Accretion Regime Research Articles

A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2

ABSTRACTNGC 1313 X-2 is one of the few known pulsating ultraluminous X-ray sources (PULXs), and so is thought to contain a neutron star that accretes at highly super-Eddington rates. However, the physics of this accretion remains to be determined. Here, we report the results of two simultaneous XMM–Newton and HST observations of this PULX taken to observe two distinct X-ray behaviours as defined from its Swift light curve. We find that the X-ray spectrum of the PULX is best described by the hard ultraluminous regime during the observation taken in the lower flux, lower variability amplitude behaviour; its spectrum changes to a broadened disc during the higher flux, higher variability amplitude epoch. However, we see no accompanying changes in the optical/UV fluxes, with the only difference being a reduction in flux in the near-infrared (NIR) as the X-ray flux increased. We attempt to fit irradiation models to explain the UV/optical/IR fluxes but they fail to provide meaningful constraints. Instead, a physical model for the system leads us to conclude that the optical light is dominated by a companion O/B star, albeit with an IR excess that may be indicative of a jet. We discuss how these results may be consistent with the precession of the inner regions of the accretion disc leading to changes in the observed X-ray properties, but not the optical, and whether we should expect to observe reprocessed emission from ULXs.

Relativistic X-Ray Reverberation from Super-Eddington Accretion Flow

Abstract X-ray reverberation is a powerful technique that uses the echoes of the coronal emission reflected by a black hole (BH) accretion disk to map out the inner disk structure. While the theory of X-ray reverberation has been developed almost exclusively for standard thin disks, reverberation lags have recently been observed from likely super-Eddington accretion sources such as the jetted tidal disruption event Swift J1644+57. In this paper, we extend X-ray reverberation studies into the regime of super-Eddington accretion with a focus on investigating the lags in the fluorescent Fe Kα line region. We find that the coronal photons are mostly reflected by the fast and optically thick winds launched from the super-Eddington accretion flow, and this funnel-like reflection geometry produces lag–frequency and lag–energy spectra with unique observable characteristics. The lag–frequency spectrum exhibits a step-function-like decline near the first zero-crossing point. As a result, the magnitude of the lag scales linearly with the BH mass for a large parameter space, and the shape of the lag–energy spectrum remains almost independent of the choice of frequency bands. Not only can these features be used to distinguish super-Eddington accretion systems from sub-Eddington systems, but they are also key for constraining the reflection geometry and extracting parameters from the observed lags. When fitting the observed reverberation lag of Swift J1644+57 to our modeling, we find that the super-Eddington disk geometry is slightly preferred over the thin disk geometry, and we obtain a BH mass of 5–6 × 106 M ☉ and a coronal height around 10 Rg .

Steady-state accretion in magnetized protoplanetary disks

Context. The transition between magnetorotational instability (MRI)-active and magnetically dead regions corresponds to a sharp change in the disk turbulence level, where pressure maxima may form, hence potentially trapping dust particles and explaining some of the observed disk substructures. Aims. We aim to provide the first building blocks toward a self-consistent approach to assess the dead zone outer edge as a viable location for dust trapping, under the framework of viscously driven accretion. Methods. We present a 1+1D global magnetically driven disk accretion model that captures the essence of the MRI-driven accretion, without resorting to 3D global nonideal magnetohydrodynamic (MHD) simulations. The gas dynamics is assumed to be solely controlled by the MRI and hydrodynamic instabilities. For given stellar and disk parameters, the Shakura–Sunyaev viscosity parameter, α, is determined self-consistently under the adopted framework from detailed considerations of the MRI with nonideal MHD effects (Ohmic resistivity and ambipolar diffusion), accounting for disk heating by stellar irradiation, nonthermal sources of ionization, and dust effects on the ionization chemistry. Additionally, the magnetic field strength is numerically constrained to maximize the MRI activity. Results. We demonstrate the use of our framework by investigating steady-state MRI-driven accretion in a fiducial protoplanetary disk model around a solar-type star. We find that the equilibrium solution displays no pressure maximum at the dead zone outer edge, except if a sufficient amount of dust particles has accumulated there before the disk reaches a steady-state accretion regime. Furthermore, the steady-state accretion solution describes a disk that displays a spatially extended long-lived inner disk gas reservoir (the dead zone) that accretes a few times 10−9 M⊙ yr−1. By conducting a detailed parameter study, we find that the extent to which the MRI can drive efficient accretion is primarily determined by the total disk gas mass, the representative grain size, the vertically integrated dust-to-gas mass ratio, and the stellar X-ray luminosity. Conclusions. A self-consistent time-dependent coupling between gas, dust, stellar evolution models, and our general framework on million-year timescales is required to fully understand the formation of dead zones and their potential to trap dust particles.

Study of timing and spectral properties of the X-ray pulsar 1A 0535+262 during the giant outburst in 2020 November–December

ABSTRACT We made a detailed study of the timing and spectral properties of the X-ray pulsar 1A 0535+262 during the recent giant outburst in 2020 November and December. The flux of the pulsar reached a record value of ∼12.5 Crab as observed by Swift/BAT (15–50 keV) and the corresponding mass accretion rate was ∼6.67 × 1017 g s−1 near the peak of the outburst. There was a transition from the subcritical to the supercritical accretion regime which allows exploring different properties of the source in the supercritical regime. A q-like feature was detected in the hardness–intensity diagram during the outburst. We observed high variability and strong energy dependence of pulse profiles during the outburst. Cyclotron Resonant Scattering Feature (CRSF) was detected at ∼44 keV from the NuSTAR energy spectrum in the subcritical regime and the corresponding magnetic field was B ≃ 4.9 × 1012 G. The energy of the CRSF was shifted towards lower energy in the supercritical regime. The luminosity dependence of the CRSF was studied and during the supercritical regime, a negative correlation was observed between the line energy and luminosity. The critical luminosity was ∼6 × 1037erg s−1 above which a state transition occurred. A reversal of correlation between the photon index and luminosity was observed near the critical luminosity. The NuSTAR spectra can be described by a composite model with two continuum components, a blackbody emission, cut-off power law, and a discrete component to account for the iron emission line at 6.4 keV. An additional cyclotron absorption feature was included in the model.

Determination of wind-fed model parameters of neutron stars in high-mass X-ray binaries

Abstract We have studied several neutron star high-mass X-ray binaries (HMXBs) with super-giant (SG) companions using a wind-fed binary model associated with the magnetic field. By using the concept of torque balance, the magnetic field parameter determines the mass accretion rate. This would help us to consider the relationship between wind velocity and mass-loss rate. These parameters significantly improve our understanding of the accretion mechanism. The wind velocity is critical in determining the X-ray features. This can be used to identify the ejection process and the stochastic variations in their accretion regimes. However, even in systems with a long orbital period, an accretion disk can be created when the wind velocity is slow. This will allow the HMXB of both types, SG and Be, to be better characterised by deriving accurate properties from these binaries. In addition, we have performed segmentation in the parameter space of donors intended for several SG-HMXB listed in our sample set. The parameter space can be categorised into five regimes, depending on the possibility of disk formation associated with accretion from the stellar wind. This can give a quantitative clarification of the observed variability and the properties of these objects. For most of the systems, we show that the derived system parameters are consistent with the assumption that the system is emitting X-rays through direct accretion. However, there are some sources (LMC X-4, Cen X-3 and OAO1657-415) that are not in the direct accretion regime, although they share similar donor parameters. This may indicate that these systems are transitioning from a normal wind accretion phase to partial RLOF regimes.

Progress in Understanding the Nature of SS433

SS433 is the first example of a microquasar discovered in the Galaxy. It is a natural laboratory for studies of extraordinarily interesting physical processes that are very important for the relativistic astrophysics, cosmic gas dynamics and theory of evolution of stars. The object has been studied for over 40 years in the optical, X-ray and radio bands. By now, it is generally accepted that SS433 is a massive eclipsing X-ray binary in an advanced stage of evolution in the supercritical regime of accretion on the relativistic object. Intensive spectral and photometric observations of SS433 at the Caucasian Mountain Observatory of the P. K. Sternberg Astronomical Institute of M. V. Lomonosov Moscow State University made it possible to find the ellipticity of the SS433 orbit and to discover an increase in the system’s orbital period. These results shed light on a number of unresolved issues related to SS433. In particular, a refined estimate of the mass ratio MxMv>0.8 was obtained (Mx and Mv are the masses of the relativistic object and optical star). Based on these estimates, the relativistic object in the SS433 system is the black hole; its mass is >8M⊙. The ellipticity of the orbit is consistent with the “slaved” accretion disc model. The results obtained made it possible to understand why SS433 evolves as the semi-detached binary instead of the common envelope system.

The extreme properties of the nearby hyper-Eddington accreting active galactic nucleus in IRAS 04416+1215

ABSTRACT The physical properties of the accretion flow and of the X-ray emitting plasma, in supermassive black holes accreting at extreme Eddington rates, are still very unclear. Here we present the analysis of simultaneous XMM–Newton and NuSTAR observations of the hyper-Eddington Seyfert 1 galaxy IRAS 04416+1215, carried out in 2020. The main goal of these observations is to investigate the properties of the X-ray corona, as well as the structure of the accretion flow and of the circumnuclear environment, in this regime of extreme accretion. IRAS 04416+1215 has one of the highest Eddington ratio (λEdd ≃ 472) in the local Universe. It shows an interesting spectral shape, very similar to the standard narrow-line Seyfert 1 galaxy’s spectra, with the presence of multiphase absorption structure composed of three phases, whose estimate of the minimum and maximum distances suggests two different interpretations, one consistent with the three X-ray winds being cospatial, and possibly driven by magnetohydrodynamical processes, the other consistent with the multiphase winds being also multiscale. The X-ray spectrum of IRAS 04416+1215 also has a prominent soft excess component and a hard X-ray emission dominated by a reflection component. Moreover, our detailed spectral analysis shows that IRAS 04416+1215 has the lowest coronal temperature measured so far by NuSTAR (kTe = 3–22 keV, depending on the model). This is consistent with a hybrid coronal plasma, in which the primary continuum emission is driven by pair production due to high-energy tail of the energy distribution of non-thermal electrons.

The Origin of Radio Emission in Black Hole X-ray Binaries

We studied the relation of accretion-jet power and disk luminosity, especially the jet efficiencies and disk radiative efficiencies for different accretion disks as well as black hole (BH) spin, in order to explore the origin of radio emission in black hole X-ray binaries (BHXBs). We found that jet efficiency increases more rapidly (efficient) than the nearly constant disk radiative efficiency for thin disk component in high accretion regime, which could account for the steep track (μ>1) in the observed radio and X-ray luminosity relations (LR∝LXμ), but the thin disk component may not be able to explain the standard track (μ≈0.6) in the BHXBs. For hot accretion flows (HAF), the resulting jet efficiency changes along with the large range of accretions from quiescent state to nearly Eddington state, which could account for the standard track in the BHXBs. The BH spin-jet is discussed for the magnetic arrested disk (MAD) state; in this state, the spin-jet power might contribute to a linear correlation between jet power and mass accretion rate for a given source. More accurate observations are required to test the results.

The Nature of Ice Intermittently Accreted at the Base of Ronne Ice Shelf, Antarctica, Assessed Using Phase‐Sensitive Radar

AbstractIn‐situ phase‐sensitive radar measurements from the Ronne Ice Shelf (RIS) reveal evidence of intermittent basal accretion periods at several sites that are melting in the long‐term mean. Periods when ice is accreted at the ice‐shelf base coincide with a decrease in the amplitude of the basal return of up to 4 dB. To quantify basal accretion we constrain simultaneously the dielectric constant, electrical conductivity, and thickness of the accreted ice. We do this by exploring the sensitivity of the received basal echo strength and phase to different transmit frequencies using the radar data in combination with a simple model. Along the western RIS, we detect episodic basal accretion events leading to ice accumulation at a rate equivalent to 1–3 mm of meteoric ice per day. The inferred accumulation rates and electromagnetic properties of the accreted ice imply that these events are caused primarily by the deposition of frazil ice crystals. Our findings offer the possibility of monitoring and studying the evolution of boundaries between ice‐shelf basal melting and accretion regimes using remote observations, collected from the ice‐shelf surface.

XMM-Newton spectrum of the radio-loud quasar 3C 215: Slim accretion disk or SMBH binary

Context. Radio-loud active galactic nuclei (RL AGN) exhibit very powerful jet emission in the radio band, while the radio-quiet (RQ) AGN do not. This RL-RQ dichotomy would imply a sharp difference existing among these two classes, however, modern theoretical models and observations suggest a common nuclear environment that is possibly characterized by different working regimes. Aims. We explore the geometrical structure and mutual interactions of the innermost components of the broad line radio galaxy 3C 215, with a particular focus on the accretion and ejection mechanisms involving the central supermassive black hole (SMBH). We compare these observational features with those of the RQ Seyfert 1 galaxies. Investigating their differences is aimed at improving our understanding of the jet launching mechanisms and devising an explanation for why this phenomenon is efficient only in a small fraction of all the AGNs. Methods. Using high-quality data from a ∼60 ks observation with XMM-Newton, we carried out a detailed X-ray spectral analysis of 3C 215 in the broad energy range of 0.5−10 keV. We modeled the spectrum with an absorbed double power-law model for the primary continuum, reprocessed by reflection from ionized and cold neutral material and modified by relativistic blurring. We also compared our results with those of earlier multi-wavelength observations. Results. We obtained a primary continuum photon index from the corona, namely, Γ1 = 1.97 ± 0.06, along with evidence of a jet contribution, modeled as a power law with photon index of Γ2 ≃ 1.29. The reflector, which is possibly attributed to the accretion disk and portions of the broad-line region (BLR), is ionized (logξ = 2.31−0.27+0.37 erg s−1 cm) and relatively distant from the SMBH (Rin > 38 Rg), where Rg = GMBH/c2 is the gravitational radius. The obscuring torus seems patchy, dust-poor, and inefficient, while the jet emission shows a twisted and knotted geometry. We propose three scenarios to describe the following characteristics: 1. An ADAF state in the inner disk; 2. A slim accretion disk; and 3. A sub-pc SMBH binary system (SMBHB). Conclusions. While the first scenario is not in agreement with the SMBH accretion regime, the slim disk scenario is consistent with the observational features of this radio galaxy, showing that 3C 215 is similar to non-jetted AGNs, accreting at a high rate. Nonetheless, the first two scenarios are unable to account for the particular shape of 3C 215 jet emission. The SMBHB scenario seems to be in agreement with almost all 3C 215 observational features, but we are not able to unequivocally determine this source as a strong SMBHB candidate. A final determination will require further analysis.

The Radiative Newtonian 1 < γ ≤ 1.66 and the Paczyński–Wiita γ = 5/3 Regime of Non-Isothermal Bondi Accretion onto a Massive Black Hole with an Accretion Disc

We investigate the non-isothermal Bondi accretion onto a supermassive black hole (SMBH) for the unexplored case when the adiabatic index is varied in the interval 1<γ≤1.66 and for the Paczyński–Wiita γ=5/3 regime, including the effects of X-ray heating and radiation force due to electron scattering and spectral lines. The X-ray/central object radiation is assumed to be isotropic, while the UV emission from the accretion disc is assumed to have an angular dependence. This allows us to build streamlines in any desired angular direction. The effects of both types of radiation on the accretion dynamics is evaluated with and without the effects of spectral line driving. Under line driving (and for the studied angles), when the UV flux dominates over the X-ray heating, with a fraction of UV photons going from 80% to 95%, and γ varies from 1.66 to 1.1, the inflow close to the gravitational source becomes more supersonic and the volume occupied by the supersonic inflow becomes larger. This property is also seen when this fraction goes from 50% to 80%. The underestimation of the Bondi radius close to the centre increases with increasing γ, while the central overestimation of the accretion rates decreases with increasing γ, for all the six studied cases.

Timing and spectral properties of the Be/X-ray pulsar 4U 1901+03 during its 2019 outburst

We have studied the timing and spectral properties of the BeXB 4U 1901+03 during the 2019 outburst using NuSTAR, Swift, and NICER observations. Flares are in all observations and were of tens to hundreds of seconds duration. Pulse profiles were changing significantly with time and the luminosity of the source. An increase in the height of the peak of the pulse profiles was observed with energy. The pulse fraction increases with energy and at the end of the outburst. The variation of the pulse profile with time indicates the transition of the pulsar in different accretion regimes. The absorption like feature at 10 keV shows a positive correlation with the luminosity and along with other spectral parameters this feature was also pulse phase dependent. As the distance to the source is not precisely known we cannot confirm this feature to be CSRF and also cannot ignore other possible explanations of the feature. Another absorption like feature about 30 keV was observed in the spectra of the last two NuSTAR observations and has line energy of about 30.37 ± 0.55 and 30.23 ± 0.62 keV, respectively. We have also studied the variation of the line energy, width, and optical depth of this feature with pulse phase. The softening of the spectrum along with the increase in pulse fraction at the end of the outburst and absence of pulsation after 58665.09 MJD suggest that the pulsar has entered a propeller phase, also an abrupt decrease in Swift-XRT flux supports the fact.

Luminosity Dependence of the Cyclotron Line Energy in 1A 0535+262 Observed by Insight-HXMT during the 2020 Giant Outburst

Abstract We report on a detailed spectral analysis of the transient X-ray pulsar 1A 0535+262, which underwent the brightest giant outburst ever recorded for this source from 2020 November to December with a peak luminosity of 1.2 × 1038 erg s−1. Thanks to the unprecedented energy coverage and high-cadence observations provided by Insight-HXMT, we were able to find for the first time evidence for a transition of the accretion regime. At high luminosity, above the critical luminosity 6.7 × 1037 erg s−1, the cyclotron absorption line energy anticorrelates with luminosity. Below the critical luminosity, a positive correlation is observed. Therefore, 1A 0535+262 becomes the second source after V0332+53, which clearly shows an anticorrelation above and transition between correlation and anticorrelation around the critical luminosity. The evolution of both the observed CRSF line energy and broadband X-ray continuum spectrum throughout the outburst exhibits significant differences during the rising and fading phases; that is, for a similar luminosity, the spectral parameters take different values, which results in hysteresis patterns for several spectral parameters including the cyclotron line energy. We argue that, similar to V0332+53, these changes might be related to the different geometry of the emission region in rising and declining parts of the outburst, probably due to changes in the accretion disk structure and its interaction with the magnetosphere of the neutron star.

Can a jumping-Jupiter trigger the Moon’s formation impact?

ABSTRACT We investigate the possibility that the Moon’s formation impact was triggered by an early dynamical instability of the giant planets. We consider the well-studied ‘jumping Jupiter’ hypothesis for the Solar system’s instability, where Jupiter and Saturn’s semimajor axes evolve in step-wise manner from their primordially compact architecture to their present locations. Moreover, we test multiple different configurations for the primordial system of terrestrial planets and the Moon-forming projectile, with particular focus on the almost equal masses impact. We find that the instability/migration of the giant planets excites the orbits of the terrestrial planets through dynamical perturbations, thus allowing collisions between them. About 10 per cent of the simulations lead to a collision with the proto-Earth which resulted in a final configuration of the terrestrial system that reproduces, to some extent, its present architecture. Most of these collisions occur in the hit-and-run domain, but about 15 per cent occur in the partial accretion regime, with the right conditions for a Moon-forming impact. In most of the simulations, there is a delay of more than ∼20 My between the time of the instability and the Moon-forming impact. This supports the occurrence of an early instability (<10 My after dissipation of the gas in the protoplanetary disc), compatible with the time of the Moon-forming impact (30–60 My) inferred from cosmochemical constraints. In general, the final states of the inner Solar system in our simulations show an excess of Angular Momentum Deficit, mostly attributed to the overexcitation of Mercury’s eccentricity and inclination.

UAV icing: the influence of airspeed and chord length on performance degradation

PurposeThe main purpose of this paper is to investigate the effects of icing on unmanned aerial vehicles (UAVs) at low Reynolds numbers and to highlight the differences to icing on manned aircraft at high Reynolds numbers. This paper follows existing research on low Reynolds number effects on ice accretion. This study extends the focus to how variations of airspeed and chord length affect the ice accretions, and aerodynamic performance degradation is investigated.Design/methodology/approachA parametric study with independent variations of airspeed and chord lengths was conducted on a typical UAV airfoil (RG-15) using icing computational fluid dynamic methods. FENSAP-ICE was used to simulate ice shapes and aerodynamic performance penalties. Validation was performed with two experimental ice shapes obtained from a low-speed icing wind tunnel. Three meteorological conditions were chosen to represent the icing typologies of rime, glaze and mixed ice. A parameter study with different chord lengths and airspeeds was then conducted for rime, glaze and mixed icing conditions.FindingsThe simulation results showed that the effect of airspeed variation depended on the ice accretion regime. For rime, it led to a minor increase in ice accretion. For mixed and glaze, the impact on ice geometry and penalties was substantially larger. The variation of chord length had a substantial impact on relative ice thicknesses, ice area, ice limits and performance degradation, independent from the icing regime.Research limitations/implicationsThe implications of this manuscript are relevant for highlighting the differences between icing on manned and unmanned aircraft. Unmanned aircraft are typically smaller and fly slower than manned aircraft. Although previous research has documented the influence of this on the ice accretions, this paper investigates the effect on aerodynamic performance degradation. The findings in this work show that UAVs are more sensitive to icing conditions compared to larger and faster manned aircraft. By consequence, icing conditions are more severe for UAVs.Practical implicationsAtmospheric in-flight icing is a severe risk for fixed-wing UAVs and significantly limits their operational envelope. As UAVs are typically smaller and operate at lower airspeeds compared to manned aircraft, it is important to understand how the differences in airspeed and size affect ice accretion and aerodynamic performance penalties.Originality/valueEarlier work has described the effect of Reynolds number variations on the ice accretion characteristics for UAVs. This work is expanding on those findings by investigating the effect of airspeed and chord length on ice accretion shapes separately. In addition, this study also investigates how these parameters affect aerodynamic performance penalties (lift, drag and stall).

Supernova Luminosity Powered by Magnetar–Disk System

Abstract Magnetars are one of the potential power sources for some energetic supernova explosions such as type I superluminous supernovae (SLSNe I) and broad-lined type Ic supernovae (SNe Ic-BL). In order to explore the possible link between these two subclasses of supernovae (SNe), we study the effect of fallback accretion disk on magnetar evolution and magnetar-powered SNe. In this scenario, the interaction between a magnetar and a fallback accretion disk would accelerate the spin of the magnetar in the accretion regime but could result in substantial spin-down of the magnetars in the propeller regime. Thus, the initial rotation of the magnetar plays a less significant role in the spin evolution. Such a magnetar–disk interaction scenario can explain well the light curves of both SNe Ic-BL and SLSNe I, for which the observed differences are sensitive to the initial magnetic field of the magnetar and the fallback mass and timescale for the disk. Compared to the magnetars powering the SNe Ic-BL, those accounting for more luminous SNe usually maintain faster rotation and have relatively lower effective magnetic fields around peak time. In addition, the association between SLSNe I and long gamma-ray bursts, if observed in the future, could be explained in the context of a magnetar–disk system.

Evidence from achondrites for a temporal change in Nd nucleosynthetic anomalies within the first 1.5 million years of the inner solar system formation

Heterogeneity in isotopic compositions within the protoplanetary disc has been demonstrated for a number of elements measured in extra-terrestrial materials, mostly based on chondrite meteorite analyses. However, precise 182Hf-182W and 26Al-26Mg ages of iron meteorites, achondrites, and chondrules show that chondrites accreted later than achondrites and therefore do not strictly represent the early (<2 Ma) solar system composition. Here we present the Nd mass-independent stable isotopic compositions of a suite of diverse achondrites to better constrain the Nd isotope evolution of the early solar system. Carbonaceous (C) achondrites are indistinguishable from their chondritic counterpart. However, early formed planetesimals as sampled by silicate-rich non-carbonaceous (NC) achondrite meteorites have higher 145Nd/144Nd and 148Nd/144Nd ratios (3.9 <μ145Nd < 11.0 and 9.1 <μ148Nd < 17.9 in part per million deviation, or μiNd) compared to NC chondrites (2.7 <μ145Nd < 3.3 and 2.2 <μ148Nd < 8.1). Moreover, the three terrestrial planets for which we have samples available (Earth, Mars, and the Moon) as well as the silicate inclusions from the non-magmatic IIE iron meteorite Miles present a systematic deficit in μ145Nd and μ148Nd compared to early-formed NC achondrites. Unlike chondrites, the Nd anomalies in achondrites are not correlated to the heliocentric distance of accretion of their respective parent bodies as inferred from redox conditions. Chronological constraints on planetesimal accretion suggest that Nd (and other elements such as Mo and Zr) nucleosynthetic compositions of the inner part of the protoplanetary disc significantly changed around 1.5 Ma after Solar System formation due to thermal processing of dust in the protoplanetary disc. This relatively late event coincides with the beginning of chondrule formation or at least their preservation. Terrestrial planets formed subsequently by a complex accretion regime during several million years. Therefore, two scenarios are envisioned considering the reported Nd isotope composition of early planetesimals: 1) Terrestrial planets accreted mostly chondritic material similar in composition to enstatite chondrites, or 2) early planetesimals constitute substantial parts of terrestrial planets building blocks mixed with highly thermally processed material enriched in s-process, still unsampled by meteorites.

Jet–Accretion System in the Nearby mJy Radio Galaxies

Abstract It is generally thought that FRII radio galaxies host thin optically thick disks, while FRIs are powered by advection-dominated accretion flows. Sources with an efficient engine are optically classified as high-excitation radio galaxies (HERGs) and those with an inefficient motor as low-excitation radio galaxies (LERGs). Recently, the study of radio galaxies down to mJy fluxes has cast serious doubts on the LERG-FRI and HERG-FRII correspondence, revealing that many LERGs show FRII radio morphologies. The FR catalogs recently compiled by Capetti et al. and Baldi et al. have allowed us to explore this issue in the local (z ≤ 0.15) mJy universe. Our statistical study shows that the majority of nearby mJy objects are in a late stage of their life. FRII-LERGs appear more similar to the old FRI-LERGs than to the young FRII-HERGs. FRII-LERGs may be aged HERGs that, having exhausted their cold fuel, have changed their accretion regime or are a separate LERG class particularly efficient in launching jets. Exploiting the empirical relations that convert L [O III] and L 1.4 GHz into accretion power and jet kinetic power, respectively, we observed that LERGs with similar masses and accretion rates seem to expel jets of different powers. We speculate that intrinsic differences related to the black hole properties (spin and magnetic field at its horizon) can determine the observed spread in jet luminosity. In this view, FRII-LERGs should have the fastest spinning black holes and/or the most intense magnetic fluxes. On the contrary, compact LERGs (i.e., FR0s) should host extremely slow black holes and/or weak magnetic fields.

The antesonic condition for the explosion of core-collapse supernovae – II. Rotation and turbulence

ABSTRACT In the problem of steady free fall on to a standing shockwave around a central mass, the ‘antesonic’ condition limits the regime of stable accretion to $c_T^2/v_\mathrm{esc}^2\le 3/16$, where cT is the isothermal sound speed in the subsonic post-shock flow, and vesc is the escape velocity at the shock radius. Above this limit, it is impossible to satisfy both the Euler equation and the shock jump conditions, and the system transitions to a wind. This physics explains the existence of a critical neutrino luminosity in steady-state models of accretion in the context of core-collapse supernovae. Here, we extend the antesonic condition to flows with rotation and turbulence using a simple one-dimensional formalism. Both effects decrease the critical post-shock sound speed required for explosion. While quite rapid rotation is required for a significant change to the critical condition, we show that the level of turbulence typically achieved in supernova simulations can greatly impact the critical value of $c_T^2/v_\mathrm{esc}^2$. A core angular velocity corresponding to a millisecond rotation period after contraction of the proto-neutron star results in only a ∼5 per cent reduction of the critical curve. In contrast, near-sonic turbulence with specific turbulent kinetic energy $K/c_T^2=0.5-1$, leads to a decrease in the critical value of $c_T^2/v_{\rm esc}^2$ by ∼20 to 40 per cent. This analysis provides a framework for understanding the role of post-shock turbulence in instigating explosions in models that would otherwise fail and helps explain why multidimensional simulations explode more easily than their one-dimensional counterparts.

The radiative efficiency of neutron stars at low-level accretion

ABSTRACT When neutron star low-mass X-ray binaries (NS-LMXBs) are in the low-level accretion regime (i.e. $L_{\rm X}\lesssim 10^{36}\ \rm erg\ s^{-1}$), the accretion flow in the inner region around the NS is expected to exist in the form of the hot accretion flow, e.g. the advection-dominated accretion flow (ADAF) as that in black hole X-ray binaries. Following our previous studies in Qiao &amp; Liu (2020a, b) on the ADAF accretion around NSs, in this paper, we investigate the radiative efficiency of NSs with an ADAF accretion in detail, showing that the radiative efficiency of NSs with an ADAF accretion is much lower than that of $\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$ despite the existence of the hard surface. As a result, given an X-ray luminosity LX (e.g. between 0.5 and 10 keV), $\dot{M}$ calculated by $\dot{M}=L_{\rm X}{R_{*}\over {GM}}$ is lower than the real $\dot{M}$ calculated within the framework of the ADAF accretion. The real $\dot{M}$ can be more than two orders of magnitude higher than that calculated by $\dot{M}=L_{\rm X}{R_{*}\over {GM}}$ with appropriate model parameters. Finally, we discuss that if applicable, the model of ADAF accretion around a NS can be applied to explain the observed millisecond X-ray pulsation in some NS-LMXBs (such as PSR J1023+0038, XSS J12270−4859, and IGR J17379−3747) at a lower X-ray luminosity of a few times of $10^{33}\ \rm erg\ s^{-1}$, since at this X-ray luminosity the calculated $\dot{M}$ with the model of ADAF accretion can be high enough to drive a fraction of the matter in the accretion flow to be channelled on to the surface of the NS forming the X-ray pulsation.