Radiatively Inefficient Accretion Flow Research Articles

Magnetic field transport in geometrically thick discs: multidimensional effects on the field strength and inclination angle

ABSTRACT We theoretically investigate the magnetic flux transport in geometrically thick accretion discs which may form around black holes. We utilize a two-dimensional (2D) kinematic mean-field model for poloidal field transport which is governed by both inward advection and outward diffusion of the field. Assuming a steady state, we analytically show that the multidimensional effects prevent the field accumulation towards the centre and reduce the field inclination angle. We also numerically investigate the radial profile of the field strength and the inclination angle for two geometrically thick discs for which (quasi-)analytical solutions exist: radiatively inefficient accretion flows (RIAFs) and super-Eddington accretion flows. We develop a 2D kinematic mean-field code and perform simulations of flux transport to study the multidimensional effects. The numerical simulations are consistent with our analytical prediction. We also discuss a condition for the external field strength that RIAF can be a magnetically arrested disc. This study could be important for understanding the origin of a large-scale magnetic field that drives jets and disc winds around black holes.

Imaging ultracompact objects with radiatively inefficient accretion flows

Context. Recent Event Horizon Telescope observations of M 87* and Sgr A* strongly suggest the presence of a supermassive black hole at their respective cores. We use the semi-analytic radiatively inefficient accretion flows (RIAF) model to investigate the resulting images of the Joshi–Malafarina–Narayan (JMN-1) naked singularity and the Schwarzschild black hole. Aims. We chose the JMN-1 naked singularity model and compared the synchrotron images with the Schwarzschild solution to search for any distinct features that can distinguish the two objects and to find an alternative to the solution with a black hole. Methods. We performed general relativistic ray-tracing and radiative transfer simulations using the Brahma code to generate synchrotron-emission images using the thermal distribution function for emissivity and absorptivity. We investigated effects in the images by varying the inclination angle, the disk width, and the frequency. Results. The shadow images simulated with the JMN-1 model closely resemble those generated by the Schwarzschild black hole. The disparities between the two images are very small. We conducted simulations using various plasma parameters, but the resulting images remained largely consistent for both scenarios. This similarity is evident in the horizontal cross-sectional brightness profiles of the two scenarios. Notably, the JMN-1 model exhibits a slightly higher intensity than the Schwarzschild black hole. Conclusions. We conclude that JMN-1 is a viable substitute for the black hole scenario. This conclusion is not solely grounded in the fact that the two scenarios are indistinguishable from their respective shadow observations, but also in the consideration that JMN-1 emerges as an end state of a continual gravitational collapse. This paradigm not only allows for constraints on spacetime, but also provides a good probe for the nature of the central compact object.

A Study of the Accretion State of Magnetically Arrested Disks across Black Hole Spins for Radiatively Inefficient Accretion Flows

The study of magnetically arrested disks (MAD) has attracted strong interest in recent years because these disk configurations were found to generate strong jets, as observed in many accreting systems. Here, we present the results of 14 general relativistic magnetohydrodynamic simulations of advection-dominated accretion flow in the MAD state across black hole (BH) spins, carried out with cuHARM. Our main findings are as follows. (i) The jets transport a significant amount of angular momentum to infinity in the form of Maxwell stresses. For positive, high spin, the rate of angular momentum transport is about five times higher than for negative spin. This contribution is nearly absent for a nonrotating BH. (ii) The mass accretion rate and the MAD parameter, both calculated at the horizon, are not correlated. However, their time derivatives are anticorrelated for every spin. (iii) For zero spin, the contribution of the toroidal component of the magnetic field to the magnetic pressure is negligible, while for a fast-spinning BH, it is on the same order as the contribution of the radial magnetic component. For high positive spin, the toroidal component even dominates. (iv) For negative spins, the jets are narrower than their positive-spin counterparts, while their fluctuations are stronger. The weak jet from the nonrotating BH is the widest with the weakest fluctuations. Our results highlight the complex nonlinear connection between the black hole spin and the resulting disk and jet properties in the MAD regime.

Magnetic Flux Transport in Radiatively Inefficient Accretion Flows and the Pathway toward a Magnetically Arrested Disk

Large-scale magnetic fields play a vital role in determining the angular momentum transport and generating jets/outflows in accreting systems, yet their origins remain poorly understood. We focus on radiatively inefficient accretion flows (RIAFs) around black holes (BHs), and conduct 3D general-relativistic magnetohydrodynamic simulations using the Athena++ code. We first reconfirm that the magnetorotational instability driven dynamo in the RIAF alone does not spontaneously form a magnetically arrested disk (MAD), conducive for strong-jet formation. We next investigate the other possibility, where the large-scale magnetic fields are advected inward from external sources (e.g., the companion star in X-ray binaries and the magnetized ambient medium in active galactic nuclei). Although the actual configurations of the external fields could be complex and uncertain, they are likely to be closed. As a first study, we treat them as closed field loops of different sizes, shapes, and field strengths. Unlike earlier studies of flux transport, where the magnetic flux is injected into the initial laminar flow, we inject the magnetic field loops into the quasi-stationary turbulent RIAF in inflow equilibrium, then follow their evolution. We find that a substantial fraction (∼15%–40%) of the flux injected at large radii reaches the BH, with a weak dependence on the loop parameters, except when the loops are injected at high latitudes, away from the midplane. The relatively high efficiency of the flux transport observed in our study hints that a MAD might easily be formed relatively close to the BH, provided that a source of the large-scale field exists at larger radii.

Comprehensive X-ray view of the active nucleus in NGC 4258

Context.The presence of water masers orbiting around the active galactic nucleus (AGN) in NGC 4258, one of the most studied extragalactic objects, has been crucial in developing a detailed picture of its nuclear environment. Nonetheless, its accretion rate and bolometric luminosity are still matter of debate, as there are indications that NGC 4258 may host a genuine radiatively inefficient accretion flow (RIAF).Aims.In this context, we present a detailed broadband X-ray spectrum of NGC 4258, with the goal of precisely measuring the coronal luminosity and accretion flow properties of the AGN, in addition to tracking any possible variation across two decades of observations.Methods.We collected archivalXMM-Newton,Chandra,Swift/BAT, andNuSTARspectroscopic observations spanning 15 years and fit them with a suite of state-of-the-art models, including a warped disk model that is suspected to provide the well known obscuration observed in the X-rays. We complemented this information with archival results from the literature.Results.A clear spectral variability is observed among the different epochs. The obscuring column density shows possibly periodic fluctuations on a timescale of 10 years, while the intrinsic luminosity displays a long-term decrease by a factor of three across a time span of 15 years (fromL2−10 keV ∼ 1041erg s−1in the early 2000s toL2−10 keV ∼ 3 × 1040erg s−1in 2016). The average absorption-corrected X-ray luminosityL2−10 keV, combined with archival determinations of the bolometric luminosity, implies a bolometric correctionkbol ∼ 20; this result is intriguingly typical for Seyferts powered by accretion through geometrically thin, radiatively efficient disks. Moreover, the X-ray photon index Γ is consistent with the typical value of the broader AGN population. However, the accretion rate in Eddington units is very low, well within the expected RIAF regime.Conclusions.Our results suggest that NGC 4258 is a genuinely low-luminosity Seyfert II, with no strong indications in its X-ray emission for a hot, RIAF-like accretion flow.

Multiwavelength observations of the black hole X-ray binary MAXI J1820+070 in the rebrightening phase

Abstract We report the results of quasi-simultaneous multiwavelength (near-infrared, optical, UV, and X-ray) observations of the Galactic X-ray black hole binary MAXI J1820+070 performed in 2019 May 10–13, ∼60 d after the onset of the first rebrightening phase. It showed a much larger optical-to-X-ray luminosity ratio (∼8) than in the initial outburst epoch. The primary components of the spectral energy distribution (SED) can be best interpreted by a radiatively inefficient accretion flow (RIAF) spectrum showing a luminosity peak in the optical band. By comparison with theoretical calculations, we estimate the mass accretion rate to be $\dot{M}/(8 L_{\rm Edd}/c^2) \sim 10^{-3}$, where c is the light speed and LEdd is the Eddington luminosity. In addition to the RIAF emission, a blue power-law component is detected in the optical–UV SED, which is most likely synchrotron radiation from the jet. The optical spectrum taken at the Seimei telescope shows a weak and narrow Hα emission line, the emitting region of which is constrained to be ≳2 × 104 times the gravitational radius. We suggest that the entire disk structure cannot be described by a single RIAF solution but cooler material responsible for the Hα emission must exist at the outermost region.

Black hole weather forecasting with deep learning: a pilot study

ABSTRACT In this pilot study, we investigate the use of a deep learning (DL) model to temporally evolve the dynamics of gas accreting on to a black hole in the form of a radiatively inefficient accretion flow (RIAF). We have trained a convolutional neural network (CNN) on a data set that consists of numerical solutions of the hydrodynamical equations for a range of initial conditions. We find that deep neural networks trained on one simulation seem to learn reasonably well the spatiotemporal distribution of densities and mass continuity of a black hole accretion flow over a duration of 8 × 104GM/c3, comparable to the viscous time-scale at r = 400GM/c2; after that duration, the model drifts from the ground truth suffering from excessive artificial mass injection. Models trained on simulations with different initial conditions show some promise of generalizing to configurations not present in the training set, but also suffer from mass continuity issues. We discuss the caveats behind this method and the potential benefits that DL models offer. For instance, once trained the model evolves an RIAF on a single GPU four orders of magnitude faster than usual fluid dynamics integrators running in parallel on 200 CPU cores. We speculate that a data-driven machine learning approach should be very promising for accelerating simulations of accreting black holes.

Deep learning Bayesian inference for low-luminosity active galactic nuclei spectra

ABSTRACT Most active supermassive black holes in present-day galaxies are underfed and consist of low-luminosity active galactic nuclei (LLAGN). LLAGNs display complex multiwavelength broadband spectral energy distributions (SED), dominated by non-thermal processes which are explained to first order by a radiatively inefficient accretion flow (RIAF) and a relativistic jet. Due to the computational cost of generating such SEDs, it has not been hitherto possible to perform statistical fits to observed broadband SEDs, since such procedures require generating many thousands of models on-the-fly. Here, we have used a deep learning (DL) method to interpolate a large grid consisting of dozens of thousands of model SEDs for RIAFs and jets covering the parameter space appropriate for LLAGNs. Not only the DL method computes accurate models, it does so hundreds of thousands of times faster than solving the underlying dynamical and radiative transfer equations. This brings RIAF and jet models to the realm of Bayesian inference. We demonstrate that the combination of a DL interpolator and a Markov chain Monte Carlo ensemble sampler can recover the ground truth parameters of Mock LLAGN data. We apply our model to existing radio-to-X-rays observations of three LLAGNs: M87, NGC 315, and NGC 4261. We demonstrate that our model can estimate the relevant parameters of these accreting black holes such as the mass accretion and outflow rate at a small fraction of the computational cost of previous approaches.

Magnetically Arrested Disks in Quiescent Black Hole Binaries: Formation Scenario, Observable Signatures, and Potential PeVatrons

Abstract We propose magnetically arrested disks (MADs) in quiescent (low-luminosity) black hole (BH) binaries as the origin of multiwavelength emission, and argue that this class of sources can dominate the cosmic-ray spectrum around the knee. X-ray luminosities of Galactic BH binaries in the quiescent state are far below the Eddington luminosity, and thus radiatively inefficient accretion flows (RIAFs) are formed in the inner region. Strong thermal and turbulent pressures in RIAFs produce outflows, which can create large-scale poloidal magnetic fields. These fields are carried to the vicinity of the BH by the rapid inflow motion, forming a MAD. Inside the MAD, nonthermal protons and electrons are naturally accelerated by magnetic reconnections or stochastic acceleration by turbulence. Both thermal and nonthermal electrons emit broadband photons via synchrotron emission, which are broadly consistent with the optical and X-ray data of the quiescent BH X-ray binaries. Moreover, protons are accelerated up to PeV energies and diffusively escape from these MADs, which can account for the cosmic-ray intensity around the knee energy.

Constraints on the Mass Accretion Rate onto the Supermassive Black Hole of Cygnus A Using the Submillimeter Array

Abstract We present the first detailed polarimetric studies of Cygnus A at 230 GHz with the Submillimeter Array (SMA) to constrain the mass accretion rate onto its supermassive black hole. We detected the polarized emission associated with the core at a fractional polarization of . This low fractional polarization suggests that the polarized emission is highly depolarized. One of the possible explanations is due to a significant variance in the Faraday rotation measure within the synthesized beam. By assuming the Faraday depolarization caused by inhomogeneous column density of the magnetized plasma associated with the surrounding radiatively-inefficient accretion flow within the SMA beam, we derived the constraint on the mass accretion rate to be larger than 0.15 yr−1 at the Bondi radius. The derived constraint indicates that an adiabatic inflow–outflow solution or an advection-dominated accretion flow should be preferable as the accretion flow model in order to explain the jet power of Cygnus A.

High-energy Neutrino and Gamma-Ray Emission from Tidal Disruption Events

Abstract Tidal disruption events (TDE) have been considered as cosmic-ray and neutrino sources for a decade. We suggest two classes of new scenarios for high-energy multi-messenger emission from TDEs that do not have to harbor powerful jets. First, we investigate high-energy neutrino and gamma-ray production in the core region of a supermassive black hole. In particular, we show that ∼1–100 TeV neutrinos and MeV gamma rays can efficiently be produced in hot coronae around an accretion disk. We also study the consequences of particle acceleration in radiatively inefficient accretion flows (RIAFs). Second, we consider possible cosmic-ray acceleration by sub-relativistic disk-driven winds or interactions between tidal streams, and show that subsequent hadronuclear and photohadronic interactions inside the TDE debris lead to GeV-PeV neutrinos and sub-GeV cascade gamma rays. We demonstrate that these models should be accompanied by soft gamma rays or hard X-rays as well as optical/UV emission, which can be used for future observational tests. Although this work aims to present models of non-jetted high-energy emission, we discuss the implications of the TDE AT2019dsg that might coincide with the high-energy neutrino IceCube-191001A, by considering the corona, RIAF, hidden sub-relativistic wind, and hidden jet models. It is not yet possible to be conclusive about their physical association and the expected number of neutrinos is typically much less than unity. We find that the most optimistic cases of the corona and hidden wind models could be consistent with the observation of IceCube-191001A, whereas jet models are unlikely to explain the multi-messenger observations.

Radiation Magnetohydrodynamic Simulations of Sub-Eddington Accretion Flows in AGNs: Origin of Soft X-Ray Excess and Rapid Time Variabilities

Abstract We investigate the origin of the soft X-ray excess component in Seyfert galaxies observed when their luminosity exceeds 0.1% of the Eddington luminosity ( ). The evolution of a dense blob in radiatively inefficient accretion flow (RIAF) is simulated by applying a radiation magnetohydrodynamic code, CANS+R. When the accretion rate onto a black hole exceeds 10% of the Eddington accretion rate ( , where c is the speed of light), the dense blob shrinks vertically because of radiative cooling and forms a Thomson thick, relatively cool (∼107–8 K) region. The cool region coexists with the optically thin, hot ( ) RIAF near the black hole. The cool disk is responsible for the soft X-ray emission, while hard X-rays are emitted from the hot inner accretion flow. Such a hybrid structure of hot and cool accretion flows is consistent with the observations of both hard and soft X-ray emissions from “changing-look” active galactic nuclei (CLAGNs). Furthermore, we find that quasi-periodic oscillations (QPOs) are excited in the soft X-ray-emitting region. These oscillations can be the origin of rapid X-ray time variabilities observed in CLAGNs.

Spectral and imaging properties of Sgr A* from high-resolution 3D GRMHD simulations with radiative cooling

ABSTRACT The candidate supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*), is known to be fed by a radiatively inefficient accretion flow (RIAF), inferred by its low accretion rate. Consequently, radiative cooling has in general been overlooked in the study of Sgr A*. However, the radiative properties of the plasma in RIAFs are poorly understood. In this work, using full 3D general–relativistic magnetohydrodynamical simulations, we study the impact of radiative cooling on the dynamical evolution of the accreting plasma, presenting spectral energy distributions and synthetic sub-millimetre images generated from the accretion flow around Sgr A*. These simulations solve the approximated equations for radiative cooling processes self-consistently, including synchrotron, bremsstrahlung, and inverse Compton processes. We find that radiative cooling plays an increasingly important role in the dynamics of the accretion flow as the accretion rate increases: the mid-plane density grows and the infalling gas is less turbulent as cooling becomes stronger. The changes in the dynamical evolution become important when the accretion rate is larger than $10^{-8}\, M_{\odot }~{\rm yr}^{-1}$ ($\gtrsim 10^{-7} \dot{M}_{\rm Edd}$, where $\dot{M}_{\rm Edd}$ is the Eddington accretion rate). The resulting spectra in the cooled models also differ from those in the non-cooled models: the overall flux, including the peak values at the sub-mm and the far-UV, is slightly lower as a consequence of a decrease in the electron temperature. Our results suggest that radiative cooling should be carefully taken into account in modelling Sgr A* and other low-luminosity active galactic nuclei that have a mass accretion rate of $\dot{M} \gt 10^{-7}\, \dot{M}_{\rm Edd}$.

Blob formation and ejection from the radiative inefficient accretion flow around massive black hole

ABSTRACT We study the small scale magnetic reconnection above the radiative inefficient accretion flow around massive black hole via 2D magnetohydrodynamics (MHD) numerical simulation, in order to model the blob formation and ejection from the accretion flow around Sgr A*. The connection of both the newly emerging magnetic field and the pre-existing magnetic field is investigated to check whether blobs could be driven in the environment of black hole accretion disc. After the magnetic connection, both the velocity and temperature of the plasma can be comparable to the inferred physical properties at the base of the observed blob ejection. For illustration, three small boxes which are located within 40 Schwarzschild radii from the central black hole are chosen as our simulation areas. At the beginning of the reconnections, the fluid is pulled toward the central black hole due to the gravitational attraction and the current sheet produced by the reconnection is also pulled toward the same direction, consequently, the resulting outflows move both upwards and towards the symmetry axis of the central black hole. Eventually, huge blobs appear, which supports the catastrophe model of episodic jets (Yuan et al. 2009a). It is also found that the closer to the black hole the magnetic connection happens, the higher the converting efficiency of the magnetic energy into the heat and kinetic energy. For these inner blobs, they have vortex structure due to the Kelvin–Helmholtz instability, which happens along the current sheet separating the fluids with different speed.

The origin of hotspots around Sgr A*: orbital or pattern motion?

ABSTRACT The Gravity Collaboration detected a near-infrared hotspot moving around Sgr A* during the 2018 July 22 flare. They fitted the partial loop the hotspot made on the sky with a circular Keplerian orbit of radius $\simeq 7.5\, r_{\rm g}$ around the supermassive black hole (BH), where rg is the gravitational radius. However, because the hotspot traversed the loop in a short time, models in which the hotspot tracks the motion of some fluid element tend to produce a best-fitting trajectory smaller than the observed loop. This is true for a circular Keplerian orbit, even when BH spin is accounted for, and for motion along a radiatively inefficient accretion flow (RIAF) streamline. A marginally bound geodesic suffers from the same problem; in addition, it is not clear what the origin of an object following the geodesic would be. The observed hotspot motion is more likely a pattern motion. Circular motion with $r\simeq 12.5\, r_\mathrm{g}$ and a super-Keplerian speed $\simeq 0.8\, c$ is a good fit. Such motion must be pattern motion because it cannot be explained by physical forces. The pattern speed is compatible with magnetohydrodynamic perturbations, provided that the magnetic field is sufficiently strong. Circular pattern motion of radius $\sim 20\, r_{\rm g}$ on a plane above the BH is an equally good alternative; in this case, the hotspot may be caused by a precessing outflow interacting with a surrounding disc. As all our fits have relatively large radii, we cannot constrain the BH spin using these observations.

Radiation Magnetohydrodynamic Simulations of Soft X-ray Emitting Regions in Active Galactic Nuclei

AbstractWe present the results of global three-dimensional radiation magnetohydrodynamic simulations of the formation of soft X-ray emitting regions in active galactic nuclei by applying a radiation magnetohydrodynamic code based on the M1-closure scheme. The effect of Compton cooling is taken into account. When the surface density of the accretion flow exceeds the upper limit of the radiatively inefficient accretion flow (RIAF), the optically thin, hot accretion flow near the black hole co-exists with the soft X-ray emitting, warm (T = 106 – 107 K) Comptonized region around r = 20 – 40rs, where rs is the Schwarzschild radius. Numerical results indicate that when the accretion rate approaches the Eddington accretion rate, the warm Comptonized region stays in optically thin for effective optical depth, Thomson thick, and radiation pressure dominant state. This region is found to oscillate between a geometrically thin, cool state and a geometrically thick state inflated by radiation pressure. The time variability of the accretion flow is consistent with that of the narrow-line Seyfert 1 galaxies.

Characterizing the dynamo in a radiatively inefficient accretion flow

ABSTRACT We explore the magneto-rotational instability (MRI)-driven dynamo in a radiatively inefficient accretion flow (RIAF) using the mean field dynamo paradigm. Using singular value decomposition (SVD) we obtain the least-squares fitting dynamo coefficients α and γ by comparing the time series of the turbulent electromotive force and the mean magnetic field. Our study is the first one to show the poloidal distribution of these dynamo coefficients in global accretion flow simulations. Surprisingly, we obtain a high value of the turbulent pumping coefficient γ, which transports the mean magnetic flux radially outwards. This would have implications for the launching of magnetized jets that are produced efficiently in presence a large-scale poloidal magnetic field close to the compact object. We present a scenario of a truncated disc beyond the RIAF where a large-scale dynamo-generated poloidal magnetic field can aid jet launching close to the black hole. Magnitude of all the calculated coefficients decreases with radius. Meridional variations of αϕϕ, responsible for toroidal to poloidal field conversion, is very similar to that found in shearing box simulations using the ‘test field’ (TF) method. By estimating the relative importance of α-effect and shear, we conclude that the MRI-driven large-scale dynamo, which operates at high latitudes beyond a disc scale height, is essentially of the α − Ω type.

The Structure of Radiatively Inefficient Black Hole Accretion Flows

Abstract We run three long-timescale general-relativistic magnetohydrodynamic simulations of radiatively inefficient accretion flows (RIAFs) onto non-rotating black holes. Our aim is to achieve steady-state behavior out to large radii and understand the resulting flow structure. A simulation with adiabatic index Γ = 4/3 and small initial alternating poloidal magnetic field loops is run to a time of 440,000 GM/c 3, reaching inflow equilibrium inside a radius of 370 GM/c 2. Variations with larger alternating field loops and with Γ = 5/3 are run to 220,000 GM/c 3, attaining equilibrium out to 170 GM/c 2 and 440 GM/c 2. There is no universal self-similar behavior obtained at radii in inflow equilibrium: the Γ = 5/3 simulation shows a radial density profile with a power-law index ranging from −1 in the inner regions to −1/2 in the outer regions, while the others have a power-law slope ranging from −1/2 to close to −2. Both simulations with small field loops reach a state with polar inflow of matter, while the more ordered initial field has polar outflows. However, unbound outflows remove only a factor of order unity of the inflowing material over a factor of ∼300 in radius. Our results suggest that the dynamics of RIAFs are sensitive to how the flow is fed from larger radii, and may differ appreciably in different astrophysical systems. Millimeter images appropriate for Sgr A* are qualitatively (but not quantitatively) similar in all simulations, with a prominent asymmetric image due to Doppler boosting.

Gamma-ray observations of low-luminosity active galactic nuclei

ABSTRACT The majority of the activity around nearby (z ≈ 0) supermassive black holes is found in low-luminosity active galactic nuclei (LLAGN), the most of them being classified as low-ionization nuclear emission regions. Although these sources are well studied from radio up to X-rays, they are poorly understood in γ-rays. In this work, we take advantage of the all sky-surveying capabilities of the Large Area Telescope on board Fermi Gamma-ray Space Telescope to study the whole Palomar sample of LLAGN in γ-rays. Precisely, the four radio-brightest LLAGN in the sample are identified as significant γ-ray emitters, all of which are recognized as powerful Fanaroff–Riley I galaxies. These results suggest that the presence of powerful radio jets is of substantial importance for observing a significant γ-ray counterpart even if these jets are misaligned with respect to the line of sight. We also find that most of the X-ray-brightest LLAGN do not have a significant γ-ray and strong radio emission, suggesting that the X-rays come mainly from the accretion flow in these cases. A detailed analysis of the spectral energy distributions (SEDs) of NGC 315 and NGC 4261, both detected in γ-rays, is provided where we make a detailed comparison between the predicted hadronic γ-ray emission from a radiatively inefficient accretion flow (RIAF) and the γ-ray emission from a leptonic jet-dominated synchrotron self-Compton (SSC) model. Both SEDs are better described by the SSC model, while the RIAF fails to explain the γ-ray observations.

Neutrino Emissions from Tidal Disruption Remnants

Abstract We study high-energy neutrino emissions from tidal disruption remnants (TDRs) around supermassive black holes. The neutrinos are produced by the decay of charged pions originating in ultrarelativistic protons that are accelerated there. In the standard theory of tidal disruption events (TDEs), there are four distinct phases from the debris circularization of stellar debris to super- and sub-Eddington to radiatively inefficient accretion flows (RIAFs). In addition, we consider the magnetically arrested disk (MAD) state in both the super-Eddington accretion and RIAF phases. We find that there are three promising cases to produce neutrino emissions: the super-Eddington accretion phase of the MAD state and the RIAF phases of both the non-MAD and MAD states. In the super-Eddington MAD state, the enhanced magnetic field makes it possible to accelerate the protons to with the other given appropriate parameters. The neutrino energy is then at the peak of the energy spectrum. For M bh ≳ 107.7 M ⊙, the neutrino light curve is proportional to , while it follows the standard decay rate for . In both cases, the large luminosity and characteristic light curves diagnose the super-Eddington MAD state in TDEs. In the RIAF phase of the non-MAD state, we find and , and its light curve is proportional to . This indicates that one can identify whether the existing RIAFs are the TDE origin or not. TDRs are potentially a population of hidden neutrino sources invisible in gamma-rays.