Accretion Disk Instability Research Articles

X-Ray Quasi-periodic Eruptions and Tidal Disruption Events Prefer Similar Host Galaxies

In the past 5 yr, six X-ray quasi-periodic eruption (QPE) sources have been discovered in the nuclei of nearby galaxies. Their origin remains an open question. We present Multi Unit Spectroscopic Explorer integral field spectroscopy of five QPE host galaxies to characterize their properties. We find that 3/5 galaxies host extended emission-line regions (EELRs) up to 10 kpc in size. The EELRs are photoionized by a nonstellar continuum, but the current nuclear luminosity is insufficient to power the observed emission lines. The EELRs are decoupled from the stars both kinematically and in projected sky position, and the low velocities and velocity dispersions (<100 km s−1 and ≲75 km s−1, respectively) are inconsistent with being driven by active galactic nuclei (AGNs) or shocks. The origin of the EELRs is likely a previous phase of nuclear activity. QPE host galaxies share several similarities with tidal disruption event (TDE) hosts, including an overrepresentation of galaxies with strong Balmer absorption and little ongoing star formation, as well as a preference for a short-lived (the typical EELR lifetime is ∼15,000 yr), gas-rich phase where the nucleus has recently faded significantly. This suggests that QPEs and TDEs may share a common formation channel, disfavoring AGN accretion disk instabilities as the origin of QPEs. If QPEs are related to extreme mass ratio inspiral systems (EMRIs), e.g., stellar-mass objects on bound orbits about massive black holes, the high incidence of EELRs and recently faded nuclei could be used to localize the hosts of EMRIs discovered by low-frequency gravitational-wave observatories.

The Calm Before the (Next) Storm: No Third Outburst in 2019–2020, and Ongoing Monitoring of the Transient AGN IC 3599

We report on follow-up observations of the Seyfert 1.9 galaxy IC 3599 with the NASA Neil Gehrels Swift mission. The detection of a second X-ray outburst in 2010 by Swift after the first discovery of a bright X-ray outburst in 1990 by ROSAT led to the suggestion of two very different explanations. The first one assumed that IC 3599 exhibits outbursts due to repeated partial tidal stripping of a star, predicting another outburst of IC 3599 in 2019/2020. The second, alternative scenario assumed that the event observed in X-rays is due to an accretion-disk instability, which would suggest a much longer period between the large outbursts. Our continued monitoring campaign by Swift allowed us to test the first scenario that predicted a repetition of high-amplitude flaring activity in 2019/2020. We do not find any evidence of dramatic flaring activity with factors of 100 since the last X-ray outburst seen in 2010. These observations support the accretion-disk scenario. Further, while IC 3599 remains in low-emission states, the long-term X-ray light curve of IC 3599 reveals ongoing strong variability of a factor of a few. The most remarkable event is a miniflare of a factor of 10 in X-rays in 2022 December. After that flare, the otherwise supersoft X-ray spectrum shows an exceptional hardening, reminiscent of a temporary corona formation.

A spectral study of GRS 1915+105 during its March 2017 NuSTAR observations

ABSTRACT The Nuclear Spectroscopic Telescope Array Mission (NuSTAR) observed the enigmatic microquasar GRS 1915+105 twice, on 2017 March 26 and 28. Using these NuSTAR observations, we performed a time-resolved spectral analysis. We detected a type C quasi-periodic oscillation (QPO) along with the harmonic and the sub-harmonic in both epochs. From our spectral modelling, we found a dual reflection component, where one of the components comes from the hot inner flow and the second component originates from the exterior region of the accretion disc. Additionally, we found strong, highly ionized accretion disc winds as indicated by Fe xxv (on 2017 March 26) and Fe xxvi (on 2017 March 28) absorption lines, likely ultrafast outflows. Finally, at the end of the second observation, we detected variability class, which marked the onset of heartbeat oscillations in the X-ray light curve. We perform time-resolved spectral analysis to comprehend the physical conditions responsible for the onset of the heartbeat. We found that the accretion disc became highly ionized (log ξ &amp;gt; 4), and the high-energy cut-off showed some signs of increase, approaching &amp;gt;100 keV before the onset of the heartbeat class. We have not detected any signature of accretion disc instabilities thought to cause the heartbeat oscillations. We also found a strong positive correlation between the QPO frequency and the power-law index.

Evolution of Negative Superhumps, Quasiperiodic Oscillations, and Outbursts in the Z Cam-type Dwarf Nova AH Her

AH Her is a Z Cam-type dwarf nova with an orbital period of ∼0.258 days. Dwarf nova oscillations and long-period dwarf nova oscillations have been detected, but no quasiperiodic oscillations (QPOs) and negative superhumps (NSHs) have been found. We investigated the association between NSHs, QPOs, and outbursts of AH Her based on Transiting Exoplanet Survey Satellite photometry. We find for the first time NSHs with a period of 0.24497(1) days in AH Her, and trace the variation in the amplitude and period of NSHs with the outburst. The amplitude of the NSHs is the most significant at quiescence, weakening as the outburst rises, becoming undetectable at the top, rebounding and weakening at the plateau, and strengthening again as the outburst declines. The variation in the accretion disk radius can explain the NSH amplitude variation except for the plateau, so we suggest that the relationship between the NSH amplitude and outburst can be used as a window to study the accretion disk instability and the origin of NSHs. In addition, we find periodic variations in the amplitude, maxima, and shape of the NSHs ranging from 2.33(2) to 2.68(5) days, which may be related to the precession of the tilted disk. Finally, we find QPOs at the top of AH Her’s long outburst with ∼2800 s similar to those of HS 2325+8205, suggesting that the presence of QPOs at the top of Z Cam’s long outburst may be a general phenomenon.

Long-term X-Ray Outburst in the TeV-detected Blazar Mrk 501 in 2021–2022: Further Clues for the Emission and Unstable Processes

This paper presents the results of a detailed X-ray timing and spectral analysis of Mrk 501, which are based mainly on the Swift data obtained during 2021 February–2022 December. The source showed strongly enhanced X-ray activity, characterized by a long-term increase in the baseline 0.3–10 keV flux level superimposed by shorter-term flares on timescales of a few weeks to about 2 months. During some time intervals, Mrk 501 was the brightest blazar in the X-ray sky and, moreover, showed an intense intraday variability, which was sometimes detected within the exposures lasting a few hundred seconds. These instances were characterized by fractional variability amplitudes of 4%–18% and were mostly observed during short-term X-ray flares. The source exhibited extreme spectral properties with dominance of the spectral curvature, frequent occurrence of hard photon indices in the 0.3–10 keV and 0.3–300 GeV bands, and the peak of a synchrotron spectral energy distribution in the hard X-ray range. These properties demonstrate the importance of relativistic magnetic reconnection, first-order Fermi mechanism within the magnetic field of different confinement efficiencies, stochastic acceleration, and hadronic processes. The X-ray and γ-ray fluxes showed a log-normal distribution, which hints at the imprint of accretion disk instabilities on the blazar jet, as well as the possible presence of hadronic cascades and random fluctuations in the particle acceleration rate. The optical–UV and γ-ray variabilities showed a weak or absent correlation with the X-ray flaring activity, which is not consistent with simple leptonic models and requires more complex particle acceleration, emission, and variability scenarios.

3D Radiation Hydrodynamic Simulations of Gravitational Instability in AGN Accretion Disks: Effects of Radiation Pressure

We perform 3D radiation hydrodynamic local shearing-box simulations to study the outcome of gravitational instability (GI) in optically thick active galactic nuclei (AGNs) accretion disks. GI develops when the Toomre parameter Q T ≲ 1, and may lead to turbulent heating that balances radiative cooling. However, when radiative cooling is too efficient, the disk may undergo runaway gravitational fragmentation. In the fully gas-pressure-dominated case, we confirm the classical result that such a thermal balance holds when the Shakura–Sunyaev viscosity parameter (α) due to the gravitationally driven turbulence is ≲0.2, corresponding to dimensionless cooling times Ωt cool ≳ 5. As the fraction of support by radiation pressure increases, the disk becomes more prone to fragmentation, with a reduced (increased) critical value of α (Ωt cool). The effect is already significant when the radiation pressure exceeds 10% of the gas pressure, while fully radiation-pressure-dominated disks fragment at t cool ≲ 50 Ω−1. The latter translates to a maximum turbulence level α ≲ 0.02, comparable to that generated by magnetorotational instability. Our results suggest that gravitationally unstable (Q T ∼ 1) outer regions of AGN disks with significant radiation pressure (likely for high/near-Eddington accretion rates) should always fragment into stars, and perhaps black holes.

Effect of Dust Evaporation and Thermal Instability on Temperature Distribution in a Protoplanetary Disk

The thermal instability of accretion disks is widely used to explain the activity of cataclysmic variables, but its manifestation in gas and dust disks in young stars has been studied in less detail. A semi-analytical stationary model is presented for calculating the equatorial temperature of a gas and dust disk around a young star. The model considers the opacity caused by dust and gas, as well as the evaporation of dust at temperatures above 1000 K. Using this model, the distributions of the equatorial temperature of the gas and dust disk are calculated under various assumptions on the source of opacity and the presence of dust. It is shown that when all the above processes are considered, the thermal balance equation in the region r1 AU has multiple temperature solutions. Thus, the conditions for thermal instability are satisfied in this region. As an illustration of the possible influence of instability on the nature of accretion in a protoplanetary disk, we consider a viscous disk model with α-parametrization of turbulent viscosity. It is shown that in such a model a non-stationary mode of disk evolution is realized with alternating phases of accumulation of matter in the inner disk and phases of its rapid accretion onto the star, which leads to a burst character of accretion. The results obtained indicate the need to take this instability into account when modeling the evolution of protoplanetary disks.

In situextreme mass ratio inspirals via subparsec formation and migration of stars in thin, gravitationally unstable AGN discs

ABSTRACTWe investigate the properties of stars born via gravitational instability in accretion discs around supermassive black holes (SMBHs) in active galactic nuclei (AGNs), and how this varies with the SMBH mass, accretion rate, or viscosity. We show with geometrically thin, steady-state disc solutions that fragmentation results in different populations of stars when one considers the initial conditions (e.g. density and temperature of the gravitationally unstable regions). We find that opacity gaps in discs around $10^6 \, {\rm M}_{\odot }$ SMBHs can trigger fragmentation at radii ≲ 10−2 pc, although the conditions lead to the formation of initially low stellar masses primarily at $0.1\!-\!0.5 \, {\rm M}_{\odot }$. Discs around more massive SMBHs ($M_{\rm BH} =10^{7-8} \, {\rm M}_{\odot }$) form moderately massive or supermassive stars (the majority at $10^{0-2} \, {\rm M}_{\odot }$). Using linear migration estimates, we discuss three outcomes: stars migrate till they are tidally destroyed, accreted as extreme mass ratio inspirals (EMRIs), or leftover after disc dispersal. For a single-AGN activity cycle, we find a lower limit for the EMRI rate $R_{\rm emri}\sim 0\!-\!10^{-4} \, \rm yr^{-1}$ per AGN assuming a star formation efficiency $\epsilon =1\!-\!30{{\ \rm per\ cent}}$. In cases where EMRIs occur, this implies a volumetric rate up to $0.5\!-\!10 \, \rm yr^{-1}\, Gpc^{-3}$ in the local Universe. The rates are particularly sensitive to model parameters for $M_{\rm BH}=10^6 \, {\rm M}_{\odot }$, for which EMRIs only occur if stars can accrete to 10s of solar masses. Our results provide further evidence that gas-embedded EMRIs can contribute a substantial fraction of events detectable by milliHz gravitational wave detectors such as the Laser Interferometer Space Antenna (LISA). Our disc solutions suggest the presence of migration traps, as has been found for more massive SMBH discs. Finally, the surviving population of stars after the disc lifetime leaves implications for stellar discs in galactic nuclei.

Long-term multi-wavelength variability and extreme spectral properties of the TeV-detected blazar 1ES 0033+595

Context. The multi-wavelength (MWL) properties of some TeV-detected blazars are still poorly understood. By studying the MWL properties of the BL Lacertae source 1ES 0033+595, we make further progress in our understanding of the instable physical processes responsible for particle acceleration to ultra-relativitic energies and the observed emission properties. Aims. This paper presents the results of a detailed X-ray timing and spectral analysis of the source 1ES 0033+595, allowing us to draw conclusions about the physical mechanisms responsible for particle acceleration and the generation of X-ray emission. We also examined the long-term MWL behaviour of the source and interband cross-correlations. Methods. Our study focused on the observations performed with the X-Ray Telescope on board the Neil Gehrels Swift Observatory (Swift-XRT) in the period 2005–2022. The MWL data sets were obtained with several space and ground-based instruments. We analysed the MWL flaring behaviour during the entire period, as well as in particular subperiods selected on the basis of the X-ray flaring activity of the target; constructed histograms characterising the distributions of the different spectral parameters and MWL fluxes; and checked cross-correlations between these parameters and/or MWL fluxes. The obtained results are compared with the predictions of the various theoretical studies and simulations presented by the different authors to date. Results. The source showed extreme spectral properties with dominance of high spectral curvature, frequent occurrence of very and extremely hard photon indices, and the presence of a synchrotron spectral energy distribution (SED) peak in the hard X-ray range (sometimes at energies beyond 10 keV). These properties reveal the importance of first-order Fermi acceleration with very low initial particle energy distribution, along with the co-existence of stochastic acceleration and hadronic processes. The source was characterised by very uneven and erratic flaring activity in diverse epochs: the period of strong flares (2013–2016) was preceded by a moderate variability and followed by a gradual long-term decrease in MWL flaring activity. We detected a number of instances of intraday 0.3–10 keV variability, which were sometimes observed within a few hundred seconds and explained by the interaction between the relativistic shock front and jet inhomogeneities with strong magnetic fields. The X-ray and γ-ray fluxes showed a lognormal distribution, which hints at the imprint of accretion disc instabilities on the blazar jet.

The 2019 Outburst of the 2005 Classical Nova V1047 Cen: A Record Breaking Dwarf Nova Outburst or a New Phenomenon?

We present a detailed study of the 2019 outburst of the cataclysmic variable V1047 Cen, which hosted a classical nova eruption in 2005. The peculiar outburst occurred 14 yr after the classical nova event and lasted for more than 400 days, reaching an amplitude of around 6 magnitudes in the optical. Early spectral follow-up revealed what could be a dwarf nova (accretion disk instability) outburst. However, the outburst duration, high-velocity (>2000 km s−1) features in the optical line profiles, luminous optical emission, and presence of prominent long-lasting radio emission together suggest a phenomenon more exotic and energetic than a dwarf nova outburst. The outburst amplitude, radiated energy, and spectral evolution are also not consistent with a classical nova eruption. There are similarities between V1047 Cen’s 2019 outburst and those of classical symbiotic stars, but pre-2005 images of the field of V1047 Cen indicate that the system likely hosts a dwarf companion, implying a typical cataclysmic variable system. Based on our multiwavelength observations, we suggest that the outburst may have started with a brightening of the disk due to enhanced mass transfer or disk instability, possibly leading to enhanced nuclear shell burning on the white dwarf, which was already experiencing some level of quasi-steady shell burning. This eventually led to the generation of a wind and/or bipolar, collimated outflows. The 2019 outburst of V1047 Cen appears to be unique, and nothing similar has been observed in a typical cataclysmic variable system before, hinting at a potentially new astrophysical phenomenon.

Combined dynamo of gravitational and magneto-rotational instability in irradiated accretion discs

Aims. We aim to assess whether magneto-rotational instability (MRI) can exist in a turbulent state generated by gravitational instability (GI). We investigated the magnetic field saturation and elucidated the ability of GI turbulence to act as a dynamo. Methods. The results were obtained by numerical simulations using the magnetohydrodynamics code Athena. A sub-routine to solve the Poisson equation for self-gravity using three-dimensional Fourier transforms was implemented for that purpose. A GI-turbulent state was then restarted, with a zero-net-flux type magnetic seed field being introduced. The seed field was chosen with β ≈ 1010 to make sure that the magnetic field of the stationary state is exclusively generated by the dynamo. Results. Shortly after introducing the magnetic seed field, a significant field amplification is observed, despite MRI not being active. This shows that GI acts as a kinematic dynamo. The growing magnetic field allows MRI to become active, which leads to the emergence of a butterfly diagram. The turbulent stress of the saturated state is found to be consistent with the superposition of GI stresses and MRI stresses. Moreover, the ratio of magnetic stress to magnetic pressure is found to lie in the 0.3−0.4 range, which is typical for MRI turbulence. Furthermore, it is found that the magnetic energy significantly decreases if self-gravity is turned off. This indicates, in accordance with the initial field amplification, that GI provides the dominant dynamo contribution and that MRI is not simply added but rather grows on the magnetic field provided by GI turbulence. Finally, it is shown that the combined GI-MRI-dynamo is consistent with an α − Ω model and that the observed oscillation frequency of the butterfly diagram roughly agrees with the model prediction.

Characterizing the γ-Ray Variability of Active Galactic Nuclei with the Stochastic Process Method

Gamma-ray astronomy in the time domain has been by now progressed further as the variabilities of active galactic nuclei (AGNs) on different timescales have been reported a lot. We study the γ-ray variabilities of 23 jetted AGNs by applying a stochastic process method to the ∼12.7 yr long-term light curve (LC) obtained by the Fermi-Large Area Telescope (Fermi-LAT). In this method, the stochastically driven damped simple harmonic oscillator (SHO) and the damped random-walk (DRW) models are used to model the long-term LCs. Our results show that the long-term variabilities of 23 AGNs can be characterized well by both SHO and DRW models. However, the SHO model is restricted in the overdamped mode, and the parameters are poorly constrained. The SHO power spectral densities (PSDs) are the same as those of the typical DRW PSD. In the plot of the rest-frame timescale that corresponds to the broken frequency in the PSD versus black hole mass, the intrinsic, characteristic γ-ray timescales of 23 AGNs occupy almost the same space with the optical variability timescales obtained from the accretion disk emission. This suggests a connection between the jet and the accretion disk. As with the optical variability of the AGN accretion disk, the γ-ray timescale is also consistent with the thermal timescale caused by the thermal instability in the standard accretion disk of AGNs.

Machine learning algorithm for minute-long burst searches

Minute-long Gravitational Wave (GW) transients are events lasting from few to hundreds of seconds. In opposition to compact binary mergers, their GW signals cover a wide range of poorly understood astrophysical phenomena such as accretion disk instabilities and magnetar flares. The lack of accurate and rapidly generated gravitational-wave emission models prevents the use of matched filtering methods. Such events are thus probed through the template-free excess-power method, consisting in searching for a local excess of power in the time-frequency space correlated between detectors. The problem can be viewed as a search for high-value clustered pixels within an image, which has been generally tackled by deep learning algorithms such as Convolutional Neural Networks (CNNs). In this work, we use a CNN as a anomaly detection tool for the long-duration searches. We show that it can reach a pixel-wise detection despite trained with minimal assumptions, while being able to retrieve both astrophysical signals and noise transients originating from instrumental coupling within the detectors. We also note that our neural network can extrapolate and connect partially disjoint signal tracks in the time-frequency plane.

The Super-Alfvénic Rotational Instability in Accretion Disks about Black Holes

The theory of instability of accretion disks about black holes, neutron stars, or protoplanets is revisited by means of the recent method of the Spectral Web. The cylindrical accretion disk differential equation is shown to be governed by the forward and backward Doppler-shifted continuous Alfvén spectra , where ω A is the static Alfvén frequency. It is crucial to take nonaxisymmetry (m ≠ 0) and super-Alfvénic rotation of the Doppler frames (∣mΩ∣ ≫ ∣ω A∣) into account. The continua and then overlap, ejecting a plethora of super-Alfvénic rotational instabilities (SARIs). In-depth analysis for small inhomogeneity shows that the two Alfvén singularities reduce the extent of the modes to sizes much smaller than the width of the accretion disk. Generalization for large inhomogeneity leads to the completely unprecedented result that, for mode numbers ∣k∣ ≫ ∣m∣, any complex ω in a wide neighborhood of the real axis is an approximate “eigenvalue.” The difference with genuine eigenmodes is that the amount of complementary energy to excite the modes is tiny, ∣W com∣ ≤ c, with c the machine accuracy of the computation. This yields a multitude of two-dimensional continua of quasi-discrete modes: quasi-continuum SARIs. We conjecture that the onset of 3D turbulence in magnetized accretion disks is governed not by the excitation of discrete axisymmetric magnetorotational instabilities but by the excitation of modes from these two-dimensional continua of quasi-discrete nonaxisymmetric SARIs.

Роль черного излучения в модификации критериев неустойчивости Джинса для экзопланетного пылевого плазменного диска при учете магнитной вязкости и лучевого теплообмена

Within the framework of the problem of modeling the evolution of an exoplanetary disk, the influence of blackbody radiation on the Jeans magneto-gravitational instability for a self-gravitating plasma disk is discussed, taking into account the influence on the critical length of the disturbing wave of rotation, magnetic field, dissipative processes of viscosity and thermal conductivity, as well as radiation pressure and radiation heat loss. Using the analysis of the normal mode, the general dispersion relation is derived. On its basis, modified criteria for the Jeans instability were obtained for a number of special cases associated with different relative orientations of the magnetic field, the axis of rotation, and the propagation vector of the disturbance wave. The stability of the environment is discussed using the Hurwitz criterion. Both in the case of longitudinal and transverse directions of propagation of the disturbance wave, the Jeans instability criterion is modified due to the presence of radiation heat losses and Stokes correction. The results obtained are aimed at solving the problem associated with the instability of exoplanetary accretion disks and with the formation of exoplanets.

Populations of Ultraluminous X-ray Sources in Galaxies: Origin and Evolution

A model of the population of ultraluminous X-ray sources (ULXs) in binary systems with black hole (BH) accretors is constructed by hybrid population synthesis and is compared with the model of the population of ULXs with magnetized neutron stars (NSs) that can be observed as pulsating ULXs (Kuranov et al. 2020). A model of the formation of BHs whereby their mass is determined by the mass of the CO core immediately before its collapse ( $$M_{\mathrm{CO}}$$ ) and ‘‘delayed’’ and ‘‘rapid’’ collapse models (Fryer et al. 2012) are considered. The possible transiency of ULXs due to accretion disk instability is taken into account. The parameters and evolution of ULXs in galaxies with a constant star formation rate (SFR) and in those with an old stellar population after an instantaneous star formation burst are computed. The maximum number of ULXs with BHs ( $${\sim}10$$ ) is reached in galaxies with a stationary $$SFR=10M_{\odot}$$ yr $${}^{-1}$$ $${\sim}1$$ Gyr after the beginning of star formation. ULXs observed after the end of star formation are close binary systems in which BHs and/or NSs formed before the end of star formation, while long-lived donors with a mass $${\sim}M_{\odot}$$ continue to overflow their Roche lobes after its end or have filled their Roche lobes even later. Several Gyr after the end of star formation the number of ULXs in galaxies with a mass $$M_{G}=10^{10}M_{\odot}$$ is no more than 0.1, most of them are ULXs with NSs. Persistent sources with a Roche-lobe-overflowing optical star dominate in ULXs with NSs, irrespective of the adopted star formation model. The transient sources are an order of magnitude fewer. The ULXs accreting from the stellar wind of the optical component are an order of magnitude fewer than the sources with accretion via Roche lobe overflow.

The stress–pressure lag in MRI turbulence and its implications for thermal instability in accretion discs

ABSTRACT The classical alpha-disc model assumes that the turbulent stress scales linearly with – and responds instantaneously to – the pressure. It is likely, however, that the stress possesses a non-negligible relaxation time and will lag behind the pressure on some time-scale. To measure the size of this lag we carry out unstratified 3D magnetohydrodynamic shearing box simulations with zero-net-magnetic-flux using the finite-volume code pluto. We impose thermal oscillations of varying periods via a cooling term, which in turn drives oscillations in the turbulent stress. Our simulations reveal that the stress oscillations lag behind the pressure by ∼5 orbits in cases where the oscillation period is several tens of orbits or more. We discuss the implication of our results for thermal and viscous overstability in discs around compact objects.

Modeling changing‐look active galactic nuclei phenomenon in 1D using accretion disk instabilities

AbstractApart from regular, low‐level stochastic variability, some active galactic nuclei (AGN) occasionally show exceptionally large changes in the luminosity, spectral shape, and/or X‐ray absorption. The most notable are the changes of the spectral type when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy or vice versa. Thus, a name was coined of “changing‐look AGN” (CL AGN). The origin of this phenomenon is still unknown, but for most of the sources, there are strong arguments in favor of the intrinsic changes. Understanding the nature of such rapid changes is a challenge to the models of black hole accretion flows since the timescales of the changes are much shorter than the standard disk viscous timescales. We aim to model the CL AGN phenomenon assuming that the underlying mechanism is the time‐dependent evolution of a black hole accretion disk unstable due to the dominant radiation pressure. We use a one‐dimensional, vertically integrated disk model, but we allow for the presence of the hot coronal layer above the disk and the presence of the inner purely hot flow. We focus on the variability timescales and amplitudes, which can be regulated by the action of large‐scale magnetic fields, the description of the disk‐corona coupling and the presence of an inner optically thin flow, like advection‐dominated accretion flow. We compare model predictions for the accretion disk around black hole mass .

A minimal model for vertical shear instability in protoplanetary accretion disks

The vertical shear instability is an axisymmetric effect suggested to drive turbulence in the magnetically inactive zones of protoplanetary accretion disks. Here we examine its physical mechanism in analytically tractable “minimal models” in three settings that include a uniform density fluid, a stratified atmosphere, and a shearing-box section of a protoplanetary disk. Each of these analyses show that the vertical shear instability's essence is similar to the slantwise convective symmetric instability in the mid-latitude Earth atmosphere, in the presence of vertical shear of the baroclinic jet stream, as well as mixing in the top layers of the Gulf Stream. We show that in order to obtain instability, the fluid parcels' slope should exceed the slope of the mean absolute momentum in the disk radial-vertical plane. We provide a detailed and mutually self-consistent physical explanation from three perspectives: in terms of angular momentum conservation, as a dynamical interplay between a fluid's radial and azimuthal vorticity components, and from an energy perspective involving a generalised Solberg-Høiland Rayleigh condition. Furthermore, we explain why anelastic dynamics yields oscillatory unstable modes and isolate the oscillation mechanism from the instability one.

A Radio Quasi-periodic Oscillation of 176 days in the Radio-loud Narrow-line Seyfert 1 Galaxy J0849+5108

Abstract We analyze the 11 yr long-term light curve of the Radio-loud Narrow-line Seyfert 1 galaxy J0849+5108 and the nearly simultaneous γ-ray data of the source. The data were obtained with the Owens Valley Radio Observatory 40 m telescope at 15 GHz and with the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope, respectively. A quasi-periodic oscillation (QPO) signal at a significance of &gt;5σ is found in the radio light curve, but no similar modulation is seen in the γ-ray light curve. The QPO signal was present for 21 cycles, the longest among the reported radio QPOs, and likely disappeared after 2019 January 16. Different mechanisms proposed to explain the observed QPOs in active galactic nuclei (AGNs) are discussed for this QPO case. Either a secular instability in the inner accretion disk or a helical structure in the jet of J0849+5108 may explain the radio QPO, while for the latter scenario the jet has to be collimated up to ∼200 pc, similar to that seen in the nearby AGN M87. It will be of interest to keep monitoring the source at radio frequencies, seeing whether similar QPO signals appear again.