Accretion Regime Research Articles

Super-Eddington accretion in the Q2237+0305 quasar?

The interband time lags between the flux variations of the Q2237+0305 quasar have been determined from light curves in the Johnson-Cousins V, R, and I spectral bands. The values of the time lags for filter pairs R − V, I − R, and I − V are significantly higher than those predicted by the standard accretion disk model by Shakura and Sunyaev. To explain the discrepancy, the idea of a supercritical accretion regime in quasars considered in 1973 by Shakura and Sunyaev is applied. This regime has been shown by them to cause an extended scattering envelope around the accretion disk. The envelope efficiently scatters and re-emits the radiation from the accretion disk and thus increases the apparent disk size. We made use of analytical expressions for the envelope radius and temperature derived by Shakura and Sunyaev in their analysis of super-Eddington accretion and show that our results are consistent with the existence of such an envelope. The corresponding parameters of the accretion regime were calculated. They provide the radii of the envelope in the V, R, and I spectral bands consistent with the inter-band time lags determined in our work.

Ultraluminous X-Ray Sources

Ultraluminous X-ray sources (ULXs) were identified as a separate class of objects in 2000 based on data from the Chandra X-Ray Observatory. These are unique objects: their X-ray luminosities exceed the Eddington limit for a typical stellar-mass black hole. For a long time, the nature of ULXs remained unclear. However, the gradual accumulation of data, new results of X-ray and optical spectroscopy, and the study of the structure and energy of nebulae surrounding ULXs led to the understanding that most of the ultraluminous X-ray sources must be supercritical accretion disks like SS 433. The discovery of neutron stars in a number of objects only increased the confidence of the scientific community in the conclusions obtained, since the presence of neutron stars in such systems clearly indicates a supercritical accretion regime. In this review, we systematize the main facts about the observational manifestations of ULXs and SS 433 in the X-ray and optical ranges and discuss their explanation from the point of view of the supercritical accretion theory.

Influences of protoplanet-induced three-dimensional gas flow on pebble accretion

Context.Pebble accretion is among the major theories of planet formation. Aerodynamically small particles, called pebbles, are highly affected by the gas flow. A growing planet embedded in a protoplanetary disk induces three-dimensional (3D) gas flow. In our previous work, Paper I, we focused on the shear regime of pebble accretion and investigated the influence of planet-induced gas flow on pebble accretion. In Paper I, we found that pebble accretion is inefficient in the planet-induced gas flow compared to that of the unperturbed flow, particularly when St ≲ 10−3, where St is the Stokes number.Aims.Following on the findings of Paper I, we investigate the influence of planet-induced gas flow on pebble accretion. We did not consider the headwind of the gas in Paper I. Here, we extend our study to the headwind regime of pebble accretion.Methods.Assuming a nonisothermal, inviscid sub-Keplerian gas disk, we performed 3D hydrodynamical simulations on the spherical polar grid hosting a planet with the dimensionless mass,m=RBondi∕H, located at its center, whereRBondiandHare the Bondi radius and the disk scale height, respectively. We then numerically integrated the equation of motion for pebbles in 3D using hydrodynamical simulation data.Results.We first divided the planet-induced gas flow into two regimes: flow-shear and flow-headwind. In the flow-shear regime, where the planet-induced gas flow has a vertically rotational symmetric structure, we find that the outcome is identical to what we obtained in Paper I. In the flow-headwind regime, the strong headwind of the gas breaks the symmetric structure of the planet-induced gas flow. In the flow-headwind regime, we find that the trajectories of pebbles with St ≲ 10−3in the planet-induced gas flow differ significantly from those of the unperturbed flow. The recycling flow, where gas from the disk enters the gravitational sphere at low latitudes and exits at high latitudes, gathers pebbles around the planet. We derive the flow transition mass analytically,mt, flow, which discriminates between the flow-headwind and flow-shear regimes. From the relation betweenm,mt, flowandmt, peb, wheremt, pebis the transition mass of the accretion regime of pebbles, we classify the results obtained in both Paper I and this study into four groups. In particular, only when the Stokes gas drag law is adopted andm< min(mt, peb,mt, flow), where the accretion and flow regime are both in the headwind regime, the accretion probability of pebbles with St ≲ 10−3is enhanced in the planet-induced gas flow compared to that of the unperturbed flow.Conclusions.Combining our results with the spacial variety of turbulence strength and pebble size in a disk, we conclude that the planet-induced gas flow still allows for pebble accretion in the early stage of planet formation. The suppression of pebble accretion due to the planet-induced gas flow occurs only in the late stage of planet formation, more specifically, in the inner region of the disk. This may be helpful for explaining the distribution of exoplanets and the architecture of the Solar System, both of which have small inner and large outer planets.

Evolution of a Viscous Protoplanetary Disk with Convectively Unstable Regions. II. Accretion Regimes and Long-Term Dynamics

In this article, we proceed to study convection as a possible factor of episodic accretion in protoplanetary disks. Within the model presented in Article~I, the accretion history is analyzed at different rates and areas of matter inflow from the envelope onto the disk. It is shown that the burst-like regime occurs in a wide range of parameters. The long-term evolution of the disk is also modeled, including the decreasing-with-time matter inflow from the envelope. It is demonstrated that the disk becomes convectively unstable and maintains burst-like accretion onto the star for several million years. Meanwhile, the instability expands to an area of several tens of astronomical units and gradually decreases with time. It is also shown that at early stages in the disk evolution, conditions arise for gravitational instability in the outer parts of the disk and for dust evaporation in the convectively unstable inner regions. The general conclusion of the study is that convection can serve as one of the mechanisms of episodic accretion in protostellar disks, but this conclusion needs to be verified using more consistent hydrodynamic models.

Growth of Jupiter: Formation in disks of gas and solids and evolution to the present epoch

The formation of Jupiter is modeled via core-nucleated accretion, and the planet's evolution is simulated up to the present epoch. Throughout the phases when the planet acquires most of its heavy-element content, the calculation of solids' accretion accounts for interactions with an evolving disk of planetesimals. The phase of growth from an embryo of a few hundred kilometers in radius until the time when the accretion of gas overtakes solids' accretion was presented by D'Angelo et al. (2014), and the same numerical methods are applied here. Those calculations followed the formation for about 4 × 105 years, until the epoch when the heavy-element and hydrogen/helium masses wereMZ ≈ 7.3 andMXY ≈ 0.15 Earth's masses (M⊕), respectively, and ṀXY≈ṀZ. Herein, the calculation is continued through the phase when MXYgrows to equalMZ, at which age, about 2.4 × 106 years, the total mass of the planet isMp ≈ 20M⊕. About 9 × 105 years later,Mpis approximately 60M⊕and MZ ≈ 16M⊕, three-quarters of which are delivered by planetesimals larger than 10km in radius. Around this epoch, the contraction of the envelope dictates gas accretion rates a few times 10−3M⊕per year, initiating the regime of disk-limited accretion, whereby the planet can accrete all the gas provided by the disk, and its evolution is therefore tied to disk's evolution. Growth is continued by constructing simplified models of protosolar accretion disks that evolve through viscous diffusion, winds, and accretion on the planet. Jupiter's formation ends after ≈ 3.4–4.2Myr, depending on the applied disk viscosity parameter, when nebula gas disperses. The young Jupiter is 4.5–5.5 times as voluminous as it is presently and thousands of times as luminous, ~10−5L⊙. The heavy-element mass is ≈ 20M⊕. The evolution proceeds through the cooling and contraction phase, in isolation except for solar irradiation. After 4570Myr, the age of the solar system, radius and luminosity of the planet are within 10% of current values, accounting also for uncertainties in the power absorbed from the Sun. During formation, and soon thereafter, the planet exhibits features, e.g., luminosity and effective temperature, that may probe aspects of the latter stages of formation, if observable. These possibly distinctive features, however, seem to disappear within a few tens of Myr

Insight-HXMT Firm Detection of the Highest-energy Fundamental Cyclotron Resonance Scattering Feature in the Spectrum of GRO J1008-57

Abstract We report on the observation of the accreting pulsar GRO J1008-57 performed by Insight-Hard X-ray Modulation Telescope (HXMT) at the peak of the source’s 2017 outburst. Pulsations are detected with a spin period of 93.283(1) s. The pulse profile shows double peaks at soft X-rays, and only one peak above 20 keV. The spectrum is well described by the phenomenological models of X-ray pulsars. A cyclotron resonant scattering feature (CRSF) is detected with very high statistical significance at a centroid energy of keV, for the reference continuum and line models, HIGHECUT and GABS, respectively. Detection is very robust with respect to different continuum models. The line energy is significantly higher than what is suggested from previous observations, which provided very marginal evidence for the line. This establishes a new record for the centroid energy of a fundamental CRSF observed in accreting pulsars. We also discuss the accretion regime of the source during the Insight-HXMT observation.

Comprehensive analysis of the transient X-ray pulsar MAXI J1409−619

ABSTRACT We probe the properties of the transient X-ray pulsar MAXI J1409−619 through RXTE and Swift follow-up observations of the outburst in 2010. We are able to phase-connect the pulse arrival times for the 25 d episode during the outburst. We suggest that either an orbital model (with Porb ≃ 14.7(4) d) or a noise process due to random torque fluctuations (with Sr ≈ 1.3 × 10−18 Hz2 s−2 Hz−1) is plausible to describe the residuals of the timing solution. The frequency derivatives indicate a positive torque–luminosity correlation, which implies temporary accretion disc formation during the outburst. We also discover several quasi-periodic oscillations in company with their harmonics whose centroid frequencies decrease as the source flux decays. The variation of the pulsed fraction and spectral power-law index of the source with X-ray flux is interpreted as the sign of transition from a critical to a sub-critical accretion regime at the critical luminosity within the range of 6 × 1037–1.2 × 1038 erg s−1. Using pulse-phase-resolved spectroscopy, we show that the phases with higher flux tend to have lower photon indices, indicating that the polar regions produce spectrally harder emission.

The Interplay between Star Formation and Black Hole Accretion in Nearby Active Galaxies

Abstract Black hole accretion is widely thought to influence star formation in galaxies, but the empirical evidence for a physical correlation between star formation rate (SFR) and the properties of active galactic nuclei (AGNs) remain highly controversial. We take advantage of a recently developed SFR estimator based on the [O ii] λ3727 and [O iii] λ5007 emission lines to investigate the SFRs of the host galaxies of more than 5800 type-1 and 7600 type-2 AGNs with z < 0.35. After matching in luminosity and redshift, we find that type-1 and type-2 AGNs have a similar distribution of internal reddening, which is significant and corresponds to ∼109 M ⊙ of cold molecular gas. In spite of their comparable gas content, type-2 AGNs, independent of stellar mass, Eddington ratio, redshift or molecular gas mass, exhibit intrinsically stronger star formation activity than type-1 AGNs, in apparent disagreement with the conventional AGN unified model. We observe a tight, linear relation between AGN luminosity (accretion rate) and SFR, one that becomes more significant toward smaller physical scales, suggesting that the link between the AGN and star formation occurs in the central kpc-scale region. This, along with a correlation between SFR and Eddington ratio in the regime of super-Eddington accretion, can be interpreted as evidence that star formation is impacted by positive feedback from the AGN.

Insight-HXMT insight into switch of the accretion mode: The case of the X-ray pulsar 4U 1901+03

We use the Insight-HXMT data collected during the 2019 outburst from X-ray pulsar 4U 1901+03 to complement the orbital parameters reported by Fermi/GBM. Using the Insight-HXMT, we examine the correlation between the derivative of the intrinsic spin frequency and bolometric flux based on accretion torque models. It was found that the pulse profiles significantly evolve during the outburst. The existence of two types of the profile's pattern discovered in the Insight-HXMT data indicates that this source experienced transition between a super-critical and a sub-critical accretion regimes during its 2019 outburst. Based on the evolution of the pulse profiles and the torque model, we derive the distance to 4U 1901+03 as 12.4±0.2kpc.

The Jet-Disk Coupling of Seyfert Galaxies from a Complete Hard X-ray Sample

We analyze the jet-disk coupling for different subsamples from a complete hard X-ray Seyfert sample to study the coupling indices and their relation to accretion rate. The results are: (1) the power-law coupling index ranges from nearly unity (linear correlation) for radio loud Seyferts to significantly less than unity for radio quiet ones. This decline trend of coupling index also holds from larger sources to compact ones; (2) the Seyferts with intermediate to high accretion rate (Eddington ratio λ ∼ 0.001 to 0.3) show a linear jet-disk coupling, but it shallows from near to super Eddington ( λ ∼ 0.3 to 10), and the former is more radio loud than the latter; (3) the Seyfert 1s are slightly steeper than the Seyfert 2s, in the jet-disk correlation. In the linear coupling regime, the ratio of jet efficiency to radiative efficiency ( η / ε ) is nearly invariant, but in low accretion or super accretion regime, η / ε varies with λ in our model. We note that a radio-active cycle of accretion-dominated active galactic nuclei would be: from a weaker jet-disk coupling in λ < 0 . 001 for low luminosity Seyferts, to a linear coupling in 0 . 001 < λ < 0 . 3 for radio-loud luminous Seyferts and powerful radio galaxies/quasars, and to a weaker coupling in 0 . 3 < λ < 10 ones.

Effects of hydrodynamical regime on morphological evolution at Cua Dai estuary and coastlines of Quang Nam province

In recent years, accretion-erosion processes are frequently recorded in the estuary of Cua Dai, Quang Nam province. Most recently, however, sand bars are formed in the estuary not far from the place where the Inland Waterways Authority of Vietnam had previously dredged. The estuary continues to be accreted by sand. These have been drawing special attention of national and international scientists. While there is accretion at the estuary, eroded processes are strongly recorded along the coastlines of Quang Nam province. Therefore, it is necessary to carry out a study in the effects of the hydrodynamic regime on morphological changes in the Cua Dai estuary and coastlines of Quang Nam province. The goal of this paper is to fully interpret the causes, regimes of accretion and erosion processes over the study area. In this study, satellite images and hydrodynamic models of Delft3D and MIKE 11 are applied. The results show a strong accretion process in the estuary of Cua Dai. Sandbar formation across the Cua Dai estuary comes from the interaction of flood, wave, and current conditions during the northeast monsoon. This mainly affects the changes in morphology in the estuary of Cua Dai, Quang Nam.

Effects of hydrodynamical regime on morphological evolution at Cua Dai estuary and coastlines of Quang Nam province

In recent years, accretion-erosion processes are frequently recorded in the estuary of Cua Dai, Quang Nam province. Most recently, however, sand bars are formed in the estuary not far from the place where the Inland Waterways Authority of Vietnam had previously dredged. The estuary continues to be accreted by sand. These have been drawing special attention of national and international scientists. While there is accretion at the estuary, eroded processes are strongly recorded along the coastlines of Quang Nam province. Therefore, it is necessary to carry out a study in the effects of the hydrodynamic regime on morphological changes in the Cua Dai estuary and coastlines of Quang Nam province. The goal of this paper is to fully interpret the causes, regimes of accretion and erosion processes over the study area. In this study, satellite images and hydrodynamic models of Delft3D and MIKE 11 are applied. The results show a strong accretion process in the estuary of Cua Dai. Sandbar formation across the Cua Dai estuary comes from the interaction of flood, wave, and current conditions during the northeast monsoon. This mainly affects the changes in morphology in the estuary of Cua Dai, Quang Nam.

Choked Accretion onto a Schwarzschild Black Hole: A Hydrodynamical Jet-launching Mechanism

Abstract We present a novel, relativistic accretion model for accretion onto a Schwarzschild black hole. This consists of a purely hydrodynamical mechanism in which, by breaking spherical symmetry, a radially accreting flow transitions into an inflow-outflow configuration. The spherical symmetry is broken by considering that the accreted material is more concentrated on an equatorial belt, leaving the polar regions relatively under-dense. What we have found is a flux-limited accretion regime in which, for a sufficiently large accretion rate, the incoming material chokes at a gravitational bottleneck and the excess flux is redirected by the density gradient as a bipolar outflow. The threshold value at which the accreting material chokes is of the order of the mass-accretion rate found in the spherically symmetric case studied by Bondi and Michel. We describe the choked accretion mechanism first in terms of a general relativistic, analytic toy model based on the assumption of an ultrarelativistic stiff fluid. We then relax this approximation and, by means of numerical simulations, show that this mechanism can operate also for general polytropic fluids. Interestingly, the qualitative inflow-outflow morphology obtained appears as a generic result of the proposed symmetry break, across analytic and numeric results covering both the Newtonian and relativistic regimes. The qualitative change in the resulting steady-state flow configuration appears even for a very small equatorial-to-polar-density contrast (∼0.1%) in the accretion profile. Finally, we discuss the applicability of this model as a jet-launching mechanism in different astrophysical settings.

Switches between accretion structures during flares in 4U 1901+03

ABSTRACT We report on our analysis of the 2019 outburst of the X-ray accreting pulsar 4U 1901+03 observed with Insight-HXMT and NICER. Both spectra and pulse profiles evolve significantly in the decaying phase of the outburst. Dozens of flares are observed throughout the outburst. They are more frequent and brighter at the outburst peak. We find that the flares, which have a duration from tens to hundreds of seconds, are generally brighter than the persistent emission by a factor of ∼1.5. The pulse-profile shape during the flares can be significantly different from that of the persistent emission. In particular, a phase shift is clearly observed in many cases. We interpret these findings as direct evidence of changes of the pulsed beam pattern, due to transitions between the sub- and supercritical accretion regimes on a short time-scale. We also observe that at comparable luminosities the flares’ pulse profiles are rather similar to those of the persistent emission. This indicates that the accretion on the polar cap of the neutron star is mainly determined by the luminosity, i.e. the mass accretion rate.

First characterization of Swift J1845.7–0037 with NuSTAR

The hard X-ray transient source Swift J1845.7–0037 was discovered in 2012 by Swift/BAT. However, at that time, no dedicated observations of the source were performed. In October 2019, the source became active again, and X-ray pulsations with a period of ∼199 s were detected with Swift/XRT. This triggered follow-up observations with NuSTAR. Here, we report on the timing and spectral analysis of the source properties using NuSTAR and Swift/XRT. The main goal was to confirm pulsations and search for possible cyclotron lines in the broadband spectrum of the source to probe its magnetic field. Despite highly significant pulsations with period of 207.379(2) s being detected, no evidence for a cyclotron line was found in the spectrum of the source. We therefore discuss the strength of the magnetic field based on the source flux and the detection of the transition to the “cold-disc” accretion regime during the 2012 outburst. Our conclusion is that the source is most likely a highly magnetized neutron star with B ≳ 1013 G at a large distance of d ∼ 10 kpc. The latter is consistent with the nondetection of a cyclotron line in the NuSTAR energy band.

XMM–Newton campaign on ultraluminous X-ray source NGC 1313 X-1: wind versus state variability

ABSTRACT Most ultraluminous X-ray sources (ULXs) are thought to be powered by neutron stars and black holes accreting beyond the Eddington limit. If the compact object is a black hole or a neutron star with a magnetic field ≲1012 G, the accretion disc is expected to thicken and launch powerful winds driven by radiation pressure. Evidence of such winds has been found in ULXs through the high-resolution spectrometers onboardXMM–Newton, but several unknowns remain, such as the geometry and launching mechanism of these winds. In order to better understand ULX winds and their link to the accretion regime, we have undertaken a major campaign with XMM–Newton to study the ULX NGC 1313 X-1, which is known to exhibit strong emission and absorption features from a mildly relativistic wind. The new observations show clear changes in the wind with a significantly weakened fast component (0.2c) and the rise of a new wind phase which is cooler and slower (0.06–0.08c). We also detect for the first time variability in the emission lines which indicates an origin within the accretion disc or in the wind. We describe the variability of the wind in the framework of variable super-Eddington accretion rate and discuss a possible geometry for the accretion disc.

Strongly magnetized accretion discs: structure and accretion from global magnetohydrodynamic simulations

ABSTRACT We use global magnetohydrodynamic simulations to study the influence of net vertical magnetic fields on the structure of geometrically thin (H/r ≈ 0.05) accretion discs in the Newtonian limit. We consider initial mid-plane gas to magnetic pressure ratios $\beta _0 = 1000,\, 300$, and 100, spanning the transition between weakly and strongly magnetized accretion regimes. We find that magnetic pressure is important for the discs’ vertical structure in all three cases, with accretion occurring at $z$/R ≈ 0.2 in the two most strongly magnetized models. The disc mid-plane shows outflow rather than accretion. Accretion through the surface layers is driven mainly by stress due to coherent large-scale magnetic field rather than by turbulent stress. Equivalent viscosity parameters measured from our simulations show similar dependencies on initial β0 to those seen in shearing box simulations, though the disc mid-plane is not magnetic pressure dominated even for the strongest magnetic field case. Winds are present but are not the dominant driver of disc evolution. Over the (limited) duration of our simulations, we find evidence that the net flux attains a quasi-steady state at levels that can stably maintain a strongly magnetized disc. We suggest that geometrically thin accretion discs in observed systems may commonly exist in a magnetically ‘elevated’ state, characterized by non-zero but modest vertical magnetic fluxes, with potentially important implications for disc phenomenology in X-ray binaries and active galactic nuclei.

The magnetic field and accretion regime of CI Tau

ABSTRACT This paper exploits spectropolarimetric data of the classical T Tauri star CI Tau collected with ESPaDOnS at the Canada–France–Hawaii Telescope, with the aims of detecting and characterizing the large-scale magnetic field that the star hosts, and of investigating how the star interacts with the inner regions of its accretion disc through this field. Our data unambiguously show that CI Tau has a rotation period of 9.0 d, and that it hosts a strong, mainly poloidal large-scale field. Accretion at the surface of the star concentrates within a bright high-latitude chromospheric region that spatially overlaps with a large dark photospheric spot, in which the radial magnetic field reaches −3.7 kG. With a polar strength of −1.7 kG, the dipole component of the large-scale field is able to evacuate the central regions of the disc up to about 50 per cent of the co-rotation radius (at which the Keplerian orbital period equals the stellar rotation period) throughout our observations, during which the average accretion rate was found to be unusually high. We speculate that the magnetic field of CI Tau is strong enough to sustain most of the time a magnetospheric gap extending to at least 70 per cent of the co-rotation radius, which would explain why the rotation period of CI Tau is as long as 9 d. Our results also imply that the 9 d radial velocity (RV) modulation that CI Tau exhibits is attributable to stellar activity, and thus that the existence of the candidate close-in massive planet CI Tau b to which these RV fluctuations were first attributed needs to be reassessed with new evidence.

On the Role of the Hot Feedback Mode in Active Galactic Nuclei Feedback in an Elliptical Galaxy

Depending on the value of the accretion rate, black hole accretion is divided into cold and hot modes. The two modes have distinctly different physics and correspond to two feedback modes. Most previous feedback works either focus only on one mode, or the accretion physics is not always properly adopted. Here, by performing hydrodynamical numerical simulations of AGN feedback in an elliptical galaxy, we show that including both is important, and gives different results from including just one or the other. We specifically focus on the wind and radiation (but neglecting the jet) feedback in the hot mode, to explore whether this particular mode of feedback can play any important feedback role. For this aim, we have run two test models. In one model, we always adopt the cold mode no matter what value the accretion rate is; in another model, we turn off the AGN once it enters into the hot mode. We have calculated the AGN light curves, black hole growth, star formation, and AGN energy duty-cycle; and compared the results with the model in which the two modes are correctly included. Important differences are found. For example, if we were to adopt only the cold mode, the total mass of newly formed stars would become two orders of magnitude smaller, and the fraction of energy ejected within the high accretion regime (i.e., $L_{\rm BH} > 2\% L_{\rm Edd}$) would be too small to be consistent with observations. We have also investigated the respective roles of wind and radiation in the hot mode.

Hot disk of the Swift J0243.6+6124 revealed by Insight-HXMT

Abstract We report on analysis of observations of the bright transient X-ray pulsar Swift J0243.6+6124 obtained during its 2017-2018 giant outburst with Insight-HXMT, NuSTAR, and Swift observatories. We focus on the discovery of a sharp state transition of the timing and spectral properties of the source at super-Eddington accretion rates, which we associate with the transition of the accretion disk to a radiation pressure dominated (RPD) state, the first ever directly observed for magnetized neutron star. This transition occurs at slightly higher luminosity compared to already reported transition of the source from sub- to super-critical accretion regime associate with onset of an accretion column. We argue that this scenario can only be realized for comparatively weakly magnetized neutron star, not dissimilar to other ultra-luminous X-ray pulsars (ULPs), which accrete at similar rates. Further evidence for this conclusion is provided by the non-detection of the transition to the propeller state in quiescence which strongly implies compact magnetosphere and thus rules out magnetar-like fields.