Massive Torus Research Articles

Is There a Relation between Duration and Eiso in Gamma-Ray Bursts?

AbstractThe system of accretion disk and black hole is usually considered as the central engine of Gamma-ray Bursts (GRBs). It is usually thought that the disk in the central engine of GRBs is the advection-dominated accretion disk, which is developed from a massive (mass Mdisk) torus at radius rdisk. We find a positive correlation between the isotropic gamma-ray energy Eiso and duration (so-called T90) for GRBs. We interpret this correlation within the advection-dominated accretion disk model, associating Eiso and T90 with Mdisk and viscous timescale respectively.

Dust shell model of the water fountain source IRAS 16342–3814

We investigate the circumstellar dust shell of the water fountain source IRAS 16342-3814. We performed two-dimensional radiative transfer modeling of the dust shell, taking into account previously observed spectral energy distributions (SEDs) and our new $J$-band imaging and $H$- and $K_S$-band imaging polarimetry obtained using the VLT/NACO instrument. Previous observations expect an optically thick torus in the equatorial plane because of a striking bipolar appearance and a large viewing angle of 30 - 40$\degr$. However, models with such a torus as well as a bipolar lobe and an AGB shell cannot fit the SED and the images simultaneously. We find that an additional optically and geometrically thick disk located inside a massive torus solves this problem. The masses of the disk and the torus are estimated to be 0.01 $M_{\sun}$ at the $a_\mathrm{max}=100 \mu$m dust and 1 $M_{\sun}$ at $a_\mathrm{max}=10 \mu$m dust, respectively. We discuss a possible formation scenario for the disk and torus based on a similar mechanism to the equatorial back flow. IRAS 16342-3814 is expected to undergo mass loss at a high rate. The radiation from the central star is shielded by the dust that was ejected in the subsequent mass loss event. As a result, the radiation pressure on dust particles cannot govern the motion of the particles anymore. The mass loss flow can be concentrated in the equatorial plane by help of an interaction, which might be the gravitational attraction by the companion, if it exists in IRAS 16342-3814. A fraction of the ejecta is captured in a circum-companion or circum-binary disk and the remains are escaping from the central star(s) and form the massive torus.

Radiation Magnetohydrodynamics for Black Hole-Torus System in Full General Relativity: A Step toward Physical Simulation

A radiation-magnetohydrodynamic simulation for the black hole-torus system is performed in the framework of full general relativity for the first time. A truncated moment formalism is employed for a general relativistic neutrino radiation transport. Several systems in which the black hole mass is MBH = 3 or 6M⊙, the black hole spin is zero, and the torus mass is ≈ 0.14–0.38M⊙ are evolved as models of the remnant formed after the merger of binary neutron stars or black hole-neutron star binaries. The equation of state and microphysics for the high-density and high-temperature matter are phenomenologically taken into account in a semi-quantitative manner. It is found that the temperature in the inner region of the torus reaches ≳ 10 MeV which enhances a high luminosity of neutrinos ∼ 1051 ergs/s for MBH = 6M⊙ and ∼ 1052 ergs/s for MBH = 3M⊙. It is shown that neutrinos are likely to be emitted primarily toward the outward direction in the vicinity of the rotational axis and their energy density may be high enough to launch a low-energy short gamma-ray burst via the neutrino-antineutrino pair-annihilation process with the total energy deposition ∼ 1047–1049 ergs. It is also shown in our model that for MBH = 3M⊙, the neutrino luminosity is larger than the electromagnetic luminosity while for MBH = 6M⊙, the neutrino luminosity is comparable to or slightly smaller than the electromagnetic luminosity.

Binary neutron star mergers: Dependence on the nuclear equation of state

We perform a numerical-relativity simulation for the merger of binary neutron stars with 6 nuclear-theory-based equations of state (EOSs) described by piecewise polytropes. Our purpose is to explore the dependence of the dynamical behavior of the binary neutron star merger and resulting gravitational waveforms on the EOS of the supernuclear-density matter. The numerical results show that the merger process and the first outcome are classified into three types; (i) a black hole is promptly formed, (ii) a short-lived hypermassive neutron star (HMNS) is formed, (iii) a long-lived HMNS is formed. The type of the merger depends strongly on the EOS and on the total mass of the binaries. For the EOS with which the maximum mass is larger than 2Msun, the lifetime of the HMNS is longer than 10 ms for a total mass m_0=2.7Msun. A recent radio observation suggests that the maximum mass of spherical neutron stars is M_max \geq 1.97\pm 0.04Msun in one \sigma level. This fact and our results support the possible existence of a HMNS soon after the onset of the merger for a typical binary neutron star with m_0=2.7Msun. We also show that the torus mass surrounding the remnant black hole is correlated with the type of the merger process; the torus mass could be large, \geq 0.1Msun, in the case that a long-lived HMNS is formed. We also show that gravitational waves carry information of the merger process, the remnant, and the torus mass surrounding a black hole.

Black hole spacetimes with self-gravitating, massive accretion tori

We review the theory of stationary, axisymmetric spacetimes with black holes surrounded by a massive torus and present a numerical scheme for finding self-consistent solutions in a compactified grid that reaches to spatial infinity. Such numerical solutions have been utilized in studying nonaxisymmetric dynamical instabilities occurring in the torus.

BLACK HOLE-NEUTRON STAR MERGERS AND SHORT GAMMA-RAY BURSTS: A RELATIVISTIC TOY MODEL TO ESTIMATE THE MASS OF THE TORUS

The merger of a binary system composed of a black hole and a neutron star may leave behind a torus of hot, dense matter orbiting around the black hole. While numerical-relativity simulations are necessary to simulate this process accurately, they are also computationally expensive and unable at present to cover the large space of possible parameters, which include the relative mass ratio, the stellar compactness, and the black hole spin. To mitigate this and provide a first reasonable coverage of the space of parameters, we have developed a method for estimating the mass of the remnant torus from black hole-neutron star mergers. The toy model makes use of an improved relativistic affine model to describe the tidal deformations of an extended tri-axial ellipsoid orbiting around a Kerr black hole and measures the mass of the remnant torus by considering which of the fluid particles composing the star are on bound orbits at the time of the tidal disruption. We tune the toy model by using the results of fully general-relativistic simulations obtaining relative precisions of a few percent and use it to extensively investigate the space of parameters. In this way we find that the torus mass is largest for systems with highly spinning black holes, small stellar compactnesses, and large mass ratios. As an example, tori as massive as ~1.33 solar masses can be produced for a very extended star with compactness of ~0.1 inspiralling around a black hole with dimensionless spin equal to 0.85 and mass ratio of about 0.3. However, for a more astrophysically reasonable mass ratio of ~0.14 and a canonical value of the stellar compactness of ~0.145, the toy model sets a considerably smaller upper limit to the torus mass of less than ~0.34 solar masses.

DUSTY STRUCTURE AROUND TYPE-I ACTIVE GALACTIC NUCLEI: CLUMPY TORUS NARROW-LINE REGION AND NEAR-NUCLEUS HOT DUST

We fitted Spitzer/IRS ~2-35 um spectra of 26 luminous QSOs in attempt to define the main emission components. Our model has three major components: a clumpy torus, dusty narrow line region (NLR) clouds and a blackbody-like dust. The models utilize the clumpy torus of Nenkova et al. (2008) and are the first to allow its consistent check in type-I AGNs. Single torus models and combined torus-NLR models fail to fit the spectra of most sources but three component models adequately fit the spectra of all sources. We present torus inclination, cloud distribution, covering factor and torus mass for all sources and compare them with bolometric luminosity, black hole mass and accretion rate. The torus covering factor and mass are found to be correlated with the bolometric luminosity of the sources. We find that a substantial amount of the ~2-7 um radiation originates from a hot dust component, which likely situated in the innermost part of the torus. The luminosity radiated by this component and its covering factor are comparable to those of the torus. We quantify the emission by the NLR clouds and estimate their distance from the center. The distances are ~700 times larger than the dust sublimation radius and the NLR covering factor is about 0.07. The total covering factor by all components is in good agreement with the known AGN type-I:type-II ratio.

Merger of black hole and neutron star in general relativity: Tidal disruption, torus mass, and gravitational waves

We study the properties of the merger of black hole--neutron star (BH-NS) binaries in fully general relativistic simulation, focusing on the case that the NS is tidally disrupted. We prepare BH-NS binaries in a quasicircular orbit as the initial condition in which the BH is modeled by a nonspinning moving puncture. For modeling the NS, we adopt the $\ensuremath{\Gamma}$-law equation of state with $\ensuremath{\Gamma}=2$ and the irrotational velocity field. We change the BH mass in the range ${M}_{\mathrm{BH}}\ensuremath{\approx}3.3{M}_{\ensuremath{\bigodot}}--4.6{M}_{\ensuremath{\bigodot}}$, while the rest mass of the NS is fixed to be ${M}_{*}=1.4{M}_{\ensuremath{\bigodot}}$ (i.e., the NS mass ${M}_{\mathrm{NS}}\ensuremath{\approx}1.3{M}_{\ensuremath{\bigodot}}$). The radius of the corresponding spherical NS is set in the range ${R}_{\mathrm{NS}}\ensuremath{\approx}12--15\text{ }\text{ }\mathrm{km}$ (i.e., the compactness $G{M}_{\mathrm{NS}}/{R}_{\mathrm{NS}}{c}^{2}\ensuremath{\approx}0.13--0.16$). We find for all the chosen initial conditions that the NS is tidally disrupted near the innermost stable circular orbit. The numerical results indicate that, for the model of ${R}_{\mathrm{NS}}=12\text{ }\text{ }\mathrm{km}$, more than $\ensuremath{\sim}95%$ of the rest mass is quickly swallowed by the BH and the resultant torus mass surrounding the BH is less than $\ensuremath{\sim}0.05{M}_{\ensuremath{\bigodot}}$. For the model of ${R}_{\mathrm{NS}}\ensuremath{\approx}14.7\text{ }\text{ }\mathrm{km}$, by contrast, the torus mass is $\ensuremath{\sim}0.15{M}_{\ensuremath{\bigodot}}$ for the BH mass $\ensuremath{\approx}4{M}_{\ensuremath{\bigodot}}$. The thermal energy of the material in the torus increases due to the shock heating that occurs in the collision between the spiral arms, resulting in the temperature ${10}^{10}--{10}^{11}\text{ }\text{ }\mathrm{K}$. Our results indicate that the merger between a low-mass BH and its companion NS may form a central engine of short gamma-ray bursts of the total energy of order ${10}^{49}\text{ }\text{ }\mathrm{ergs}$ if the compactness of the NS is fairly small, $\ensuremath{\lesssim}0.145$. However, for the canonical values ${M}_{\mathrm{NS}}=1.35{M}_{\ensuremath{\bigodot}}$ and ${R}_{\mathrm{NS}}=12\text{ }\text{ }\mathrm{km}$, the merger results in small torus mass, and hence it can be a candidate only for the low-energy, short gamma-ray bursts of total energy of order ${10}^{48}\text{ }\text{ }\mathrm{ergs}$. We also present gravitational waveforms during the inspiral, tidal disruption of the NS, and subsequent evolution of the disrupted material. We find that the amplitude of gravitational waves quickly decreases after the onset of tidal disruption. Although the quasinormal mode is excited, its gravitational wave amplitude is much smaller than that of the late inspiral phase. This is reflected in the fact that the spectrum amplitude sharply falls above a cutoff frequency which is determined by the tidal disruption process. We also find that the cutoff frequency is 1.25--1.4 times larger than the frequency predicted by the study for the sequence of the quasicircular orbits, and this factor of the deviation depends on the compactness of the NS.

Massive Expanding Torus and Fast Outflow in Planetary Nebula NGC 6302

We present interferometric observations of 12CO and 13CO J = 2–1 emission from the butterfly-shaped, young planetary nebula NGC 6302. The high angular resolution and high sensitivity achieved in our observations allow us to resolve the nebula into two distinct kinematic components: (1) a massive expanding torus seen almost edge-on and oriented in the north-south direction (roughly perpendicular to the optical nebula axis), which exhibits very complex and fragmented structure; and (2) high-velocity molecular knots moving at more than 20 km s−1 and located in the optical bipolar lobes. These knots show a linear position-velocity gradient (Hubble-like flow), which is characteristic of fast molecular outflow in young planetary nebulae. From the low but variable 12CO/13CO J = 2–1 line intensity ratio, we conclude that the 12CO J = 2–1 emission is optically thick over much of the nebula. Using the optically thinner line 13CO J = 2–1, we estimate a total molecular gas mass of ~0.1 M☉, comparable to the ionized gas mass; the total gas mass of the NGC 6302 nebula, including the massive ionized gas from the photon dominated region, is found to be ~0.5 M☉. From radiative transfer modeling, we infer that the torus is seen at an inclination angle of 75° with respect to the plane of the sky, and is expanding at a velocity of 15 km s−1. Comparison with recent observations of molecular gas in NGC 6302 is also discussed.

The massive expanding molecular torus in the planetary nebula NGC 6302

Aims.We measure the mass and kinematics of the massive molecular torus in the the proto-typical butterfly planetary nebula NGC 6302. Determining the mass-loss history of the source is an important step in understanding the origin and formation of the wing-like morphology. Methods. Using the SMA interferometer we have imaged both the continuum emission and the J=2-1 transitions of 12CO and 13CO at arcsecond resolution. These data are analysed in combination with observations of both the J=2-1 and J=3-2 transitions of 12CO and 13CO made with JCMT. Results. The 12CO and 13CO emission match the dark lane seen in absorption in the H${\alpha}$ image of the object and trace an expanding torus of material. The CO indicates a mass of the torus of ~2 $M_\odot \pm 1$ $M_\odot$. The torus is expanding with a velocity of ~8 km s-1, centred at $V_{\rm lsr}=-31.5\rm\,km\,s^{-1}$. The size and expansion velocity of the torus indicates that it was ejected from ~7500 yr to 2900 yr ago, with a mass-loss rate of $5\times10^{-4}\,M_\odot\,\rm yr^{-1}$. In addition we detect a ballistic component in the CO images which has a velocity gradient of 140 km s-1 pc-1. Conclusions.The derived mass-loss history of the torus favours binary interaction as the cause of the ejection of the torus and we predict the existence of a companion with an orbital period $P\lesssim 1\,$month.

Anticorrelation between the Mass of a Supermassive Black Hole and the Mass Accretion Rate in Type 1 Ultraluminous Infrared Galaxies and Nearby QSOs

We discovered a significant anti-correlation between the mass of a supermassive black hole (SMBH), $M_{\rm BH}$, and the luminosity ratio of infrared to active galactic nuclei (AGN) Eddington luminosity, $L_{\rm IR}/L_{\rm Edd}$, over four orders of magnitude for ultraluminous infrared galaxies with type I Seyfert nuclei (type I ULIRGs) and nearby QSOs. This anti-correlation ($M_{\rm BH}$ vs. $L_{\rm IR}/L_{\rm Edd}$) can be interpreted as the anti-correlation between the mass of a SMBH and the rate of mass accretion onto a SMBH normalized by the AGN Eddington rate, $\dot{M}_{\rm BH}/\dot{M}_{\rm Edd}$. In other words, the mass accretion rate $\dot{M}_{\rm BH}$ is not proportional to that of the central BH mass. Thus, this anti-correlation indicates that BH growth is determined by the external mass supply process, and not the AGN Eddington-limited mechanism. Moreover, we found an interesting tendency for type I ULIRGs to favor a super-Eddington accretion flow, whereas QSOs tended to show a sub-Eddington flow. On the basis of our findings, we suggest that a central SMBH grows by changing its mass accretion rate from super-Eddington to sub-Eddington. According to a coevolution scenario of ULIRGs and QSOs based on the radiation drag process, it has been predicted that a self-gravitating massive torus, whose mass is larger than a central BH, exists in the early phase of BH growth (type I ULIRG phase) but not in the final phase of BH growth (QSO phase). At the same time, if one considers the mass accretion rate onto a central SMBH via a turbulent viscosity, the anti-correlation ($M_{\rm BH}$ vs. $L_{\rm IR}/L_{\rm Edd}$) is well explained by the positive correlation between the mass accretion rate $\dot{M}_{\rm BH}$ and the mass ratio of a massive torus to a SMBH.

Relativistic neutron star merger simulations with non-zero temperature equations of state

An extended set of binary neutron star (NS) merger simulations is performed with an approximative treatment of general relativity to systematically investigate the influence of the nuclear equation of state (EoS), the NS masses, and the NS spin states prior to merging. The general relativistic hydrodynamics simulations are based on a conformally flat approximation to the Einstein equations and a Smoothed Particle Hydrodynamics code for the gas treatment. We employ the two non-zero temperature EoSs of Shen et al. (1998a, Nucl. Phys. A, 637, 435; 1998b, Prog. Theor. Phys., 100, 1013) and Lattimer & Swesty (1991, Nucl. Phys. A, 535, 331), which represent a “harder” and a “softer” behavior, respectively, with characteristic differences in the incompressibility at supernuclear densities and in the maximum mass of nonrotating, cold neutron stars. In addition, we use the cold EoS of Akmal et al. (1998, Phys. Rev. C, 58, 1804) with a simple ideal-gas-like extension according to Shibata & Taniguchi (2006, Phys. Rev. D, 73, 064027), in order to compare with their results, and an ideal-gas EoS with parameters fitted to the supernuclear part of the Shen-EoS. We estimate the mass sitting in a dilute “torus” around the future black hole (BH) by requiring the specific angular momentum of the torus matter to be larger than the angular momentum of the ISCO around a Kerr BH with the mass and spin parameter of the compact central remnant. The dynamics and outcome of the models is found to depend strongly on the EoS and on the binary parameters. Larger torus masses are found for asymmetric systems (up to ~ for a mass ratio of 0.55), for large initial NSs, and for a NS spin state which corresponds to a larger total angular momentum. We find that the postmerger remnant collapses either immediately or after a short time when employing the soft EoS of Lattimer& Swesty, whereas no sign of post-merging collapse is found within tens of dynamical timescales for all other EoSs used. The typical temperatures in the torus are found to be about MeV depending on the strength of the shear motion at the collision interface between the NSs and thus depending on the initial NS spins. About of NS matter become gravitationally unbound during or right after the merging process. This matter consists of a hot/high-entropy component from the collision interface and (only in case of asymmetric systems) of a cool/low-entropy component from the spiral arm tips.

Three-dimensional simulations of non-stationary accretion by remnant black holes of compact object mergers

(abridged) Three-dimensional hydrodynamic simulations with an Eulerian PPM code are presented for the time-dependent evolution of accretion tori around nonrotating and rotating stellar-mass black holes (BHs), using a pseudo-Newtonian gravitational potential to approximate the effects of general relativity. The initial configurations are assumed to be remnants of binary neutron star (NS) or NS+BH mergers and consist of a 4 solar mass BH with varied spin, girded by a torus with a mass between 0.01 and 0.2 solar masses. The evolution of tori without and with physical shear viscosity is simulated, using a realistic equation of state and following the energy loss and lepton number change due to neutrino emission by a neutrino-trapping scheme. The time-dependent efficiency of converting rest-mass energy to neutrinos is found to reach 10 percent, the efficiency of converting neutrino energy into a pair-photon fireball by neutrino annihilation can reach several percent. The rate of the latter process declines with time much less steeply than the total neutrino luminosity, because the ongoing protonization of the torus ensures a rather stable product of neutrino and antineutrino luminosities. The neutrino emission increases steeply with higher viscosity, larger torus mass, and larger BH spin in corotation with the torus. For rotation rates as expected for post-merger BHs (a > 0.5) and reasonable values of the alpha viscosity (alpha ~ 0.1), the considered tori release sufficient energy in neutrinos to account for the energetics of the well-localized short gamma-ray bursts recently detected by Hete and Swift, if collimation is invoked as predicted by hydrodynamic jet simulations.

Collapse of Magnetized Hypermassive Neutron Stars in General Relativity

Hypermassive neutron stars (HMNSs)--equilibrium configurations supported against collapse by rapid differential rotation--are possible transient remnants of binary neutron-star mergers. Using newly developed codes for magnetohydrodynamic simulations in dynamical spacetimes, we are able to track the evolution of a magnetized HMNS in full general relativity for the first time. We find that secular angular momentum transport due to magnetic braking and the magnetorotational instability results in the collapse of an HMNS to a rotating black hole, accompanied by a gravitational wave burst. The nascent black hole is surrounded by a hot, massive torus undergoing quasistationary accretion and a collimated magnetic field. This scenario suggests that HMNS collapse is a possible candidate for the central engine of short gamma-ray bursts.

A WHITE DWARF-NEUTRON STAR RELATIVISTIC BINARY MODEL FOR SOFT GAMMA-RAY REPEATERS

A scenario for soft gamma-ray repeaters (SGRs) is introduced in which gravitational radiation reaction (GRR) effects drive the dynamics of an ultrashort orbital period X-ray binary embracing a high-mass donor white dwarf (WD) to a rapidly rotating low magnetized massive neutron star (NS) surrounded by a thick, dense and massive accretion torus. Driven by GRR, over timescales of ΔTrep~ 10 years, the binary separation reduces, the WD overflows its Roche lobe and the mass transfer drives unstable the accretion disk around the NS. As the binary circular orbital period is a multiple integer number (m) of the period of the WD fundamental mode,37the WD is since long pulsating at its fundamental mode; and most of its harmonics, due to the tidal interaction with its NS orbital companion. Hence, when the powerful irradiation glows onto the WD; from the fireball ejected as part of the disk matter slumps onto the NS, it is partially absorbed. This huge energy excites other WD radial (p-mode) pulsations.34,35After each mass-transfer episode the binary separation (and orbital period) is augmented significantly1,5due to the binary's angular momentum redistribution. Thus a new adiabatic inspiral phase driven by GRR reaction starts which brings the binary close again, and the process repeats after a time span ΔTrep. This model allows to explain most of SGRs observational features: their recurrent activity, energetics of giant superoutbursts and quiescent stages, and particularly the intriguing subpulses discovered by BeppoSAX,10which are suggested here to be overtones of the WD radial fundamental mode (see the accompanying paper).31

From the Circumnuclear Disk in the Galactic Center to thick, obscuring tori of AGNs

We compare three different models of clumpy gas disk and show that the Circumnuclear Disk (CND) in the Galactic Center and a putative, geometrically thick, obscuring torus are best explained by a collisional model consisting of quasi-stable, self-gravitating clouds. Kinetic energy of clouds is gained by mass inflow and dissipated in cloud collisions. The collisions give rise to a viscosity in a spatially averaged gas dynamical picture, which connects them to angular momentum transport and mass inflow. It is found that CND and torus share the same gas physics in our description, where the mass of clouds is 20-50 M ○. and their density is close to the limit of disruption by tidal shear. We show that the difference between a transparent CND and an obscuring torus is the gas mass and the velocity dispersion of the clouds. A change in gas supply and the dissipation of kinetic energy can turn a torus into a CND-like structure and vice versa. Any massive torus will naturally lead to sufficiently high mass accretion rates to feed a luminous AGN. For a geometrically thick torus to obscure the view to the center even super-Eddington accretions rates with respect to the central black hole are required.

Oscillations of vertically integrated relativistic tori - I. Axisymmetric modes in a Schwarzschild space-time

This is the first of a series of papers investigating the oscillation properties of relativistic, non-self-gravitating tori orbiting around a black hole. In this initial paper we consider the axisymmetric oscillation modes of a torus constructed in a Schwarzschild space-time. To simplify the treatment and make it as analytical as possible, we build our tori with vertically integrated and vertically averaged quantities, thus transforming the eigenvalue problem into a set of coupled ordinary differential equations. The tori are also modelled with a number of different non-Keplerian distributions of specific angular momentum, and we discuss how the oscillation properties change when different distributions of angular momentum are considered. Our investigation progresses by steps. We first consider a local analysis in Newtonian gravity and determine the properties of acoustic wave propagation within these objects, as well as the relations between acoustic and epicyclic oscillations. Next, we extend the local analysis to a general relativistic framework. Finally, we perform a global analysis and determine both the eigenfunctions and the eigenfrequencies of the axisymmetric oscillations corresponding to the p modes of relativistic tori. These behave as sound waves globally trapped in the torus and possess eigenfrequencies appearing in the simple sequence 2:3:4:..., independently of the distribution of angular momentum considered. The properties of the modes investigated here are in good agreement with those observed in recent numerical simulations, and could have a number of different applications. In X-ray binary systems containing a black hole candidate, for instance, p-mode oscillations could be used to explain the harmonic relations in the high-frequency quasi-periodic oscillations observed. In systems comprising a massive torus orbiting a black hole, on the other hand, p-mode oscillations could be used to explain the development or the suppression of the runaway instability.

Gravitational-Wave Radiation from Magnetized Accretion Disks

Abstract The detectability of gravitational-wave (GW) radiation from accretion disks is discussed based on various astrophysical contexts. In order to emit GW radiation, the disk shape should lose axial symmetry. We point out that a significant deformation is plausible in non-radiative hot accretion disks because of enhanced magnetic activity, whereas it is unlikely for standard-type cool disks. We have analyzed the 3D magnetohydrodynamical (MHD) simulation data of magnetized accretion flow, finding non-axisymmetric density patterns. The corresponding ellipticity is $ \epsilon \sim 0.01$. The expected time variations of GW radiation are overall chaotic, but there is a hint of quasi-periodicity. GW radiation has no interesting consequence, however, in the case of close binaries, because of very tiny disk masses. GW radiation is not significant, either, for AGN because of very slow rotation velocities. The most promising case can be found in gamma-ray bursts or supernovae, in which a massive torus (or disk) with a solar mass or so may be formed around a stellar-mass compact object as the result of a merger of compact objects, or by the fallback of exploded material towards the center in a supernova. Although much more intense GW radiation is expected before the formation of the torus, the detection of GW radiation in the subsequent accretion phase is of great importance, since it will provide a good probe to investigating their central engines.

Can Neutrino‐cooled Accretion Disks Be an Origin of Gamma‐Ray Bursts?

It is often proposed that a massive torus with approximately solar mass surrounding a stellar-mass black hole could be a central engine of gamma-ray bursts. We study the properties of such massive accretion tori (or disks) based on the α viscosity model. For surface density exceeding about 1020 g cm-2, which is realized when ~1 M☉ of material is contained within a disk of size ~5 × 106 cm, we find that (1) the luminosity of photons is practically zero because of significant photon trapping, (2) neutrino cooling dominates over advective cooling, (3) the pressure of degenerate electrons dominates over the pressure of gas and photons, and (4) the magnetic field strength exceeds the critical value of about 4 × 1013 G, even if we take 0.1% of the equipartition value. The possible observable quantum electrodynamical (QED) effects arising from supercritical fields are discussed. Most interestingly, photon splitting may occur, producing a significant number of photons of energies below ~511 keV, thereby possibly suppressing e± pair creation.

Extreme Ultraviolet Explorer spectra of the Io plasma torus: Improved spectral resolution and new results

Spectral images of the Io plasma torus obtained from ∼165 hours of observation by the Extreme Ultraviolet Explorer (EUVE) spacecraft during the 124 encounter of the Galileo spacecraft with the torus and at an earlier epoch in 1996 are analyzed by a spectral‐resolution‐enhancing deconvolution algorithm in order to estimate averaged torus properties. Because of our analysis's improved spectral resolution, we are better able to separate lines arising from different ion species as compared to earlier EUVE studies. We exploit this capability to estimate emission‐weighted whole‐torus averages of electron temperature and ion fraction. Comparing the 1996 results for the torus to those of 1999, we estimate that (1) the electrons were hotter (5.4 versus 4.6 eV), (2) the luminosity was higher (375 versus 245 GW in the 350–730 Å region), (3) the mass was lower (by 10–15%), (4) the sulfur/oxygen ratio was higher (0.8 versus 0.5), and (5) the ionization level was greater (1.2 versus 0.8 for [S++]/[S+], 0.22 versus 0.15 for [O++]/[O+]) in the 1996 observation than during 124. Considering the constraints of formation, chemistry, transport, and mass loss, the torus has fewer degrees of freedom than observables, so these two snapshots give us clues as to how torus characteristics may covary. For example, our data are consistent with the hypothesis that electron temperature and density are anticorrelated. We also confirm that the Io‐correlated enhancement of torus brightness discovered by Voyager still persists. However, we raise new doubts that the torus's dawn‐dusk brightness asymmetry is the result of torus mass loss that is controlled by the total mass of the torus. We also estimate corrections to the electron impact collision strengths for many EUV multiplets of interest for understanding the torus and other sulfur and oxygen plasmas of astrophysical interest, finding, for example, that the O+/S++ 484 Å feature is about an order of magnitude brighter than that predicted by available electron collision strength data.