Accretion Rate Research Articles

He-accreting Oxygen–Neon White Dwarfs and Accretion-induced Collapse Events

It has been widely accepted that mass-accreting white dwarfs (WDs) are the progenitors of Type Ia supernovae (SNe) or electron-capture supernovae. Previous work has shown that the accretion rate could affect the elemental abundance on the outer layers of CO WDs, and therefore affect the observational characteristics after they exploded as SNe Ia. However, it has not been well studied how elemental abundance changes on the outer layers of He-accreting ONe WDs as they approach the Chandrasekhar mass limit. In this paper, we investigated the evolution of He-accreting ONe WDs with MESA. We found that a CO-rich mantle will accumulate beneath the He layers resulting from the He burning, after which the ignition of the CO-rich mantle could transform carbon into silicon (Si). The amount of Si produced by carbon burning is strongly anticorrelated with the accretion rate. As the ONe WD nearly approaches the Chandrasekhar mass limit (M ch) through accretion, it is likely to undergo accretion-induced collapse, resulting in the formation of a neutron star.

Modeling the Energy-dependent Broadband Variability in the Black Hole Transient GX 339–4 Using AstroSat and NICER

We present a spectro-timing analysis of the black hole X-ray transient GX 339–4 using simultaneous observations from AstroSat and the Neutron Star Interior Composition Explorer (NICER) during the 2021 outburst period. The combined spectrum obtained from NICER, Large Area X-ray Proportional Counter and SXT data is effectively described by a model comprising a thermal disk component, hard Comptonization component, and reflection component with an edge. Our analysis of the AstroSat and NICER spectra indicates the source to be in a low/hard state, with a photon index of ∼1.64. The power density spectra obtained from both AstroSat and NICER observations exhibit two prominent broad features at 0.22 Hz and 2.94 Hz. We generated energy-dependent time lag and fractional root mean square (frms) at both frequencies in a broad energy range of 0.5–30 keV and found the presence of hard lags along with a decrease in variability at higher energy levels. Additionally, we discovered that the correlated variations in accretion rate, inner disk radius, coronal heating rate, and the scattering fraction, along with a delay between them, can explain the observed frms and lag spectra for both features.

An X-ray flaring event and a variable soft X-ray excess in the Seyfert LCRS B040659.9–385922 as detected with eROSITA

Context. Extreme continuum variability in extragalactic nuclear sources can indicate extreme changes in accretion flows onto supermassive black holes. Aims. We explore the multiwavelength nature of a continuum flare in the Seyfert LCRS B040659.9−385922. The all-sky X-ray surveys conducted by the Spectrum Roentgen Gamma (SRG)/extended ROentgen Survey with an Imaging Telescope Array (eROSITA) showed that its X-ray flux increased by a factor of roughly five over six months, and concurrent optical photometric monitoring with the Asteroid Terrestrial-impact Last Alert System (ATLAS) showed a simultaneous increase. Methods. We complemented the eROSITA and ATLAS data by triggering a multiwavelength follow-up monitoring program (X-ray Multi-Mirror Mission: XMM-Newton, Neutron star Interior Composition Explorer: NICER; optical spectroscopy) to study the evolution of the accretion disk, broad-line region, and X-ray corona. During the campaign, X-ray and optical continuum flux subsided over roughly six months. Our campaign includes two XMM-Newton observations, one taken near the peak of this flare and the other taken when the flare had subsided. Results. The soft X-ray excess in both XMM-Newton observations was power law-like (distinctly nonthermal). Using a simple power law, we observed that the photon index of the soft excess varies from a steep value of Γ ∼ 2.7 at the flare peak to a relatively flatter value of Γ ∼ 2.2 as the flare subsided. We successfully modeled the broadband optical/UV/X-ray spectral energy distribution at both the flare peak and post-flare times with the AGNSED model, incorporating thermal disk emission into the optical/UV and warm thermal Comptonization in the soft X-rays. The accretion rate falls by roughly 2.5, and the radius of the hot Comptonizing region increases from the flaring state to the post-flare state. Additionally, from the optical spectral observations, we find that the broad He IIλ4686 emission line fades significantly as the optical/UV/X-ray continuum fades, which could indicate a substantial flare of disk emission above 54 eV. We also observed a redshifted broad component in the Hβ emission line that is present during the high flux state of the source and disappears in subsequent observations. Conclusions. A sudden strong increase in the local accretion rate in this source manifested itself via an increase in accretion disk emission and in thermal Comptonized emission in the soft X-rays, which subsequently faded. The redshifted broad Balmer component could be associated with a transient kinematic component distinct from that comprising the rest of the broad-line region.

Electromagnetic signatures of black hole clusters in the center of super-Eddington galaxies

Supermassive black holes (SMBHs) at the centers of active galaxies are fed by accretion disks that radiate from the infrared or optical to the X-ray bands. Several types of objects can orbit SMBHs, including massive stars, neutron stars, clouds from the broad- and narrow-line regions, and X-ray binaries. Isolated black holes with a stellar origin (BHs of ∼10 M⊙) should also be present in large numbers within the central parsec of the galaxies. These BHs are expected to form a cluster around the SMBH as a result of the enhanced star formation rate in the inner galactic region and the BH migration caused by gravitational dynamical friction. However, except for occasional microlensing effects on background stars or gravitational waves from binary BH mergers, the presence of a BH population is hard to verify. In this paper, we explore the possibility of detecting electromagnetic signatures of a central cluster of BHs when the accretion rate onto the central SMBH is greater than the Eddington rate. In these supercritical systems, the accretion disk launches powerful winds that interact with the objects orbiting the SMBH. Isolated BHs can capture matter from this dense wind, leading to the formation of small accretion disks around them. If jets are produced in these single microquasars, they could be sites of particle acceleration to relativistic energies. These particles in turn are expected to cool by various radiative processes. Therefore, the wind of the SMBH might illuminate the BHs through the production of both thermal and nonthermal radiation.

The VANDELS Survey: Star formation and quenching in two over-densities at 3<z<4

Exploring galaxy evolution in dense environments, such as proto-clusters, is pivotal for understanding the mechanisms that drive star formation and the quenching of star formation. This study provides insights into how two over-densities could have impacted the physical properties, such as the star formation rate, stellar mass, morphology, and the evolution of their members, particularly members characterised by a quenching of star formation. We focus on the over-densities identified at $3<z<4$ in the Chandra Deep Field South (CDFS) and in the Ultra Deep Survey (UDS) regions of the VIMOS (VIsible MultiObject Spectrograph) Ultra Deep Survey (VANDELS). Our methodology involves the analysis of the spectral energy distribution of the members of the over-densities and of the galaxies in the field. We relied on Bayesian analysis techniques BEAGLE and BAGPIPES to study the best-fit physical parameters and the rest-frame U-V and V-J colours (UVJ). This approach allowed us to separate quenched and star-forming galaxies based on the UVJ diagram and by estimating their specific star formation rate (sSFR). We used the TNG300 simulation to interpret our results. We find that two out of 13 over-densities host quenched galaxies, with red rest-frame U-V colour and low sSFR. The physical properties of them are consistent with those of massive passive galaxies from the literature. The quenched members are redder, older, more massive, and show a more compact morphology than the other galaxy members. The two over-densities, with the highest-density peaks at $z and $z respectively, have dark matter halo masses consistent with being proto-clusters at $z and they each host an active galactic nucleus (AGN). We found five AGNs in the structure at $z and three AGNs in the one at $z In comparison to quenched galaxies in the field, our massive quenched members show a higher local density environment. By using the IllustrisTNG simulation (TNG300), we find that proto-cluster structures with quenched galaxies at high redshift are likely to evolve into a structure with a higher fraction of passive galaxies by $z = The two over-densities studied here host massive quenched galaxies in their highest-density peaks and AGNs. By following the evolution of the passive galaxies in the simulated proto-clusters at $z = 3$ from the TNG300 simulation, we find that the median of their sSFRs was larger than 10$^ $ yr$^ $ at $z=6$ and the median mass growth rate was 96<!PCT!> from $z=6$ to $z=3$. In 20<!PCT!> of the simulated proto-clusters, the passive galaxy had already accreted 10-20<!PCT!> of the mass at $z=6$, with SFRs $>100$ M$_ odot $ yr$^ $ at $z=8$. The conditions for this favorable mass assembly could be the galaxy interactions and the high gas accretion rate in the dense environment. As a consequence, the quenching of the star formation at $z=3$ could be driven by the black hole mass growth and AGN feedback. This scenario is consistent with the properties of the two quenched galaxies we find in our two over-densities at $z sim3$.

Twins in Diversity: Understanding circumstellar disk evolution in the twin clusters of W5 complex

Abstract Young star-forming regions in massive environments are ideal test beds to study the influence of surroundings on the evolution of disks around low-mass stars. We explore two distant young clusters, IC 1848-East and West located in the massive W5 complex. These clusters are unique due to their similar (distance, age, and extinction) yet distinct (stellar density and FUV radiation fields) physical properties. We use deep multi-band photometry in optical, near-IR, and mid-IR wavelengths complete down to the substellar limit in at least five bands. We trace the spectral energy distribution of the sources to identify the young pre-main sequence members in the region and derive their physical parameters. The disk fraction for the East and West clusters down to 0.1 M⊙ was found to be ∼27 ±2% (Ndisk=184, Ndiskless=492) and ∼17 ±1% (Ndisk=173, Ndiskless=814), respectively. While no spatial variation in the disk fraction is observed, these values are lower than those in other nearby young clusters. Investigating the cause of this decrease, we find a correlation with the intense feedback from massive stars throughout the cluster area. We also identified the disk sources undergoing accretion and observed the mass accretion rates to exhibit a positive linear relationship with the stellar host mass and an inverse relationship with stellar age. Our findings suggest that the environment significantly influences the dissipation of disks in both clusters. These distant clusters, characterized by their unique attributes, can serve as templates for future studies in outer galaxy regions, offering insights into the influence of feedback mechanisms on star and planetary formation.

Spherical accretion onto higher-dimensional Reissner–Nordström black hole

Abstract We obtain relativistic solutions of spherically symmetric accretion by a dynamical analysis of a generalised Hamiltonian for higher-dimensional Reissner–Nordström (RN) Black Hole (BH). We consider two different fluids namely, an isotropic fluid and a non-linear polytropic fluid to analyse the critical points in a higher-dimensional RN BH. The flow dynamics of the fluids are studied in different spacetime dimensions in the framework of Hamiltonian formalism. The isotropic fluid is found to have both transonic and non-transonic flow behaviour, but in the case of polytropic fluid, the flow behaviour is found to exhibit only non-transonic flow, determined by a critical point that is related to the local sound speed. The critical radius is found to change with the spacetime dimensions. Starting from the usual four dimensions it is noted that as the dimension increases the critical radius decreases, attains a minimum at a specific dimension (D > 4) and thereafter increases again. The mass accretion rate for isotropic fluid is determined using Hamiltonian formalism. The maximum mass accretion rate for RN BH with different equations of state parameters is studied in addition to spacetime dimensions. The flow behaviour and mass accretion rate for a change in BH charge is also studied analytically. It is noted that the maximum mass accretion rate in a higher-dimensional Schwarzschild BH is the lowest, which however, increases with the increase in charge parameter in a higher-dimensional RN BH.

The Interplay between the Disk and Corona of the Changing-look Active Galactic Nucleus 1ES 1927+654

Time-domain studies of active galactic nuclei (AGNs) offer a powerful tool for understanding black hole accretion physics. Prior to the optical outburst on 2017 December 23, 1ES 1927+654 was classified as a “true” type 2 AGN, an unobscured source intrinsically devoid of broad-line emission in polarized spectra. Through our 3 yr monitoring campaign spanning X-ray to ultraviolet/optical wavelengths, we analyze the post-outburst evolution of the spectral energy distribution (SED) of 1ES 1927+654. Examination of the intrinsic SED and subsequent modeling using different models reveal that the post-outburst spectrum is best described by a combination of a disk, blackbody, and corona components. We detect systematic SED variability and identify four distinct stages in the evolution of these components. During the event the accretion rate is typically above the Eddington limit. The correlation between ultraviolet luminosity and optical to X-ray slope (α OX) resembles that seen in previous studies of type 1 AGNs, yet exhibits two distinct branches with opposite slopes. The optical bolometric correction factor (κ 5100) is ∼10 times higher than typical AGNs, again displaying two distinct branches. Correlations among the corona optical depth, disk surface density, and α OX provide compelling evidence of a disk–corona connection. The X-ray corona showcases systematic variation in the compactness-temperature plot. Between 200 and 650 days, the corona is “hotter when brighter,” whereas after 650 days, it becomes “cooler when brighter.” This bimodal behavior, in conjunction with the bifurcated branches of α OX and κ 5100, offers strong evidence of a transition from a slim disk to a thin disk ∼650 days after the outburst.

ExoplaNeT accRetion mOnitoring sPectroscopic surveY (ENTROPY)

Context. Accretion among planetary mass companions is a poorly understood phenomenon, due to the lack of both observational and theoretical studies. The detection of emission lines from accreting gas giants facilitates detailed investigations into this process. Aims. This work presents a detailed analysis of Balmer lines from one of the few known young, planetary-mass objects with observed emission, the isolated L2γ dwarf 2MASS J11151597+1937266 with a mass between 7 and 21 MJup and an age of 5–45 Myr, located at 45 ± 2 pc. Methods. We obtained the first high-resolution (R ~ 50 000) spectrum of the target with VLT/UVES, an echelle spectrograph operating in the near-ultraviolet to visible wavelengths (3200–6800 Å). Results. We report several resolved hydrogen (H I; H3–H6) and helium (He I; λ5875.6) emission lines in the spectrum. Based on the asymmetric line profiles of Hα and Hβ, the 10% width of Hα (199 ± 1 km s−1), tentative He I λ6678 emission, and indications of a disk from mid-infrared excess, we confirm ongoing accretion at this object. Using the Gaia update of the parallax, we revise its temperature to 1816 ± 63 K and radius to 1.5 ± 0.1 RJup. Analysis of observed H I profiles using a 1D planet-surface shock model implies a pre-shock gas velocity, v0 = 120−40+ 80 km s−1, and a pre-shock density, log(n0/cm−3) = 14−5+ 0. The pre-shock velocity points to a mass, Mp = 6−4+ 8 MJup, for the target. Combining H I line luminosities (Lline) and planetary Lline−Lacc (accretion luminosity) scaling relations, we derived a mass accretion rate, Ṁacc = 1.4−0.9+ 2.8 × 10−8 MJup yr−1. Conclusions. The line-emitting area predicted from the planet-surface shock model is very small (~0.03%), and points to a shock at the base of a magnetospherically induced funnel. The Hα profile exhibits a much stronger flux than predicted by the model that best fits the rest of the H I profiles, indicating that another mechanism than shock emission contributes to the Hα emission. Comparison of line fluxes and Ṁacc from archival moderate-resolution SDSS spectra indicate variable accretion at 2MASS J11151597+1937266.

X-ray reverberation as an explanation for UV/optical variability in nearby Seyferts

Context. Active galactic nuclei (AGNs) are known to be variable across all wavelengths. Significant observational efforts have been invested in the last decade in studying their ultraviolet (UV) and optical variability. Long and densely sampled, multi-wavelength monitoring campaigns of numerous Seyfert galaxies have been conducted with the aim of determining the X-ray/UV/optical continuum time lags. Time-lag studies can be used to constrain theoretical models. The observed time lags can be explained by thermal reprocessing of the X-rays illuminating the accretion disc (known as the X-ray reverberation model). However, the observed light curves contain more information that can be used to further constrain physical models. Aims. Our primary objective is to investigate whether, in addition to time lags, the X-ray reverberation model can also explain the UV/optical variability amplitude of nearby Seyferts. Methods. We measured the excess variance of four sources (namely Mrk 509, NGC 4151, NGC 2617, and Mrk 142) as a function of wavelength using data from archival long, multi-wavelength campaigns with Swift, and ground-based telescopes. We also computed the model excess variance in the case of the X-ray reverberation model by determining the disc’s transfer function and assuming a bending power law for the X-ray power spectrum. We tested the validity of the model by comparing the measured and model variances for a range of accretion rates and X-ray source heights. Results. Our main result is that the X-ray thermal reverberation model can fit both the continuum, UV/optical time lags, as well as the variance (i.e. the variability amplitude) in these AGNs, for the same physical parameters. Our results suggest that the accretion disc is constant and that all the observed UV/optical variations, on timescales of days and up to a few weeks, can be fully explained by the variable X-rays as they illuminate the accretion disc.

Evolution of Tidal Disruption Event Disks with Magnetically Driven Winds

We present a time-dependent, one-dimensional, magnetically driven disk wind model based on magnetohydrodynamic (MHD) equations, in the context of tidal disruption events (TDEs). We assume that the disk is geometrically thin, is gas pressure dominated, and explicitly accounts for magnetic braking and turbulent viscosity through an extended α-viscosity prescription. We find a particular wind solution for a set of basic equations that satisfies the necessary and sufficient conditions for vertically unbound MHD flows. The solution shows that the disk evolves with mass loss due to wind and accretion from the initial Gaussian density distribution. We confirm that the mass accretion rate follows the power law of time t −19/16 at late times in the absence of wind, which matches the classical solution of J. K. Cannizzo et al. We find that the mass accretion rate is steeper than the t −19/16 curve when the wind is present. Mass accretion is also induced by magnetic braking, known as the wind-driven accretion mechanism, which results in a faster decay with time of both the mass accretion and mass-loss rates. In the disk emission, the ultraviolet (UV) luminosity is the highest among the optical, UV, and X-ray luminosities. While the optical and X-ray emission is observationally insignificant without magnetic braking, the X-ray emission is brighter at late times, especially in the presence of magnetic braking. This provides a possible explanation for observed delayed X-ray flares. Our model predicts that late-time bolometric light curves steeper than t −19/16 in UV-bright TDEs are potentially compelling indicators of magnetically driven winds.

Accretion of Vlasov gas onto a black hole in the Kalb–Ramond field

Abstract We investigate the accretion of Vlasov gas onto a static, spherically symmetric black hole (BH) influenced by the Kalb–Ramond (KR) field, focusing on the effects of Lorentz symmetry breaking (LSB) parameters. We employ the Maxwell–Jüttner distribution to model the gas at infinity and derive key quantities such as the particle current density and mass accretion rate. Our findings reveal that an increase in the LSB parameter results in a decrease in the mass accretion rate. We also present explicit formulations of the accretion rate in the high-temperature limit and find the result is quite different from the result in Bumblebee model.

A Two-zone Accretion Disk in the Changing-look Active Galactic Nucleus 1ES 1927+654: Physical Implications for Tidal Disruption Events and Super-Eddington Accretion

The properties of slim accretion disks, while crucial for our understanding of black hole growth, have yet to be studied extensively observationally. We analyze the multiepoch broadband spectral energy distribution of the changing-look active galactic nucleus 1ES 1927+654 to derive the properties of its complex, time-dependent accretion flow. The accretion rate decays as Ṁ∝t−1.53 , consistent with the tidal disruption of a 1.1 M ⊙ star. Three components contribute to the spectral energy distribution: a central overheated zone resembling a slim disk, an outer truncated thin disk, and a hot corona. Photon trapping in the slim disk triggered by the high initial Ṁ was characterized by a low radiation efficiency (3%), which later more than doubled (8%) after Ṁ dropped sufficiently low for the disk to transition to a geometrically thin state. The blackbody temperature profile T ∝ R −0.60 for the inner overheated zone matches the theoretical expectations of a slim disk, while the effective temperature profile of T ∝ R −0.69 for the outer zone is consistent with the predictions of a thin disk. Both profiles flatten toward the inner boundary of the disk as a result of Compton cooling in the corona. Our work presents compelling observational evidence for the existence of slim accretion disks and elucidates the key parameters governing their behavior, paving the way for further exploration in this area.

Constraining Accreted Neutron Star Crust Shallow Heating with the Inferred Depth of Carbon Ignition in X-ray Superbursts

Abstract Evidence has accumulated for an as-yet unaccounted for source of heat located at shallow depths within the accreted neutron star crust. However, the nature of this heat source is unknown. I demonstrate that the inferred depth of carbon ignition in X-ray superbursts can be used as an additional constraint for the magnitude and depth of shallow heating. The inferred shallow heating properties are relatively insensitive to the assumed crust composition and carbon fusion reaction rate. For low accretion rates, the results are weakly dependent on the duration of the accretion outburst, so long as accretion has ensued for enough time to replace the ocean down to the superburst ignition depth. For accretion rates at the Eddington rate, results show a stronger dependence on the outburst duration. Consistent with earlier work, it is shown that urca cooling does not impact the calculated superburst ignition depth unless there is some proximity in depth between the heating and cooling sources.

X-ray reverberation modelling of the continuum, optical/UV time-lags in quasars

Extensive, multi-wavelength monitoring campaigns of nearby and higher redshift active galactic nuclei (AGN) have shown that the UV/optical variations are well correlated with time delays which increase with increasing wavelength. Such behaviour is expected in the context of the X-ray thermal reverberation of the accretion disc in AGN. Our main objective is to use time-lag measurements of luminous AGN and fit them with sophisticated X-ray reverberation time-lags models. In this way we can investigate whether X-ray reverberation can indeed explain the observed continuum time lags, and whether time-lag measurements can be used to measure physical parameters such as the X-ray corona height and the spin of the black hole (BH) in these systems. We use archival time-lag measurements for quasars from different surveys, and we compute their rest frame, mean time-lags spectrum. We fit the data with analytical X-ray reverberation models, using $ statistics, and fitting for both maximal and non spinning BHs, for various colour correction values and X-ray corona heights. We found that X-ray reverberation can explain very well the observed time lags, assuming the measured BH mass, accretion rate and X-ray luminosity of the quasars in the sample. The model agrees well with the data both for non-rotating and maximally rotating BHs, as long as the corona height is larger than $ 40$ gravitational radii. This is in agreement with previous results which showed that X-ray reverberation can also explain the disc radius in micro-lensed quasars, for the same corona heights. The corona height we measure depends on the model assumption of a perfectly flat disc. More realistic disc models may result in lower heights for the X-ray corona.

Sequential formation of supermassive stars and heavy seed BHs through the interplay of cosmological cold accretion and stellar radiative feedback

ABSTRACT Supermassive stars (SMSs) and heavy seed black holes, as their remnants, are promising candidates for supermassive black hole (SMBH) progenitors, especially for ones observed in the early universe $z\simeq 8.5-10$ by recent JWST observations. Expected cradles of SMSs are the atomic cooling haloes ($M_{\rm halo}\simeq 10^7\ \mathrm{M}_{\odot }$), where ‘cold accretion’ emerges and possibly forms SMSs. We perform a suit of cosmological radiation hydrodynamics simulations and investigate star formation after the emergence of cold accretion, solving radiative feedback from stars inside the halo. We follow the mass growth of the protostars for $\sim 3\ \mathrm{Myr}$, resolving the gas inflow down to $\sim 0.1\ \mathrm{pc}$ scales. We discover that, after cold accretion emerges, multiple SMSs of $m_{\star }\gtrsim 10^5\ \mathrm{M}_{\odot }$ form at the halo centre with the accretion rates maintained at $\dot{m}_{\star }\simeq 0.04\ \mathrm{M}_\odot \mathrm{yr}^{-1}$ for $\lesssim 3\ \mathrm{Myr}$. Cold accretion supplies gas at a rate of $\dot{M}_{\rm gas}\gtrsim 0.01-0.1\ \mathrm{M}_\odot \mathrm{yr}^{-1}$ from outside the halo virial radius to the central gas disc. Gravitational torques from spiral arms transport gas further inwards, which feeds the SMSs. Radiative feedback from stars suppresses $\mathrm{H}_2$ cooling and disc fragmentation, while photoevaporation is prevented by a dense envelope, which attenuates ionizing radiation. Our results suggest that cold accretion can bring efficient BH mass growth after seed formation in the later universe. Moreover, cold accretion and gas migration inside the central disc increase the mass concentration and provide a promising formation site for the extremely compact stellar clusters observed by JWST.

The eROSITA Final Equatorial Depth Survey (eFEDS). The hard X-ray selected sample

During its calibration and performance verification phase, the eROSITA instrument aboard the Spectrum-RG satellite performed a uniform wide-area X-ray survey of approximately 140 deg2, known as the eROSITA Final Equatorial Depth Survey (eFEDS). The primary aim of eFEDS is to demonstrate the scientific performance to be expected at the end of the eight--pass eROSITA all--sky survey. This survey will provide the first focussed image of the whole sky in the hard X-ray ($>2$ keV) bandpass. The expected source population in this energy range is thus of great interest, particularly for AGN studies. We used a 2.3--5 keV selection to construct a sample of 246 point-like hard X-ray sources for further study and characterisation. These sources are classified as either extragalactic ($ 90$ <!PCT!>) or Galactic ($ 10$ <!PCT!>), with the former consisting overwhelmingly of AGN and the latter active stars. We concentrated our further analysis on the extragalactic AGN sample, describing their X-ray and multi-wavelength properties and comparing them to the eFEDS main AGN sample selected in the softer 0.2--2.3 keV band. The eROSITA hard band selects a subsample of sources that is a factor of more than ten brighter than the eFEDS main sample. The AGN within the hard population reach up to $z=3.2$ but on the whole, they are relatively nearby, with median $z$=0.34 compared to $z$=0.94 for the main sample. The hard survey probes typical luminosities in the range $ X = 43-46$. The X-ray spectral analysis shows significant intrinsic absorption (with $ H >21$) in $ 20$ <!PCT!> of the sources, with a hard X-ray power law continuum with mean $< which is typical of AGN, but slightly harder than the soft-selected eROSITA sample. Around $10$ <!PCT!> of the hard sample show a significant `soft excess' component. The sampled black hole mass distribution in the eFEDS broad-line AGN population is consistent with that of the deeper COSMOS survey that probes a higher redshift population. On the other hand, the Eddington ratios appear systematically lower, which is consistent with the idea that the decline in SMBH activity since $z 1$ is due to a reduction in the typical accretion rate, rather than a shift towards activity in lower-mass black holes. The eFEDS hard sample provides a preview of what can be expected from the eRASS final survey in terms of data quality. This pilot survey indicates the power of eROSITA to shed new light on the demographics and evolution of AGN, and the potential for discovery of new and rare populations.

Origin of transition disk cavities: Pebble-accreting protoplanets vs super-Jupiters

Protoplanetary disks surrounding young stars are the birth places of planets. Among them, transition disks with inner dust cavities of tens of au are sometimes suggested to host massive companions. Yet, such companions are often not detected. Some transition disks exhibit a large amount of gas inside the dust cavity and relatively high stellar accretion rates, which contradicts typical models of gas-giant-hosting systems. Therefore, we investigate whether a sequence of low-mass planets can create the appearance of cavities in the dust disk. We evolve the disks with low-mass growing embryos in combination with 1D dust transport and 3D pebble accretion, to investigate the reduction of the pebble flux at the embryos' orbits. We vary the planet and disk properties to understand the resulting dust profile. We find that multiple pebble-accreting planets can efficiently decrease the dust surface density, resulting in dust cavities consistent with transition disks. The number of low-mass planets necessary to sweep up all pebbles decreases with decreasing turbulent strength and is preferred when the dust Stokes number is $10^ $. Compared to dust rings caused by pressure bumps, those by efficient pebble accretion exhibit more extended outer edges. We also highlight the observational reflections: the transition disks with rings featuring extended outer edges tend to have a large gas content in the dust cavities and rather high stellar accretion rates. We propose that planet-hosting transition disks consist of two groups. In Group A disks, planets have evolved into gas giants, opening deep gaps in the gas disk. Pebbles concentrate in pressure maxima, forming dust rings. In Group B, multiple Neptunes (unable to open deep gas gaps) accrete incoming pebbles, causing the appearance of inner dust cavities and distinct ring-like structures near planet orbits. The morphological discrepancy of these rings may aid in distinguishing between the two groups using high-resolution ALMA observations.

Long-term evolution of spin and other properties of neutron star low-mass X-ray binaries: implications for millisecond X-ray pulsars

ABSTRACT A neutron star (NS) accreting matter from a companion star in a low-mass X-ray binary (LMXB) system can spin up to become a millisecond pulsar (MSP). Properties of many such MSP systems are known, which is excellent for probing fundamental aspects of NS physics when modelled using the theoretical computation of NS LMXB evolution. Here, we systematically compute the long-term evolution of NS, binary, and companion parameters for NS LMXBs using the stellar evolution code mesa. We consider the baryonic to gravitational mass conversion to calculate the NS mass evolution and show its cruciality for the realistic computation of some parameters. With computations using many combinations of parameter values, we find the general nature of the complex NS spin frequency ($\nu$) evolution, which depends on various parameters, including accretion rate, fractional mass-loss from the system, and companion star magnetic braking. Further, we utilize our results to precisely match some main observed parameters, such as $\nu$, orbital period ($P_{\rm orb}$), etc., of four accreting millisecond X-ray pulsars (AMXPs). By providing the $\nu$, $P_{\rm orb}$, and the companion mass spaces for NS LMXB evolution, we indicate the distribution and plausible evolution of a few other AMXPs. We also discuss the current challenges in explaining the parameters of AMXP sources with brown dwarf companions and indicate the importance of modelling the transient accretion in LMXBs as a possible solution.

MASTER OT J030227.28+191754.5: An unprecedentedly energetic dwarf nova outburst

Abstract We present a detailed study of the MASTER OT J030227.28$+$191754.5 outburst in 2021–2022, which reached an amplitude of $10.2\:$mag and a duration of $60\:$d. The detections of (1) the double-peaked optical emission lines, and (2) the early and ordinary superhumps, established that MASTER OT J030227.28$+$191754.5 is an extremely energetic WZ Sge-type dwarf nova (DN). Based on the superhump observations, we obtained its orbital period and mass ratio as $0.05986(1)\:$d and 0.063(1), respectively. These values are within a typical range for low-mass-ratio DNe. According to the binary parameters derived based on the thermal–tidal instability model, our analyses showed that (1) the standard disk model requires an accretion rate $\simeq\!\! 10^{20}\:$g$\:$s$^{-1}$ to explain its peak optical luminosity, and (2) large mass was stored in the disk at the outburst onset. These factors cannot be explained solely by the impact of its massive ($\gtrsim\!\! 1.15\, M_{\odot }$) primary white dwarf implied by Kimura et al. (2023, ApJ, 951, 124). Instead, we propose that the probable origin of this enormously energetic DN outburst is the even lower quiescence viscosity than other WZ Sge-type DNe. This discussion is qualitatively valid for most possible binary parameter spaces unless the inclination is low enough ($\lesssim\!\! 40^\circ$) for the disk to be bright, explaining the outburst amplitude. Such low inclinations, however, would not allow detectable amplitude of early superhumps in the current thermal–tidal instability model. The optical spectra at outburst maximum showed strong emission lines of the Balmer, He i, and He ii series, the core of which is narrower than $\sim \! 800\:$km$\:$s$^{-1}$. Considering its binary parameters, a Keplerian disk cannot explain this narrow component, but the presumable origin is disk winds.