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We present steady-state solutions for a one-dimensional, magnetically-driven accretion disk wind model based on magnetohydrodynamic equations. We assume a geometrically thin, gas-pressure-dominated accretion disk, incorporating both magnetic braking and turbulent viscosity introduced by an extended alpha-viscosity prescription. Additionally, the vertical stress parameter is assumed to scale with the disk aspect ratio. We confirm that the derived solutions result in standard disk solutions when the wind is absent. We find that the mass accretion rate decreases as the disk mass falls inward, while the mass loss rate increases with radius. The disk spectrum emitted from the magnetically-driven disk wind can be observed without interference from the wind medium because the wind is significantly optically thin. The spectral luminosity is proportional to $\nu^{1/3}$ in the intermediate, multicolor-blackbody wavebands, in the absence of wind, as predicted by standard disk theory. However, in the presence of wind, it follows a different power-law dependence on frequency over the same range. A deviation from the spectral slope of $1/3$, particularly a negative spectral slope, is a clear indicator of the presence of a magnetically driven wind. We also discuss an observational strategy to test our model with multi-wavelength observations., Comment: 24 pages, 4 figures, submitted to Progress of Theoretical and Experimental Physics (PTEP)

We study the effect of irradiation from two accretion disks (minidisks) around respective black holes of stellar to intermediate masses in a circular binary on the spectrum of a circumbinary disk (CBD) surrounding them. We assume the CBD to be a standard disk and adopt the orbit-averaged irradiation flux because the viscous timescale is much longer than the orbital period. We then solve the energy equation both analytically and numerically to compute the CBD temperature distribution and the corresponding disk spectrum. We find that the analytically calculated spectra are in good agreement with the numerical ones. The CBD spectrum is almost independent of the binary mass ratio. We also find that the combined spectra of two minidisks and the CBD have double peaks, one peak in the soft X-ray band and the other in the infrared (IR) band. The former peak comes from the two minidisks, while the latter peak from the CBD. The observed flux density increases with frequency as $\nu^{1/3}$ towards the soft X-ray peak, while it decreases with frequency away from the IR peak as $\nu^{-5/3}$. The latter feature is testable with near-IR observations with Subaru and JWST., Comment: 19 pages, 7 figures. Submitted to ApJ

Tidal disruption event (TDE) iPTF16fnl shows a relatively low optical flare with observationally very weak X-ray emission and the spectroscopic property that the helium emission line from the source dominates over the hydrogen emission line at early times. We explore these observed signatures by calculating spectral emission lines with the publicly available code, CLOUDY. We estimate five physical parameters by fitting the observed optical UV spectra on multiple days to a theoretical model of a steady-state, slim disk with a spherical outflow. The resultant key parameters among them are black hole mass $M_{\bullet} = (6.73 \pm 0.44) \times 10^5 M_{\odot}$, stellar mass $M_{\star} = (2.59 \pm 0.17) M_{\odot}$, and wind velocity $v_{\rm w} = 7447.43 \pm 183.9~{\rm km~s^{-1}}$. The disk-wind model also estimates the radiative efficiency to be $0.01\lesssim\eta\lesssim0.02$ over the observational time, resulting in the disk being radiatively inefficient, and the disk X-ray luminosity is consistent with the observed low luminosity. In our CLOUDY model, the filling factor of the wind is also estimated to be 0.8, suggesting that the wind is moderately clumpy. We reveal that the helium-to-hydrogen number density ratio of the wind lies between 0.1 and 0.15, which is nearly the same as the solar case, suggesting the tidally disrupted star is originally a main sequence star. Because the optical depth of the helium line is lower than the hydrogen line by two orders of magnitude, the helium line is significantly optically thinner than the hydrogen line. Consequently, our results indicate that the helium line luminosity dominates the hydrogen line luminosity due to the optical depth effect despite a small helium-to-hydrogen number density ratio value., Comment: 15 pages, 8 figures, submitted to Physical Review D

We present a time-dependent, one-dimensional, magnetically-driven disk wind model based on magnetohydrodynamic (MHD) equations, in the context of tidal disruption events (TDEs). We assume that the disk is geometrically thin and gas-pressure dominated, and explicitly accounts for magnetic braking and turbulent viscosity through an extended alpha-viscosity prescription. We find a particular wind solution for a set of basic equations that satisfies the necessary and sufficient conditions for vertically unbound MHD flows. The solution shows that the disk evolves with mass loss due to wind and accretion from the initial Gaussian density distribution. We confirm that the mass accretion rate follows the power law of time $t^{-19/16}$ at late times in the absence of wind, which matches the classical solution of Cannizzo et al. (1990). We find that the mass accretion rate is steeper than the $t^{-19/16}$ curve when the wind is present. Mass accretion is also induced by magnetic braking, known as the wind-driven accretion mechanism, which results in a faster decay with time of both the mass accretion and loss rates. In the disk emission, the ultraviolet (UV) luminosity is the highest among the optical, UV, and X-ray luminosities. While the optical and X-ray emission is observationally insignificant without magnetic braking, the X-ray emission is brighter at late times, especially in the presence of magnetic braking. This provides a possible explanation for observed delayed X-ray flares. Our model predicts that late-time bolometric light curves steeper than $t^{-19/16}$ in UV-bright TDEs are potentially compelling indicators of magnetically driven winds., Comment: 27 pages, 17 figures, Accepted for publication in The Astrophysical Journal

Circinus X-1 (Cir X-1) is a neutron star binary with an elliptical orbit of 16.6~days. The source is unique for its extreme youth, providing a key to understanding early binary evolution. However, its X-ray variability is too complex to reach a clear interpretation. We conducted the first high cadence (every 4 hours on average) observations covering one entire orbit using the NICER X-ray telescope. The X-ray flux behavior can be divided into stable, dip, and flaring phases. The X-ray spectra in all phases can be described by a common model consisting of a partially covered disk black body emission and the line features from a highly-ionized photo-ionized plasma. The spectral change over the orbit is attributable to rapid changes of the partial covering medium in the line-of-sight and gradual changes of the disk black body emission. Emission lines of the H-like and He-like Mg, Si, S, and Fe are detected, most prominently in the dip phase. The Fe emission lines change to absorption in the course of the transition from the dip phase to the flaring phase. The estimated ionization degree indicates no significant changes, suggesting that the photo-ionized plasma is stable over the orbit. We propose a simple model in which the disk black body emission is partially blocked by such a local medium in the line-of-sight that has spatial structures depending on the azimuth of the accretion disk. Emission lines upon the continuum emission are from the photo-ionized plasma located outside of the blocking material., Comment: Accepted for publication in ApJ, 16 pages, 7 figures

We construct a numerical model of steady-state, general relativistic (GR) super-Eddington accretion flows in an optically thick, advection-dominated regime, motivated by tidal disruption events wherein super-Eddington accretion assumes a pivotal role. Our model takes into account the loss of angular momentum due to radiation and the scale-height derivative in the basic equations of the GR slim disk. For comparison purposes, we also provide a new analytical solution for a radiation-pressure-dominant GR slim disk, which neglects the angular momentum loss due to radiation and the scale-height derivative. We find that the radiation pressure enhances by incorporating the scale height derivative into the basic equations. As a result, the surface density near the disk's inner edge decreases, whereas the disk temperature and scale height increase, brightening the disk spectrum in the soft X-ray waveband. Notably, an extremely high mass accretion rate significantly enhances the effect of the scale-height derivative, affecting the entire disk. In contrast, the inclusion of the radiation-driven angular momentum loss only slightly influences the disk surface density and temperature compared with the case of the scale-height derivative inclusion. The X-ray luminosity increases significantly due to scale height derivative for $\dot{M}/\dot{M}_{\rm Edd} \gtrsim 2$. In addition, the increment is higher for the non-spinning black hole than the spinning black hole case, resulting in a one-order of magnitude difference for $\dot{M}/\dot{M}_{\rm Edd}\gtrsim100$. We conclude that incorporating the scale-height derivative into a GR slim disk model is crucial as it impacts the disk structure and its resultant spectrum, particularly on a soft-X-ray waveband., Comment: 21 pages, 15 figures, 1 table, accepted for publication in Physical Review D

We study the evolution of a non-relativistically expanding thin shell in radio-emitting tidal disruption events (TDEs) based on a one-dimensional spherically symmetric model considering the effect of both a time-dependent mass loss rate of the disk wind and the ambient mass distribution. The analytical solutions are derived in two extreme limits: one is the approximate solution near the origin in the form of the Taylor series, and the other is the asymptotic solution in which the ambient matter is dominant far away from the origin. Our numerical solutions are confirmed to agree with the respective analytical solutions. We find that no simple power-law of time solution exists in early to middle times because the mass loss rate varies over time, affecting the shell dynamics. We also discuss the application of our model to the observed radio-emitting TDE, AT2019dsg., Comment: 28 pages, 6 figures, Accepted for publication in ApJ

The origin of cosmic high-energy neutrinos remains largely unexplained. For high-energy neutrino alerts from IceCube, a coincidence with time-variable emission has been seen for three different types of accreting black holes: (1) a gamma-ray flare from a blazar (TXS 0506+056), (2) an optical transient following a stellar tidal disruption event (TDE; AT2019dsg), and (3) an optical outburst from an active galactic nucleus (AGN; AT2019fdr). For the latter two sources, infrared follow-up observations revealed a powerful reverberation signal due to dust heated by the flare. This discovery motivates a systematic study of neutrino emission from all supermassive black hole with similar dust echoes. Because dust reprocessing is agnostic to the origin of the outburst, our work unifies TDEs and high-amplitude flares from AGN into a population that we dub accretion flares. Besides the two known events, we uncover a third flare that is coincident with a PeV-scale neutrino (AT2019aalc). Based solely on the optical and infrared properties, we estimate a significance of 3.6$\sigma$ for this association of high-energy neutrinos with three accretion flares. Our results imply that at least ~10% of the IceCube high-energy neutrino alerts could be due to accretion flares. This is surprising because the sum of the fluence of these flares is at least three orders of magnitude lower compared to the total fluence of normal AGN. It thus appears that the efficiency of high-energy neutrino production in accretion flares is increased compared to non-flaring AGN. We speculate that this can be explained by the high Eddington ratio of the flares., Comment: Accepted for publication in MNRAS

We study the nature of the changing-look Active Galactic Nucleus NGC 1566 during its June 2018 outburst. During the outburst, the X-ray intensity of the source rises up to ~25-30 times compared to its quiescent state intensity. We perform timing and spectral analysis of the source during pre-outburst, outburst, and post-outburst epochs using semi-simultaneous observations with the XMM-Newton, Nuclear Spectroscopic Telescope Array (NuSTAR), and Neil Gehrels Swift Observatories. We calculate variance, normalized variance, and fractional rms amplitude in different energy bands to study the variability. The broad-band 0.5-70 keV spectra are fitted with phenomenological models, as well as physical models. A strong soft X-ray excess is detected in the spectra during the outburst. The soft excess emission is found to be complex and could originate in the warm Comptonizing region in the inner accretion disc. We find that the increase in the accretion rate is responsible for the sudden rise in luminosity. This is supported by the q-shape of the hardness-intensity diagram that is generally found in outbursting black hole X-ray binaries. From our analysis, we find that NGC 1566 most likely harbours a low-spinning black hole with the spin parameter a* ~ 0.2. We also discuss a scenario where the central core of NGC 1566 could be a merging supermassive black hole., Comment: 18 pages, 10 figures, Accepted for publication in MNRAS

We propose a model to explain the time delay between the peak of the optical and X-ray luminosity, \dt hereafter, in UV/optically-selected tidal disruption events (TDEs). The following picture explains the observed \dt in several TDEs as a consequence of the circularization and disk accretion processes as long as the sub-Eddington accretion. At the beginning of the circularization, the fallback debris is thermalized by the self-crossing shock caused by relativistic precession, providing the peak optical emission. During the circularization process, the mass fallback rate decreases with time to form a ring around the supermassive black hole (SMBH). The formation timescale corresponds to the circularization timescale of the most tightly bound debris, which is less than a year to several decades, depending mostly on the penetration factor, the circularization efficiency, and the black hole mass. The ring will subsequently evolve viscously over the viscous diffusion time. We find that it accretes onto the SMBH on a fraction of the viscous timescale, which is $2$ years for given typical parameters, leading to X-ray emission at late times. The resultant \dt\,is given by the sum of the circularization timescale and the accretion timescale and significantly decreases with increasing penetration factor to several to $\sim10$ years typically. Since the X-ray luminosity substantially decreases as the viewing angle between the normal to the disk plane and line-of-sight increases from $0^\circ$ to $90^\circ$, a low late-time X-ray luminosity can be explained by an edge-on view. We also discuss the super-Eddington accretion scenario, where \dt\,is dominated by the circularization timescale., Comment: 21 pages, 4 figures, accepted for publication in ApJ

Tidal disruption events are an excellent probe for supermassive black holes in distant inactive galaxies because they show bright multi-wavelength flares lasting several months to years. AT2019dsg presents the first potential association with neutrino emission from such an explosive event., Comment: Published in Nature Astronomy, News & Views

Tidal disruption events (TDEs) provide a clue to the properties of a central supermassive black hole (SMBH) and an accretion disk around it, and to the stellar density and velocity distributions in the nuclear star cluster surrounding the SMBH. Deviations of TDE light curves from the standard occurring at a parabolic encounter with the SMBH depends on whether the stellar orbit is hyperbolic or eccentric (Hayasaki et al. 2018) and the penetration factor ($\beta$, tidal disruption radius to orbital pericenter ratio). We study the orbital parameters of bound and unbound stars being tidally disrupted by comparison of direct $N$-body simulation data with an analytical model. Starting from the classical steady-state Fokker-Planck model of Cohn & Kulsrud (1978), we develop an analytical model of the number density distribution of those stars as a function of orbital eccentricity ($e$) and $\beta$. To do so fittings of the density and velocity distribution of the nuclear star cluster and of the energy distribution of tidally disrupted stars are required and obtained from $N$-body data. We confirm that most of the stars causing TDEs in a spherical nuclear star cluster originate from the full loss-cone region of phase space, derive analytical boundaries in eccentricity-$\beta$ space, and find them confirmed by $N$-body data. Since our limiting eccentricities are much smaller than critical eccentricities for full accretion or full escape of stellar debris, we conclude that those stars are only very marginally eccentric or hyperbolic, close to parabolic., Comment: 28 pages, 10 figures, 1 table. Accepted for publication in ApJ

We study the mass fallback rate of tidally disrupted stars on marginally bound and unbound orbits around a supermassive black hole (SMBH) by performing three-dimensional smoothed particle hydrodynamic (SPH) simulations with three key parameters: the star is modeled by a polytrope with two different indexes ($n=1.5$ and $3$). The stellar orbital properties are characterized by five orbital eccentricities ranging from $e=0.98$ to $1.02$ and five different penetration factors ranging from $\beta=1$ to $3$, where $\beta$ represents the ratio of the tidal disruption to pericenter distance radii. We derive analytic formulae for the mass fallback rate as a function of the stellar density profile, orbital eccentricity, and penetration factor. Moreover, two critical eccentricities to classify tidal disruption events (TDEs) into five different types: eccentric ($e

We show results from the radiation hydrodynamics (RHD) simulations of tidal disruption of a star on a parabolic orbit by a supermassive black hole (SMBH) based on a three-dimensional smoothed particle hydrodynamics code with radiative transfer. We find that such a tidally disrupted star fragment and form clumps soon after its tidal disruption. The fragmentation results from the endothermic processes of ionization and dissociation that reduce the gas pressure, leading to local gravitational collapse. Radiative cooling is less effective because the stellar debris is still highly optically thick in such an early time. Our simulations reveal that a solar-type star with a stellar density profile of n=3 disrupted by a 10^6 solar mass black hole produces $\sim20$ clumps of masses in the range of 0.1 to 12 Jupiter masses. The mass fallback rate decays with time, with pronounced spikes from early to late time. The spikes provide evidence for the clumps of the returning debris, while the clumps on the unbound debris can be potentially freely-floating planets and brown dwarfs. This ionization and dissociation induced fragmentation on a tidally disrupted star are a promising candidate mechanism to form low-mass stars to planets around an SMBH., Comment: 13 pages, 4 figures, submitted for publication, comments welcome