arXiv subjects

Shin Mineshige

Publications and source records attributed to Shin Mineshige.

At least 37 records · Page 2Linked to original sources

arXiv2019

Black hole Spin Measurement Based on Time-domain VLBI Observations of Infalling Gas Cloud

The black hole spacetime is described by general relativity and characterized by two quantities: the black hole mass and spin. Black hole spin measurement requires information from the vicinity of the event horizon, which is spatially resolved for the Galactic center SagittariusA* (SgrA*) and nearby radio galaxy M87 by means of very long baseline interferometry (VLBI) observations with the Event Horizon Telescope (EHT). In this paper, we simulate EHT observations for a gas cloud intermittently falling onto a black hole, and construct a method for spin measurement based on its relativistic flux variation. The light curve of the infalling gas cloud is composed of peaks formed by photons which directly reach a distant observer and by secondary ones reaching the observer after more than one rotation around the black hole. The time interval between the peaks is determined by a period of photon rotation near the photon circular orbit which uniquely depends on the spin. We perform synthetic EHT observations for SgrA* under a more realistic situation that a number of gas clouds intermittently fall towards the black hole with various initial parameters. Even for this case, the black hole spin dependence is detectable in correlated flux densities which are accurately calibrated by baselines between sites with redundant stations. The synthetic observations indicate that our methodology can be applied to EHT observations of Sgr A* since April 2017.

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arXiv2019

Super-Eddington growth of black holes in the early Universe: effects of disk radiation spectra

We investigate the properties of accretion flows onto a black hole (BH) with a mass of $M_{\rm BH}$ embedded in an initially uniform gas cloud with a density of $n_{\infty}$ in order to study rapid growth of BHs in the early Universe. In previous work, the conditions for super-Eddington accretion from outside the Bondi radius were studied by assuming that radiation produced at the vicinity of the central BH has a single-power-law spectrum $ν^{-α}$ at $hν\geq 13.6~{\rm eV}$ ($α\sim 1.5$). However, radiation spectra depends on the BH mass and accretion rate. Here, we perform two-dimensional multi-frequency radiation hydrodynamical simulations taking into account more realistic radiation spectra associated with the properties of nuclear accretion disks. We find that the condition for a transition to super-Eddington accretion is alleviated for a wide range of masses ($10\lesssim M_{\rm BH}/M_{\odot} \lesssim 10^6$) because photoionization for accretion disk spectra are less efficient than those for single-power-law spectra. For disk spectra, the transition to super-Eddington is more likely to occur for lower BH masses because the radiation spectra become too hard to ionize the gas. Even when accretion flows are exposed to anisotropic radiation, the effect due to radiation spectra shrinks the ionized region and likely leads to the transition to a wholly neutral accretion phase. Finally, by generalizing our simulation results, we construct a new analytical criterion required for super-Eddington accretion; $(M_{\rm BH}/10^5~M_{\odot}) (n_{\infty}/10^4~{\rm cm^{-3}}) \gtrsim 2.4~ (\langleε\rangle /100~{\rm eV})^{-5/9}$, where $\langleε\rangle$ is the mean energy of ionizing radiation from the central BH.

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arXiv2018

Systematic two-dimensional radiation hydrodynamic simulations of super-Eddington accretion flow and outflow: Comparison with the slim disk model

To what extent can the one-dimensional slim disk model reproduce the multi-dimensional results of global radiation-hydrodynamic simulations of super-Eddington accretion? With this question in mind, we perform systematic simulation study of accretion flow onto a non-spinning black hole for a variety of black hole masses of $(10 - 10^7) M_{\odot}$ and mass accretion rates of $(1.4 \times 10^2 - 5.6 \times 10^3)L_{\rm Edd}/c^2$ (with $L_{\rm Edd}$ and $c$ being the Eddington luminosity and speed of light). In order to adequately resolve large-scale outflow structure, we extensively expand a simulation box to cover the space of $3000 r_{\rm S}$ (with $r_{\rm S}$ being the Schwarzschild radius), larger than those in most previous studies, so that we can put relatively large angular momentum to the gas injected from the outer simulation boundary. The adopted Keplerian radius, at which the centrifugal force balances with the gravitational force, is $r_{\rm K}=300 r_{\rm S}$. The injected mass first falls and is accumulated at around this radius and then slowly accretes towards the central black hole via viscosity. We simulate such accretion processes, taking inverse and bulk Compton scattering into account. The simulated accretion flow is in a quasi-steady state inside $r_{\rm qss}\sim 200 r_{\rm S}$. Within this radius the flow properties are, as a whole, in good agreement with those described by the slim disk model except that the radial density profile of the underlying disk is much flatter, $ρ\propto r^{-0.73}$ (cf. $ρ\propto r^{-3/2}$ in the slim disk model), due probably to efficient convection. We find very weak outflow from inside $r\sim 200 r_{\rm S}$ unlike the past studies.

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arXiv2018

Can the relativistic light bending model explain X-ray spectral variations of Seyfert galaxies?

Many Seyfert galaxies are known to exhibit Fe-K broad emission line features in their X-ray energy spectra. The observed lines have three distinct features; (1) the line profiles are skewed and show significant low-energy tails, (2) the Fe-K band have low variability, which produces a broad and deep dip in the root-mean-square (rms) spectra, and (3) photons in this band have time lags behind those in the adjacent energy bands with amplitudes of several $R_g/c$, where $R_g$ is the gravitational radius. The "relativistic light bending model" is proposed to explain these observed features, where a compact X-ray source (lamp post) above an extreme Kerr black hole illuminates the innermost area of the accretion disc. In this paper, we critically examine the relativistic light bending model by computing the rms spectra and the lag features using a ray-tracing technique, when a lamp post moves vertically on the black hole spin axis. As a result, we found that the observed deep rms dip requires that the iron is extremely overabundant ($\gtrsim10$ solar), whereas the observed lag amplitude is consistent with the normal iron abundance. Furthermore, disappearance of the lag in the high-flux state requires a source height as high as $\sim40\,R_g$, which contradicts the relativistically broad emission line feature. Our simulations agree with the data that the reverberation feature moves to lower frequencies with larger source height, however, if this scenario is correct, the simulations predict detection of a clear Fe-K lag at low frequencies, which is not constrained in the data. Therefore, we conclude that the relativistic light bending model may not explain the characteristic Fe-K spectral variations in Seyfert galaxies.

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arXiv2018

Three-dimensional structure of clumpy outflow from supercritical accretion flow onto black holes

We perform global three-dimensional (3D) radiation-hydrodynamic (RHD) simulations of out- flow from supercritical accretion flow around a 10 Msun black hole. We only solve the outflow part, starting from the axisymmetric 2D simulation data in a nearly steady state but with small perturbations in a sinusoidal form being added in the azimuthal direction. The mass accretion rate onto the black hole is ~10^2 L_E/c^2 in the underlying 2D simulation data and the outflow rate is ~10 L_E/c^2 (with LE and c being the Eddington luminosity and speed of light, respectively). We first confirm the emergence of clumpy outflow, which was discovered by the 2D RHD simulations, above the photosphere located at a few hundreds of Schwarzschild radii (r_S) from the central black hole. As prominent 3D features we find that the clumps have the shape of a torn sheet, rather than a cut string, and that they are rotating around the central black hole with a sub-Keplerian velocity at a distance of ~10^3 r_S from the center. The typical clump size is ~30 r_S or less in the radial direction, and is more elongated in the angular directions, ~hundreds of r_S at most. The sheet separation ranges from 50 to 150 r_S. We expect stochastic time variations when clumps pass across the line of the sight of a distant observer. Variation timescales are estimated to be several seconds for a black hole with mass of ten to several tens of Msun, in rough agreement with the observations of some ultra-luminous X-ray sources.

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arXiv2017

Supercritical Accretion onto a Non-Magnetized Neutron Star: Why is it Feasible?

To understand why supercritical accretion is feasible onto a neutron star, we carefully examine the accretion flow dynamics by 2.5-dimensional general relativistic (GR) radiation magnetohydrodynamic (RMHD) simulations, comparing the cases of accretion onto a non-magnetized neutron star (NS) and that onto a black hole (BH). Supercritical BH accretion is relatively easy, since BH can swallow excess radiation energy, so that radiation flux can be inward in its vicinity. This mechanism can never work for NS which has a solid surface. In fact, we find that the radiation force is always outward. Instead, we found significant reduction in the mass accretion rate due to strong radiation-pressure driven outflow. The radiation flux $F_\mathrm{rad}$ is self-regulated such that the radiation force balances with the sum of gravity and centrifugal forces. Even when the radiation energy density much exceeds that expected from the Eddington luminosity $E_\mathrm{rad} \simeq F_\mathrm{rad}τ/c> 10^2 L_\mathrm{Edd}/(4πr^2 c)$, the radiation flux is always kept below the certain value which makes it possible not to blow all the gas away from the disk. These effects make supercritical accretion feasible. We also find that a settling region, where accretion is significantly decelerated by radiation cushion, is formed around the NS surface. In the settling region, the radiation temperature and mass density roughly follow $T_\mathrm{rad} \propto r^{-1}$ and $ρ\propto r^{-3}$, respectively. No settling region appears around the BH so that matter can be directly swallowed by the BH with supersonic speed.

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arXiv2017

High-frequency quasi-periodic light variations from arc-shaped gas clouds falling to a black hole

We investigate dynamical and radiative properties of arc-shaped gas clouds falling onto a stellar mass black hole based on the three-dimensional general relativistic radiation-magnetohydrodynamics (3D-GRRMHD) simulation data. Assuming that the gas clouds radiate due mainly to the free-free emission and/or optically thick, inverse Compton scattering, we calculate how the emissivity distributions develop with time. We find that (1) gas clouds, each of which has a ring-like or arc shape, are intermittently formed, and that (2) they slowly fall to the black hole, keeping nearly the Keplerian orbital velocity. These features support the dynamical properties of the gas clouds assumed in the spin measurement method proposed by Moriyama & Mineshige 2015, but the radius of the inner edge of the accretion disk is larger than that of the marginally stable orbit (ISCO). Next, we examine how each gas cloud is observed by a distant observer by calculating the photon trajectories in the black hole space-time. The luminosity of the accretion flow exhibits significant time variations on different timescales, reflecting the time evolution of the gas density distributions. The relatively slow variations on the time durations of 0.08-0.10 sec is due to the formation and fall of gas clouds, while quasi-periodic flux peaks with short time intervals (0.01 sec) are due to the quasi-periodic enhancement of light from the non-axisymmetric arc-shaped clouds through the beaming effect. This may account for the high-frequency quasi-periodic oscillations (HF QPOs) observed in black hole binaries. The observational implications and future issues are briefly discussed.

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arXiv2017

Measurements of resonant scattering in the Perseus cluster core with Hitomi SXS

Thanks to its high spectral resolution (~5 eV at 6 keV), the Soft X-ray Spectrometer (SXS) on board Hitomi enables us to measure the detailed structure of spatially resolved emission lines from highly ionized ions in galaxy clusters for the first time. In this series of papers, using the SXS we have measured the velocities of gas motions, metallicities and the multi-temperature structure of the gas in the core of the Perseus cluster. Here, we show that when inferring physical properties from line emissivities in systems like Perseus, the resonant scattering (RS) effect should be taken into account. In the Hitomi waveband, RS mostly affects the FeXXV He$α$ line ($w$) - the strongest line in the spectrum. The flux measured by Hitomi in this line is suppressed by a factor ~1.3 in the inner ~30 kpc, compared to predictions for an optically thin plasma; the suppression decreases with the distance from the center. The $w$ line also appears slightly broader than other lines from the same ion. The observed distortions of the $w$ line flux, shape and distance dependence are all consistent with the expected effect of the resonant scattering in the Perseus core. By measuring the ratio of fluxes in optically thick ($w$) and thin (FeXXV forbidden, He$β$, Ly$α$) lines, and comparing these ratios with predictions from Monte Carlo radiative transfer simulations, the velocities of gas motions have been obtained. The results are consistent with the direct measurements of gas velocities from line broadening described elsewhere in this series, although the systematic and statistical uncertainties remain significant. Further improvements in the predictions of line emissivities in plasma models, and deeper observations with future X-ray missions will enable RS measurements to provide powerful constraints on the amplitude and anisotropy of clusters gas motions.

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arXiv2017

Theoretical modeling of Comptonized X-ray spectra of super-Eddington accretion flow: origin of hard excess in Ultraluminous X-Ray Sources

X-ray continuum spectra of super-Eddington accretion flow are studied by means of Monte Carlo radiative transfer simulations based on the radiation hydrodynamic simulation data, in which both of thermal and bulk Compton scatterings are taken into account. We compare the calculated spectra of accretion flow around black holes with masses of $M_{\rm BH} = 10, 10^2, 10^3$, and $10^4 M_\odot$ for a fixed mass injection rate (from the computational boundary at $10^3 r_{\rm s}$) of $10^3 L_{\rm Edd}/c^2$ (with $r_{\rm s}$, $L_{\rm Edd}$, and $c$ being the Schwarzschild radius, the Eddington luminosity, and the speed of light, respectively). The soft X-ray spectra exhibit mass dependence in accordance with the standard-disk relation; the maximum surface temperature is scaled as $T \propto M_{\rm BH}^{-1/4}$. The spectra in the hard X-ray bands, by contrast, look quite similar among different models, if we normalize the radiation luminosity by $M_{\rm BH}$. This reflects that the hard component is created by thermal and bulk Compton scattering of soft photons originating from an accretion flow in the over-heated and/or funnel regions, the temperatures of which have no mass dependence. The hard X-ray spectra can be reproduced by a Wien spectrum with temperature of $T\sim 3$ keV accompanied by a hard excess at photon energy above several keV. The excess spectrum can be well fitted with a power law with a photon index of $Γ\sim 3$. This feature is in good agreement with that of the recent NuSTAR observations of ULX (Ultra-Luminous X-ray sources).

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arXiv2017

Search for Thermal X-ray Features from the Crab nebula with Hitomi Soft X-ray Spectrometer

The Crab nebula originated from a core-collapse supernova (SN) explosion observed in 1054 A.D. When viewed as a supernova remnant (SNR), it has an anomalously low observed ejecta mass and kinetic energy for an Fe-core collapse SN. Intensive searches were made for a massive shell that solves this discrepancy, but none has been detected. An alternative idea is that the SN1054 is an electron-capture (EC) explosion with a lower explosion energy by an order of magnitude than Fe-core collapse SNe. In the X-rays, imaging searches were performed for the plasma emission from the shell in the Crab outskirts to set a stringent upper limit to the X-ray emitting mass. However, the extreme brightness of the source hampers access to its vicinity. We thus employed spectroscopic technique using the X-ray micro-calorimeter onboard the Hitomi satellite. By exploiting its superb energy resolution, we set an upper limit for emission or absorption features from yet undetected thermal plasma in the 2-12 keV range. We also re-evaluated the existing Chandra and XMM-Newton data. By assembling these results, a new upper limit was obtained for the X-ray plasma mass of <~ 1Mo for a wide range of assumed shell radius, size, and plasma temperature both in and out of the collisional equilibrium. To compare with the observation, we further performed hydrodynamic simulations of the Crab SNR for two SN models (Fe-core versus EC) under two SN environments (uniform ISM versus progenitor wind). We found that the observed mass limit can be compatible with both SN models if the SN environment has a low density of <~ 0.03 cm-3 (Fe core) or <~ 0.1 cm-3 (EC) for the uniform density, or a progenitor wind density somewhat less than that provided by a mass loss rate of 10-5 Mo yr-1 at 20 km s-1 for the wind environment.

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arXiv2016

Understanding the General Feature of Microvariability in ${\it Kepler}$ Blazar W2R 1926$+$42

We analyze the ${\it Kepler}$ monitoring light curve of a blazar W2R 1926$+$42 to examine features of microvariability by means of the "shot analysis" technique. We select 195 intra-day, flare-like variations (shots) for the continuous light curve of Quarter 14 with a duration of 100 d. In the application of the shot analysis, an averaged profile of variations is assumed to converge with a universal profile which reflects a physical mechanism generating the microvariability in a blazar jet, although light-variation profiles of selected shots show a variety. A mean profile, which is obtained by aligning the peaks of the 195 shots, is composed of a spiky-shape shot component at $\pm$0.1 d (with respect to the time of the peak), and two slow varying components ranging from $-$0.50 d to $-$0.15 d and from 0.10 d to 0.45 d of the peak time. The former spiky feature is well represented by an exponential rise of 0.043$\pm$0.001 d and an exponential decay of 0.061$\pm$0.002 d. These timescales are consistent with that corresponding to a break frequency of a power spectrum density calculated from the obtained light curve. After verification with the Monte-Carlo method, the exponential shape, but not the observed asymmetry, of the shot component can be explained by noise variation. The asymmetry is difficult to explain through a geometrical effect (i.e. changes of the geometry of the emitting region), but is more likely to be caused by the production and dissipation of high-energy accelerated particles in the jet. Additionally, durations of the detected shots show a systematic variation with a dispersion caused by a statistical randomness. A comparison with the variability of Cygnus X-1 is also briefly discussed.

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arXiv2016

A radiation-hydrodynamic model of accretion columns for ultra-luminous X-ray pulsars

Prompted by the recent discovery of pulsed emission from an ultra-luminous X-ray source, M82 X-2 ("ULX-pulsar"), we perform a two-dimensional radiation-hydrodynamic simulation of a super-critical accretion flow onto a neutron star through a narrow accretion column. We set an accretion column with a cone shape filled with tenuous gas with density of $10^{-4} {\rm g}~ {\rm cm}^{-3}$ above a neutron star and solve the two dimensional gas motion and radiative transfer within the column. The side boundaries are set such that radiation can freely escape, while gas cannot. Since the initial gas layer is not in a hydrostatic balance, the column gas falls onto the neutron-star surface, thereby a shock being generated. As a result, the accretion column is composed of two regions: an upper, nearly free-fall region and a lower settling region, as was noted by Basko \& Sunyaev (1976). The average accretion rate is very high; ${\dot M}\sim 10^{2-3} L_{\rm E}/c^2$ (with $L_{\rm E}$ being the Eddington luminosity), and so radiation energy dominates over gas internal energy entirely within the column. Despite the high accretion rate, the radiation flux in the laboratory frame is kept barely below $L_{\rm E}/(4πr^2)$ at a distance $r$ in the settling region so that matter can slowly accrete. This adjustment is made possible, since large amount of photons produced via dissipation of kinetic energy of matter can escape through the side boundaries. The total luminosity can greatly exceed $L_{\rm E}$ by several orders of magnitude, whereas the apparent luminosity observed from the top of the column is much less. Due to such highly anisotropic radiation fields, observed flux should exhibit periodic variations with the rotation period, provided that the rotation and magnetic axes are misaligned.

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arXiv2015

New method for black-hole spin measurement based on flux variation from an infalling gas ring

We propose a new method for black hole spin measurement. In this method, we consider a gas blob or ring falling onto a black hole from the marginally stable orbit, keeping its initial orbital angular momentum. We calculate the gas motion and photon trajectories in the Kerr space-time and, assuming that the gas blob or ring emits monochromatic radiation, carefully examine how it is observed by a distant observer. The light curve of the orbiting gas blob is composed of many peaks because of periodic enhancement of the flux due to the gravitational lensing and beaming effects. Further, the intensity of each peak first gradually increases with time due to the focusing effect around the photon circular orbit and then rapidly decreases due to the gravitational redshift, as the gas blob approaches the event horizon. The light curve of the gas ring is equivalent to a superposition of those of the blobs with various initial orbital phases, and so it is continuous and with no peaks. The flux first gradually increases and then rapidly decays, as in the blob model. The flux variation timescale depends on the black hole spin and is independent from the inclination angle, while time averaged frequency shift have dependences of both effects. We can thus, in principle, determine spin and inclination angle from observations. The observational implications and future issues are briefly discussed.

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arXiv2015

How does a Secular Instability Grow in a Hyperaccretion Flow?

Hyperaccretion flows with mass accretion rates far above the Eddington rate have an N-shaped equilibrium curve on the $Σ$-$\dot{M}$ plane (with $Σ$ and $\dot{M}$ being surface density and mass accretion rate, respectively). The accretion flow on the lower $Σ$ branch of the N-shape is optically thick, advection-dominated accretion flow (ADAF) while that on the upper one is neutrino-dominated accretion flow (NDAF). The middle branch has a negative slope on the $Σ$-$\dot{M}$ plane, meaning that the flow on this branch is secularly unstable. To investigate how the instability affects the flow structure and what observable signatures are produced, we study the time evolution of the unstable hyperaccretion flow in response to variable mass injection rates by solving the height-averaged equations for viscous accretion flows. When a transition occurs from the lower branch to the upper branch (or from the upper branch to the lower branch), the surface density rapidly increases (decreases) around that transition region, which induces locally enhanced mass flow (referred to as non-steady mass flow) into (out of) that region. We confirm that the non-steady flow can create a kind of disturbance and that it propagates over the whole disk. However, the non-steady mass flow is not strong enough to induce coherent transition over the whole disk, unless the mass injection rate varies with time. When the injection rate continuously changes, the neutrino luminosity varies intermittently, thus producing step-function-like light curves, as the radiation efficiency discontinuously changes every time the local transition occurs. The effects of changing the N-shape and possible observational consequences on the gamma-ray burst emission are briefly discussed in relation to gamma-ray bursts.

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arXiv2015

Comparison between RHD simulation of supercritical accretion flows and steady model with outflows

We apply our two-dimensional (2D), radially self-similar steady-state accretion flow model to the analysis of hydrodynamic simulation results of supercritical accretion flows. Self-similarity is checked and the input parameters for the model calculation, such as advective factor and heat capacity ratio, are obtained from time-averaged simulation data. Solutions of the model are then calculated and compared with the simulation results. We find that in the converged region of the simulation, excluding the part too close to the black hole, the radial distribution of azimuthal velocity $v_ϕ$, density $ρ$ and pressure $p$ basically follows the self-similar assumptions, i.e. they are roughly proportional to $r^{-0.5}$, $r^{-n}$, and $r^{-(n+1)}$, respectively, where $n\sim0.85$ for the mass injection rate of $1000L_\mathrm{E}/c^2$, and $n\sim0.74$ for $3000L_\mathrm{E}/c^2$. The distribution of $v_r$ and $v_θ$ agrees less with self-similarity, possibly due to convective motions in the $rθ$ plane. The distribution of velocity, density and pressure in $θ$ direction obtained by the steady model agrees well with the simulation results {within the calculation boundary of the steady model}. Outward mass flux in the simulations is overall directed toward polar angle of 0.8382 rad ($\sim 48.0^\circ$) for $1000L_\mathrm{E}/c^2$, and 0.7852 rad ($\sim 43.4^\circ$) for $3000L_\mathrm{E}/c^2$, and $\sim$94\% of the mass inflow are driven away as outflow, while outward momentum and energy fluxes are focused around the polar axis. Part of these fluxes lie in the region that are not calculated by the steady model, and special attention should be paid when the model is applied.

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arXiv2014

25 Years of Self-Organized Criticality: Solar and Astrophysics

Shortly after the seminal paper {\sl "Self-Organized Criticality: An explanation of 1/f noise"} by Bak, Tang, and Wiesenfeld (1987), the idea has been applied to solar physics, in {\sl "Avalanches and the Distribution of Solar Flares"} by Lu and Hamilton (1991). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme.

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arXiv2013

The Discovery of a New Instability in a Hyperaccretion Flow and its Implication for Gamma-ray Bursts

A hyperaccretion flow around a stellar mass black hole is thought to be the most plausible engine that powers gamma-ray bursts (GRBs). The flow efficiently cools via neutrino emission at >~ 0.003-0.01 M_sun s^{-1} (corresponding to a luminosity of ~10^50 erg s^{-1}), while neither neutrino nor photon emission is efficient below this rate, so the flow should be advection dominated. We carefully solve how a transition occurs from the advection-dominated to the neutrino-dominated branches, and find that the slope of the thermal equilibrium curve is negative in the surface density - accretion rate (Sigma-Mdot) plane, a condition for viscous instability, at radii smaller than ~12R_g (with R_g being the gravitational radius). We also confirm that the flow is thermally stable. The consequence of this instability is the formation of a clumpy structure in the flow. This is because the larger (respectively smaller) surface density is, the smaller (respectively larger) the mass accretion rate from the region in question becomes, leading to growth of the density contrast. The timescale for clump formation is estimated to be shorter than 0.1s. The observational implication is discussed in the context of GRBs. We suggest that this might explain the origin of the large variability observed in the prompt emission of GRBs.

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arXiv2012

Two Power-Law States of the Ultraluminous X-ray Source IC342 X-1

In order to elucidate the emission properties of ultraluminous X-ray sources (ULXs) during their power-law (PL) state, we examined long-term X-ray spectral data of IC342 X-1 during its PL state by using our own Suzaku data and the archival data by XMM-Newton, Chandra, and Swift observations. The PL state of this source seems to be classified into two sub-states in terms of the X-ray luminosities in 0.5-10 keV: the low luminosity PL state with 4-6*10^{39} erg/s and the high luminosity one with 1.1-1.4*10^{40} erg/s. During the Suzaku observations which were made in 2010 August and 2011 March, X-1 stayed in the low luminosity PL state. The observed X-ray luminosity (4.9-5.6*10^{39} erg/s) and the spectral shape (photon index = 1.67-1.83) slightly changed between the two observations. Using the Suzaku PIN detector, we for the first time confirmed a PL tail extending up to at least 20 keV with no signatures of a high-energy turnover in both of the Suzaku observations. In contrast, a turnover at about 6 keV was observed during the high luminosity PL state in 2004 and 2005 with XMM-Newton. Importantly, photon indices are similar between the two PL states and so is the Compton y-parameters of y ~ 1, which indicates a similar energy balance (between the corona and the accretion disk) holding in the two PL states despite different electron temperatures. From spectral similarities with recent studies about other ULXs and the Galactic black hole binary GRS1915+105, IC342 X-1 is also likely to be in a state with a supercritical accretion rate, although more sensitive higher energy observations would be necessary to conclude.

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