Accretion Disk Model Research Articles

Magnetically activated accretion outbursts of pre-main-sequence discs

ABSTRACT We investigate whether triggering of the magnetorotational instability (MRI) in protoplanetary discs can account for the wide diversity of observed accretion outbursts. We show that short-lived, relatively low accretion rate events probably result from triggering in the inner disc and can occur at low surface densities, comparable to or smaller than the minimum mass solar nebula, and thus are very unlikely to result from MRI triggering by gravitational instability. We develop time-dependent accretion disc models using an α-viscosity approach and calculate light curves to compare with observations. Our modelling indicates that the lag time between infrared and optical bursts seen in Gaia 17bpi can be explained with an outside-in propagation with an α ∼ 0.1 in the MRI-active region, consistent with other estimates. While outbursts in inner discs can show time delays of a few yr between infrared and optical light curves, our models indicate that large, FU Ori-like bursts can exhibit infrared precursors decades before optical bursts. Detecting such precursors could enable analysis of the central star before it is overwhelmed by the rapid accreting material, as well as constraining outburst physics. Our results emphasize the importance of near-infrared monitoring of young stellar objects in addition to optical surveys. In addition, our findings emphasize the need for more sophisticated, three-dimensional, non-ideal magnetohydrodynamic simulations to fully exploit observational results.

Far-ultraviolet Spectroscopy of Active Galactic Nuclei with ASTROSAT/UVIT

We study accretion disk emission from eight Seyfert 1–1.5 active galactic nuclei (AGN) using far-ultraviolet (FUV) (1300–1800 Å) slitless grating spectra acquired with AstroSat/UVIT. We correct for the Galactic and intrinsic extinction, contamination from the host galaxies, narrow and broad-line regions, Fe ii emission, and Balmer continuum, and derive the intrinsic continua. We use Hubble Space Telescope COS/FOS spectra to account for the emission/absorption lines in the low-resolution UVIT spectra. We find generally redder power-law (f ν ∝ ν α ) slopes (α ∼ −1.1 to 0.3) in the FUV band than predicted by the standard accretion disk model in the optical/UV band. We fit accretion disk models such as the multitemperature disk blackbody (DISKBB) and relativistic disk (ZKERRBB, OPTXAGNF) models to the observed intrinsic continuum emission. We measure the inner disk temperatures using the DISKBB model for seven AGN. These temperatures in the range ∼3.6–5.8 eV are lower than the peak temperatures predicted for standard disks around maximally spinning supermassive black holes accreting at Eddington rates. The inner disks in two AGN, NGC 7469, and Mrk 352, appear to be truncated at ∼35–125 and 50–135 r g , respectively. While our results show that the intrinsic FUV emission from the AGN is consistent with the standard disks, it is possible that UV continua may be affected by the presence of soft X-ray excess emission, X-ray reprocessing, and thermal Comptonization in the hot corona. Joint spectral modeling of simultaneously acquired UV/X-ray data may be necessary to further investigate the nature of accretion disks in AGN.

Radial velocity and light curves analysis of eclipsing binary system V448 Cyg and investigation of its accretion disk model

Radial velocity and light curves analysis of eclipsing binary system V448 Cyg and investigation of its accretion disk model

Optical/UV emission in the Tidal Disruption Event ASASSN-14li: implications of disc modelling

ABSTRACT We predict late-time optical/UV emission from tidal disruption events (TDEs) from our slim accretion disc model (Wen et al. 2020) and explore the impact of the black hole mass M•, black hole spin a•, and accretion disc size. We use these synthetic spectra to successfully fit the multiband Swift observations of ASASSN-14li at >350 d, setting only the host galaxy extinction and outer disc radius as free parameters and employing the M•, a•, disc inclination, and disc accretion rates derived from fitting 10 epochs of ASASSN-14li’s X-ray spectra with the slim disc. To address the nature of the early-time optical/UV emission, we consider two models: shock dissipation and reprocessing. We find that (1) the predicted late-time optical/UV colour (e.g. u − w2) is insensitive to black hole and disc parameters unless the disc spreads quickly; (2) a starburst galaxy extinction model is required to fit the data, consistent with ASASSN-14li’s post-starburst host; (3) surprisingly, the outer disc radius is ≈2 × the tidal radius and ∼constant at late times, showing that viscous spreading is slow or non-existent; (4) the shock model can be self-consistent if M• ≲ 106.75 M⊙, i.e. on the low end of ASASSN-14li’s M• range (106.5–7.1 M⊙; 1σ CL); larger black hole masses require disruption of an unrealistically massive progenitor star; (5) the gas mass needed for reprocessing, whether by a quasi-static or an outflowing layer, can be <0.5 M⊙, consistent with a (plausible) disruption of a solar-mass star.

Characterising SMSS J2157–3602, the most luminous known quasar, with accretion disc models

ABSTRACT We develop an accretion disc (AD)-fitting method, utilizing thin and slim disc models and Bayesian inference with the Markov-Chain Monte Carlo approach, testing it on the most luminous known quasar, SMSS J215728.21-360215.1, at redshift z = 4.692. With a spectral energy distribution constructed from near-infrared spectra and broad-band photometry, the AD models find a black hole mass of $\log (M_{\rm {AD}}/{\rm M}_{\odot }) = 10.31^{+0.17}_{-0.14}$ with an anisotropy-corrected bolometric luminosity of $\log {(L_{\rm {bol}}/\rm {erg\, s^{-1}})} = 47.87 \pm 0.10$ and derive an Eddington ratio of $0.29^{+0.11}_{-0.10}$ as well as a radiative efficiency of $0.09^{+0.05}_{-0.03}$. Using the near-infrared spectra, we estimate the single-epoch virial black hole mass estimate to be log (MSE/M⊙) = 10.33 ± 0.08, with a monochromatic luminosity at 3000 Å of $\log {(L(\rm {3000{\AA}})/\rm {erg\, s^{-1}})} = 47.66 \pm 0.01$. As an independent approach, AD fitting has the potential to complement the single-epoch virial mass method in obtaining stronger constraints on properties of massive quasar black holes across a wide range of redshifts.

Observational predictions of LQG motivated polymerized black holes and constraints from Sgr A* and M87*

Loop quantum gravity inspired partial polymer quantization in four-dimensional spacetime leads to a globally regular black hole with a single horizon. The polymerized black hole metric is characterized by the minimum length parameter k, and mimics the Schwarzschild black hole in the weak-field limit. We present an analytic and numerical investigation of the strong gravitational lensing and shadow morphology to determine the observational impacts of quantum effects. Interestingly, the light deflection angle, the angular separation between the outermost relativistic image, and magnification are significantly larger than those for the Schwarzschild black hole. Using the ray-tracing technique, we simulate the black hole shadows under three distinct optically thin accretion models: static spherical accretion, radially infalling spherical accretion, and the thin accretion disk model. Polymerized black holes' shadow morphology strongly depends on k. We derive constraints on k from the M87* and Sgr A* black hole shadow observations from the Event Horizon Telescope.

The jet emitting disk‐standard accretion disk model applied to the active galactic nuclei ultra violet–X‐ray correlation

AbstractThe non‐linear correlation between the UV and X‐ray emission observed in active galactic nuclei remains a puzzling question that challenges accretion models. While the UV emission originates from the cold disk, the X‐ray emission is emitted by a hot corona whose physical characteristics and geometry are still highly debated. The jet emitting disk‐standard accretion disk (JED‐SAD) is a spectral model stemming from self‐similar accretion‐ejection solutions. It is composed of an inner highly magnetized and hot accretion flow launching jets, the JED, and an outer SAD. The model has been successfully applied to X‐ray binaries outbursts. The AGN UV–X‐ray correlation represents another essential test for the JED‐SAD model. We use multiple AGN samples to explore the parameter space and identify the regions able to reproduce the observations. In this first paper, we show that the JED‐SAD model is able to reproduce the UV–X‐ray correlation.

Long-term Quasiperiodicity in LMXB 4U 1636–536

We present the results of the low-mass X-ray binary 4U 1636–536 study. We performed a temporal analysis of all available RXTE/All-Sky Monitor, RXTE/Proportional Counter Array, Swift/Burst Alert Telescope, and MAXI data collected until the middle of 2022. Within our analysis we confirmed the previously discovered quasiperiodicity of ≃45 days present during ∼2004, and we found that it continued until 2007. Moreover, we showed that similar periodicity also appeared in 2010 and 2011. At other epochs, the quasiperiodicity is only transient. We applied a time-dependent accretion disk model to the interval with the most significant X-ray quasiperiodicity. Although 4U 1636–536 is persistent, the observed quasiperiodicity can be well modeled by the hydrogen thermal-ionization instability occurring in the outer regions of the accretion disk. For our best model, the theoretical light curve’s period and amplitude agree well with those observed.

Similar Signatures of Coplanar Gas Inflow and Disk Warps in Galactic Gas Kinematic Maps

Hydrodynamic simulations suggest that galactic gas disks form when coplanar gas spirals into the inner regions of the disk. We recently presented a simple modified accretion disk model of viscous galactic disks in which star formation is fed by a radial flow of gas. However, little observational evidence has been presented for such inflows, which are expected to be only a few kilometers per second in the central regions of the disk, i.e., within four disk scale lengths, but could reach of order 50–100 km s−1 in the very outer disk. The effects of systematic inflow on the 2D velocity field are examined and it is shown that these are quite similar to those produced by geometric warps of the disks, with twist distortions of both the kinematic major and minor axes. This makes it potentially difficult to distinguish between these in practice. By comparing the handedness of the observed twisting of the kinematic axes and of the spiral arms for a sample of nearby galaxies, we find (assuming that the spiral arms are generally trailing) that the effects of warps are in fact likely to dominate over the effects of radial inflows. However, the common practice of treating these twist distortions of the kinematic major and minor axes as being due only to warps can lead, for galaxies of low-to-intermediate inclinations, to substantial underestimates of any systematic inflow.

Testing relativistic accretion disk models with GRO J1655‐40

AbstractBlack hole X‐ray binaries are ideal environments to test the accretion phenomena in the presence of strong gravitational potentials. KERRBB held an important place in the X‐ray spectral continuum method for measuring the black hole spin modeling the emission from the innermost regions of the accretion disk. In this work, we present the results of X‐ray spectral analysis using publicly available RXTE data of GRO J1655‐40 obtained during the 2005 outburst with the two relativistic accretion disk models, KERRBB and KYNBB. Our analysis showed that both models provide identical results with black hole spin measurements, disk temperature, and disk luminosity when the inner edge of the accretion disk is set at the innermost stable circular orbit (ISCO) for the same accretion rates. We could not obtain reasonable fits for ∼89% of the observations with a fixed black hole spin value at a* = 0.7 using both models. Allowing the spin parameter to vary improved the fit statistic significantly with reduced χ2 values being reduced from ∼10 to 100 to below 2. Both models revealed black hole spin values varying between 0.52 < a* < 0.94, which can be interpreted as a variable inner edge of the disk throughout different accretion states.

The rapidly spinning intermediate-mass black hole 3XMM J150052.0+015452

ABSTRACT A star tidally disrupted by a black hole can form an accretion disc with a super-Eddington mass accretion rate; the X-ray emission produced by the inner disc provides constraints on the black hole mass M• and dimensionless spin parameter a•. Previous studies have suggested that the M• responsible for the tidal disruption event 3XMM J150052.0+015452 (hereafter J150052) is ∼105 M⊙ in the intermediate-mass black hole (IMBH) regime. Fitting multi-epoch XMM–Newton and Chandra X-ray spectra obtained after 2008 during the source’s decade-long decay, with our latest slim accretion disc model, gives $M_\bullet = 2.0^{+1.0}_{-0.3}\times 10^{5}$ M⊙ (at 68 per cent confidence) and a• > 0.97 (an 84.1 per cent confidence lower limit). The spectra obtained between 2008 and 2014 are significantly harder than those after 2014, an evolution that can be well explained by including the effects of inverse Comptonization by a corona on the early-time spectra. The corona is present when the source accretion rate is super-Eddington, while there is no evidence for its effect in data obtained after 2014, when the mass accretion rate is around the Eddington limit. Based on our spectral study, we infer that the corona is optically thick and warm ($kT_\mathrm{ e}=2.3^{+2.7}_{-0.8}$ keV). Our mass and spin measurements of J150052 confirm it as an IMBH and point to a rapid, near-extremal, spin. These M• and a• values rule out both vector bosons and axions of masses ∼10−16 eV.

From X-rays to physical parameters: a comprehensive analysis of thermal tidal disruption event X-ray spectra

ABSTRACT We perform a comprehensive analysis of a population of 19 X-ray bright tidal disruption events (TDEs), fitting their X-ray spectra with a new, physically self-consistent, relativistic accretion disc model. Not all of the TDEs inhabit regions of parameter space where the model is valid, or have sufficient data for a detailed analysis, and physically interpretable parameters for a subsample of 11 TDEs are determined. These sources have thermal (power-law free) X-ray spectra. The radial sizes measured from these spectra lie at values consistent with the innermost stable circular orbit of black holes with masses given by the MBH–σ relationship, and can be used as an independent measurement of MBH. The bolometric disc luminosity can also be inferred from X-ray data. All of the TDEs have luminosities that are sub-Eddington (Lbol, disc ≲ Ledd), and larger than the typical hard-state transitional luminosity of X-ray binary discs (Lbol, disc ≳ 0.01Ledd). The peak bolometric luminosity is found to be linearly correlated with the MBH–σ mass. The TDE X-ray-to-bolometric correction can reach values up to ∼100, and grows exponentially at late times, resolving the missing energy problem. We show that the peak disc luminosities of some TDEs are smaller than their observed optical luminosities, implying that not all of the early time optical emission can be sourced from reprocessed disc emission. Our results are supportive of the hypothesis that thermal X-ray bright TDEs are in accretion states analogous to the ‘soft’ accretion state of X-ray binaries, and that black hole accretion processes are scale (mass) invariant.

Rimmed and rippled accretion disc models to explain AGN continuum lags

ABSTRACT We propose a solution to the problem of accretion disc sizes in active galactic nuclei being larger when measured by reverberation mapping than predicted by theory. Considering the disc’s exposed-surface thickness profile H(r), our solution invokes a steep rim or rippled structures irradiated by the central lamp-post. We model the continuum lags and the faint and bright disc spectral energy distribution (SED) in the best-studied case NGC 5548 (black hole mass $M_\bullet =7\times 10^7\, \mathrm{M}_\odot$, disc inclination i = 45°). With the lamp-post off, the faint-disc SED fixes a low accretion rate $\dot{M}\simeq 0.0014~\mathrm{M}_\odot \, {\rm yr}^{-1}$ and high prograde black hole spin a• ≃ 0.93, for which $r_{\rm in}=2\, G\, M_\bullet /c^2$ and $L_{\rm disc}=0.25\, \dot{M}\, c^2$. The bright-disc SED then requires a lamp-post luminosity ${L_{\rm LP}}\simeq 5\, \dot{M}\, c^2/(1-A)$ for disc albedo A. Reprocessing on the thin disc with T ∝ r−3/4 gives time lags τ ∝ λ4/3 but three times smaller than observed. Introducing a steep H(r) rim, or multiple crests, near r ∼ 5 light days, reprocessing on the steep centre-facing slope increases temperatures from ∼1500 to ∼6000 K, and this increases optical lags to match the lag data. Most of the disc surface maintains the cooler T ∝ r−3/4 profile that matches the SED. The bright lamp-post may be powered by magnetic links tapping the black hole spin. The steep rim occurs near the disc’s dust sublimation radius as in the ‘failed disc wind model for broad-line clouds’. Lens–Thirring torques aligning the disc and black hole spin may also raise a warp and associated waves. In both scenarios, the small density scale height implied by the inferred value of H(r) suggests possible marginal gravitational instability in the disc.

Outer disc edge: properties of low-frequency aperiodic variability in ultracompact interacting binaries

ABSTRACT Flickering, and more specifically aperiodic broad-band variability, is an important phenomenon used in understanding the geometry and dynamics of accretion flows. Although the inner regions of accretion flows are known to generate variability on relatively fast time-scales, the broad-band variability generated in the outer regions has mostly remained elusive due to its long intrinsic variability time-scales. Ultracompact AM CVn systems are relatively small when compared to other accreting binaries and are well suited to search and characterize low-frequency variability. Here, we present the first low-frequency power spectral analysis of the ultracompact accreting white dwarf system SDSS J1908+3940. The analysis reveals a low-frequency break at ∼6.8 × 10−7 Hz in the time-averaged power spectrum as well as a second higher frequency component with characteristic frequency of ∼1.3 × 10−4 Hz. We associate both components with the viscous time-scales within the disc through empirical fits to the power spectrum as well as analytical fits using the fluctuating accretion disc model. Our results show that the low-frequency break can be associated with the outer disc regions of a geometrically thin accretion flow. The detection of the low-frequency break in SDSS J1908+3940 provides a precedent for further detection of similar features in other ultracompact accreting systems. More importantly, it provides a new observable that can help constrain simulations of accretion flows.

Thick accretion disk configurations in the Born-Infeld teleparallel gravity

The main goal of this paper is to investigate one of the important astrophysical systems, namely Thick accretion disks, in the background of the spherically symmetric solution in Born-Infeld teleparallel gravity to examine observable predictions of the theory in the vicinity of black holes. Thus, the properties of the non-self-gravitating equilibrium surfaces characterising the Thick accretion disks model are studied. In addition, we find an observational bound on the parameter of the model as $\lambda\gtrsim 140$. We show this analytical accretion disk model for different values of $\lambda$ and compare the result with the corresponding Schwarzschild solution in the general theory of relativity.

Spectral characteristics of the black hole binary 4U 1957+115: a multi mission perspective

ABSTRACT We report spectral analysis of the persistent black hole X-ray binary, 4U 1957+115, using AstroSat, Swift, and NuSTAR observations carried out between 2016 and 2019. Modelling with a disc emission, thermal Comptonization, and blurred reflection components revealed that the source was in the high-soft state with the disc flux ∼87 per cent of the total and high-energy photon index ∼2.6. There is an evidence that either the inner disc radius varied by ∼25 per cent or the colour hardening factor changed by ∼12 per cent. The values of the inner disc radius imply that for a non-spinning black hole, the black hole mass is < 7 M ⊙ and the source is located > 30 kpc away. On the other hand, a rapidly spinning black hole would be consistent with the more plausible black hole mass of < 10 M ⊙ and a source distance of ∼10 kpc. Fixing the distance to 10 kpc and using a relativistic accretion disc model, constrained the black hole mass to 6 M⊙ and inclination angle to 72°. A positive correlation is detected between the accretion rate and inner radii or equivalently between the accretion rate and colour factor.

Stochastic dynamical description of κ and ρ bursting classes of the microquasar GRS 1915+105

ABSTRACT We applied a stochastic analysis based on the Langevin equation to some bursting X-ray light curves of the bright microquasar GRS 1915+105. These time series, obtained with the PCA instrument onboard Rossi-XTE, are typical of the ρ and κ classes, which are characterized by several recurring bursts with different profiles and recurrence times. We computed the state functions which are able to capture the main dynamical properties of these light curves. In the case of the κ class, the state functions in four energy bands have a double-well structure (two stable states with an unstable one in between) and the system oscillates from one to the other minimum describing a typical hysteresis cycle, as in the case of a stochastic resonance under a forcing input. For the ρ class, the functions have only one stable state followed by an unstable interval, and the system describes a limit cycle. We also performed a new spectral analysis of these data with a multitemperature zero torque accretion disc model and used the resulting parameters for interpreting the main properties of the stochastic description.

Large-scale dynamo model for accretion disks

Context. Magnetic fields in accretion disks play an important role in the rich dynamics of these systems. A dynamo theory describing the generation of these magnetic field is in general very complex and requires many assumptions in order to be of practical use. In this respect, a theory with as few assumptions as possible is desirable. Aims. To investigate the generation of magnetic fields in accretion disks around magnetized central objects, a large-scale dynamo model is employed that includes feedback effects on the mass motion due to the Lorentz force. The dynamo model was developed from the fundamental magnetohydrodynamics equations with a minimum of hypothesis, and was tested in the case of the Sun and other stars. It is applied to accretion disks for the first time. Methods. The magnetic field in the disk, generated by the mentioned dynamo theory, was matched to that of the central object, considered dipolar, and to that of a magnetosphere described with the Grad–Shafranov equation. The relation between axial current and magnetic flux required in the Grad–Shafranov equation was not imposed, but was self-consistently determined along with the full solution. Results. The model is able to reproduce the patterns of magnetic field lines obtained in several works, such as closed magnetic lines near the central object and open lines for larger radii. The maximum value of the field is located near the internal radius of the accretion disk, where the currents in the disk force the concentration of field lines of the central object in the magnetosphere around this region. By varying the values of stellar mass, stellar magnetic field, mass accretion rate, and internal radius of the disk, it is found that the stellar magnetic field is the most important parameter in the determination of the disk magnetic field. The stellar mass is of secondary importance. It affects the azimuthal component of the disk magnetic field. The internal radius of the disk affects the disk zonal magnetic field and is likewise less important.

An elliptical accretion disk following the tidal disruption event AT 2020zso

Aims. The modelling of spectroscopic observations of tidal disruption events (TDEs) to date suggests that the newly formed accretion disks are mostly quasi-circular. In this work we study the transient event AT 2020zso, hosted by an active galactic nucleus (AGN; as inferred from narrow emission line diagnostics), with the aim of characterising the properties of its newly formed accretion flow. Methods. We classify AT 2020zso as a TDE based on the blackbody evolution inferred from UV/optical photometric observations and spectral line content and evolution. We identify transient, double-peaked Bowen (N III), He I, He II, and Hα emission lines. We model medium-resolution optical spectroscopy of the He II (after careful de-blending of the N III contribution) and Hα lines during the rise, peak, and early decline of the light curve using relativistic, elliptical accretion disk models. Results. We find that the spectral evolution before the peak can be explained by optical depth effects consistent with an outflowing, optically thick Eddington envelope. Around the peak, the envelope reaches its maximum extent (approximately 1015 cm, or ∼3000–6000 gravitational radii for an inferred black hole mass of 5−10 × 105 M⊙) and becomes optically thin. The Hα and He II emission lines at and after the peak can be reproduced with a highly inclined (i = 85 ± 5 degrees), highly elliptical (e = 0.97 ± 0.01), and relatively compact (Rin = several 100 Rg and Rout = several 1000 Rg) accretion disk. Conclusions. Overall, the line profiles suggest a highly elliptical geometry for the new accretion flow, consistent with theoretical expectations of newly formed TDE disks. We quantitatively confirm, for the first time, the high inclination nature of a Bowen (and X-ray dim) TDE, consistent with the unification picture of TDEs, where the inclination largely determines the observational appearance. Rapid line profile variations rule out the binary supermassive black hole hypothesis as the origin of the eccentricity; these results thus provide a direct link between a TDE in an AGN and the eccentric accretion disk. We illustrate for the first time how optical spectroscopy can be used to constrain the black hole spin, through (the lack of) disk precession signatures (changes in inferred inclination). We constrain the disk alignment timescale to > 15 days in AT2020zso, which rules out high black hole spin values (a < 0.8) for MBH ∼ 106 M⊙ and disk viscosity α ≳ 0.1.

Improving black hole accretion treatment in hydrodynamical simulations

ABSTRACT The large-Galactic scales are connected to the many orders of magnitude smaller supermassive black hole (SMBH) scales by an episodic cycle of feeding and feedback. Active Galactic nuclei (AGN) are powered by accretion on to SMBH and the majority of AGN energy in near-Eddington regime is produced in thin subpc accretion discs. Currently, it is very difficult to model processes that occur on vastly different scales, ranging from the circumnuclear gas reservoirs at tens to hundreds of parsecs down to the accretion disc scales at <0.01 pc. While subgrid prescriptions used in large-scale or cosmological simulations are able to reproduce large-scale feedback, we propose using a more realistic model in parsec-scale simulations, where it is important to get accurate time-scales to understand how feedback affects gas dynamics and star formation in the vicinity of the AGN. To test our approach we use a subresolution thin accretion disc model coupled to the SMBH in a set of hydrodynamical simulations of a retrograde collision between a gas ring and a molecular cloud in an environment similar to the Galactic Centre using the SPH code Gadget-3. The disc-mediated feeding of the SMBH is relatively smooth and delayed compared to an instantaneous feeding prescription. While the reduction of accretion due to feedback is present in both accretion disc and instantaneous feeding simulations, a clear central cavity appears only in accretion disc runs – hinting that a less volatile accretion phase could have a greater impact on the surrounding gas.