ωτ cool to be a function of temperature,gravitational instability can occur for this value of mass accretion rate or even less in small radii.Also,by studying the viscosity parameter α,we find that the strength of turbulence in the inner part of self-gravitating protoplanetary disks is very low.These results are qualitatively consistent with direct numerical simulations of protoplanetary disks.Also,in the case of cooling with temperature dependence,the effect of physical parameters on the structure of the disk is investigated.These solutions demonstrate that disk thickness and the Toomre parameter decrease by adding the ratio of disk mass to central object mass.However,the disk thickness and the Toomre parameter increase by adding mass accretion rate.Furthermore,for typical input parameters such as mass accretion rate 10 6 M yr 1,the ratio of the specific heat γ = 5/3 and the ratio of disk mass to central object mass q = 0.1,gravitational instability can occur over the whole radius of the disk excluding the region very near the central object.

The effects of viscosity on circumplanetary disks

The effects of viscosity on the circumplanetary disks residing in the vicinity of protoplanets are investigated through two-dimensional hydrodynamical simulations with the shearing sheet model. We find that viscosity can considerably affect properties of the circumplanetary disk when the mass of the protoplanet Mp ~ 33 Me, where Me is the Earth＇s mass. However, effects of viscosity on the circumplanetary disk are negligibly small when the mass of the protoplanet Mp 〉 33 Me. We find that when Mp ~ 33 Me, viscosity can markedly disrupt the spiral structure of the gas around the planet and smoothly distribute the gas, which weakens the torques exerted on the protoplanet. Thus, viscosity can slow the migration speed of a protoplanet. After including viscosity, the size of the circumplanetary disk can be decreased by a factor of 〉~ 20%. Viscosity helps to transport gas into the circumplanetary disk from the differentially rotating circumstellar disk. The mass of the circumplanetary disk can be increased by a factor of 50% after viscosity is taken into account when Mp ~ 33 Me. Effects of viscosity on the formation of planets and satellites are briefly discussed.