Search

This paper presents a comprehensive spectroscopic analysis of the quiescent stage of the recurrent nova RS Ophiuchi between its 2006 and 2021 outbursts. The spectra shows prominent low-ionization emission features, including hydrogen, helium, iron emissions, and TiO absorption features. The H |$\alpha$| and H |$\beta$| lines showed double-peaked emission profiles, indicating that both originate from the accretion disc. The central peaks of the H |$\alpha$| and H |$\beta$| emission profiles exhibited subtle shifts towards the blue or red side, attributed to orbital motion and fluctuations in the accretion rate. Using the double-peak features observed in the H |$\alpha$| and H |$\beta$| lines, we have estimated the accretion disc size to be |$R_{\mathrm{AD}} = 3.10 \pm 0.04 \times 10^{12} \, \text{cm}$|⁠. The cloudy photoionization code is employed to model the quiescent phase spectra, allowing us to study the evolution of various physical parameters such as temperature, luminosity, hydrogen density, elemental abundances, accreted mass, and accretion rate. The central ionizing sources exhibit temperatures in the range of |$1.05\!-\!1.80~\times 10^4$| K and luminosities between |$0.10\!-\!7.94~\times 10^{30}$|  erg s |$^{-1}$|⁠. The mean accretion rate, calculated from the model, is |$\sim$| |$1.25 \times 10^{-8} \,{\rm M}_{\odot }$| yr |$^{-1}$|⁠. The model results reveal that the accretion rate rose substantially in the later phase. The accreted mass in the 16 months, preceding the 2021 outburst exceeds 47 per cent of the critical mass, and more than 88 per cent of the critical mass was accreted in the last 3 yr. [ABSTRACT FROM AUTHOR]

Changing-look active galactic nuclei (CLAGNs) are known to change their spectral type between 1 and 2 (changing-state) or change their absorption between Compton-thick and Compton-thin (changing-obscuration) on time-scales of years or less. The physical mechanism and possible connection between the two types of CLAGNs are still unclear. We explore the evolution of the broad-band X-ray spectra from Nuclear Spectroscopic Telescope Array and column density in five CLAGNs with moderate inclination viewing angles, which have shown significant variations of both optical types and X-ray absorption. Based on a phenomenological and two clumpy torus models, we find that the X-ray photon index (⁠|$\Gamma$|⁠) and the Eddington-scaled X-ray |$2{\!-\!}10$|  keV luminosity (⁠|$L_{\rm X}/L_{\rm Edd}$|⁠) are positively correlated for the five sources, which are similar to other bright AGNs and optical CLAGNs at type 1 phase. We find a significant negative correlation between log |$N_\mathrm{H,los}$| and log |$L_{\rm X}/L_{\rm Edd}$| except for ESO 362-G18. Similar to changing-state AGNs, changing-obscuration AGNs may be also triggered by the evolution of the accretion disc. Our results support the disc wind scenario, where the disc wind proportional to the accretion rate and formed at moderate inclination angles would push the obscuration material further away and decrease the column density from the line of sight observed in the changing-look AGNs. [ABSTRACT FROM AUTHOR]

We present a search for luminous long-duration ambiguous nuclear transients (ANTs) similar to the unprecedented discovery of the extreme ambiguous event AT2021lwx with a |$\gt 150$|  d rise time and luminosity |$10^{45.7}$|  erg s |$^{-1}$|⁠. We use the Lasair transient broker to search Zwicky Transient Facility (ZTF) data for transients lasting more than one year and exhibiting smooth declines. Our search returns 59 events, 7 of which we classify as ANTs assumed to be driven by accretion onto supermassive black holes. We propose the remaining 52 are stochastic variability from regular supermassive black hole accretion rather than distinct transients. We supplement the seven ANTs with three nuclear transients in ZTF that fail the light curve selection but have clear single flares and spectra that do not resemble typical active galactic nucleus. All of these 11 ANTs have a mid-infrared flare from an assumed dust echo, implying the ubiquity of dust around the black holes giving rise to ANTs. No events are more luminous than AT2021lwx, but one (ZTF19aamrjar) has twice the duration and a higher integrated energy release. On the other extreme, ZTF20abodaps reaches a luminosity close to AT2021lwx with a rise time |$\lt 20$|  d and that fades smoothly in |$\gt 600$|  d. We define a portion of rise-time versus flare amplitude space that selects ANTs with |$\sim 50$|  per cent purity against variable AGNs. We calculate a volumetric rate of |$\gtrsim 3\times 10^{-11}$|  Mpc |$^{-1}$|  yr |$^{-1}$|⁠ , consistent with the events being caused by tidal disruptions of intermediate and high-mass stars. [ABSTRACT FROM AUTHOR]

This study explores the formation and implications of mini-active galactic nucleus (mAGN) discs around intermediate-mass black holes (IMBHs) embedded in gas-rich globular/nuclear clusters (GCs). We examine the parameter space for stable mAGN discs, considering the influence of IMBH mass, disc radius, and gas density on disc stability. The dynamics of stars and black holes within the mAGN disc are modelled, with a focus on gas-induced migration and gas dynamical friction. These dynamical processes can lead to several potentially observable phenomena, including the enhancement of gravitational wave mergers (particularly intermediate-mass ratio inspirals and extreme-mass ratio inspirals), and the occurrence of milli/centi-tidal disruption events with unique observational signatures. We find that gas hardening can significantly accelerate the inspiral of binaries within the disc, potentially leading to a frequency shift in the emitted gravitational waves. Additionally, we explore the possibility of forming accreting IMBH systems from captured binaries within the mAGN disc, potentially resulting in the formation of ultraluminous X-ray sources. The observational implications of such accreting systems, including X-ray emission, optical signatures, and transient phenomena, are discussed. Furthermore, we investigate the possibility of large-scale jets emanating from gas-embedded IMBHs in GCs. While several caveats and uncertainties exist, our work highlights the potential for mAGN discs to provide unique insights into IMBH demographics, accretion physics, and the dynamics of GCs. [ABSTRACT FROM AUTHOR]

The accretion of material on to a black hole modifies the properties of that hole owing to the capture of both matter and radiation. Adding matter to the hole through an accretion disc generally acts to increase the hole's spin parameter, while the capture of radiation generally provides a retarding torque. The balance between the torques provided by adding matter and radiation leads to a maximum spin parameter that can be obtained by a black hole which grows by accretion, known as the Thorne limit. In the simplest theory of thin disc accretion this Thorne limit has the value |$a_{\bullet , {\rm lim}} \simeq 0.998$|⁠. The purpose of this paper is to highlight that any modification to theories of accretion flows also modify this limiting value, and to compute precisely the modification arising from a particular extension of accretion theory: the inclusion of bright emission from within the plunging region which is sourced from the magnetohydrodynamic stresses ubiquitously observed in simulations. This extra emission further suppresses black hole spin-up and results in new, lower, limits on the final black hole spin. These limits depend on the details of the magnetic stresses acting within the plunging region, but typical values seen in simulations and observations would lower the limit to |$a_{\bullet , {\rm lim}} \simeq 0.99$|⁠ , a subtle but not negligible deviation. [ABSTRACT FROM AUTHOR]

Observations of supermassive black holes (BHs) at high redshift challenge our understanding of the evolution of the first generation of BHs in proto-galactic environments. One possibility is that they grow much more rapidly than current estimates of feedback and accretion efficiency permit. Following our previous analysis of super-Eddington accretion on to stellar-mass BHs in mini-haloes under no-feedback conditions, we now investigate whether this can be sustained when thermal feedback is included. We use four sets of cosmological simulations at sub-pc resolution with initial BH masses varying from |$1 \times 10^{3} \ \mathrm{ to} \ 6 \times 10^{4} \ {\rm M_\odot }$|⁠ , exploring a range of feedback efficiencies. We also vary the feedback injection radius to probe the threshold of numerical overcooling. We find that super-Eddington growth sustained of the order of |$\sim$| |$100 \ \rm kyr$| is possible with weak thermal feedback efficiency in all environments and moderate efficiency for two of the BHs. Trans-Eddington growth is possible for a |$3 \times 10^{3}\!\! - \!\! 6 \times 10^{3}\ {\rm M_\odot }$| BH at moderate feedback efficiencies. We discuss the effectiveness of thermal feedback in heating the gas, suppressing accretion, and driving outflows at these parameter configurations. Our results suggest that super-Eddington growth may be possible in the presence of thermal feedback for BHs formed from the first stars. [ABSTRACT FROM AUTHOR]

Quasi-periodic eruptions (QPEs) are a recently identified class of X-ray transient associated with tidal disruption events by supermassive black holes, and for which there are multiple possible explanations. In this paper, we present a simple model which requires the black hole be spinning, be misaligned with the accretion flow (both conditions of which are almost certainly met), and that the accretion rate is a few times the Eddington limit. We speculate that the resulting Lense–Thirring torques force the disc and entrained outflows to precess, leading to increased X-ray flux when the wind-cone is oriented at lower inclinations to the observer. We test the range of parameters for which this model could explain the period and brightness of the QPE events discovered thus far, and make qualitative comparisons between the observed X-ray spectra and light curves to those extracted from general relativistic radiation magnetohydrodynamic simulations. Overall, we find some areas of promising concordance, and identify challenges related to the details of current simulations. [ABSTRACT FROM AUTHOR]

Massive black hole (MBH) binaries in galactic nuclei are one of the leading sources of |${\sim}$| mHz gravitational waves (GWs) for future missions such as Laser Interferometer Space Antenna (LISA). However, the poor sky localization of GW interferometers will make it challenging to identify the host galaxy of MBH mergers absent an electromagnetic counterpart. One such counterpart is the tidal disruption of a star that has been captured into mean motion resonance with the inspiralling binary. Here we investigate the production of tidal disruption events (TDEs) through capture into, and subsequent evolution in, orbital resonance. We examine the full non-linear evolution of planar autoresonance for stars that lock into autoresonance with a shrinking MBH binary. Capture into the 2:1 resonance is guaranteed for any realistic astrophysical parameters (given a relatively small MBH binary mass ratio), and the captured star eventually attains an eccentricity |$e\approx 1$|⁠ , leading to a TDE. Stellar discs can be produced around MBHs following an active galactic nucleus episode, and we estimate the TDE rates from resonant capture produced when a secondary MBH begins inspiralling through such a disc. In some cases, the last resonant TDE can occur within a decade of the eventual LISA signal, helping to localize the GW event. [ABSTRACT FROM AUTHOR]

We present a comprehensive spectral and timing analysis of 15 outbursts and 18 mini-outbursts from nine dynamically confirmed black hole X-ray transients with light curve and spectral data from RXTE, MAXI , and NuSTAR obtained from 1996 to 2024. Departed from the canonical fast-rise exponential decay (FRED) morphology, the most common morphology within our sample is triangular with similar rise and decay time-scale. In most outbursts, the spectral evolutions indicate the presence of limit-cycle instability, as predicted by the disc instability model (DIM). Even though almost all of the outbursts showed a similar canonical pattern, unique transition patterns are found in FRED outbursts. On the other hand, no spectral transition is found in any mini-outburst, which was observed in either hard or thermal-dominant (TD) state only. The Fe K  |$\alpha$| emission line is the most prominent feature in the hard state of the rising phase but none is found in the decay phase. Triangular outbursts are always in transition to the TD state, following a standard accretion disc, before the peak proceeds to match DIM prediction. This is unlike the FRED outburst which directly transitioned to the steep power law (SPL) state or high Eddington ratio TD state, resembling a slim accretion disc. Non-canonical spectral evolution as well as the rarity of FRED outburst in our sample, seem to add more challenges for DIM. Studying the morphology of outburst light curve may reveal more clues on the evolution of the disc instability at least during the time relevant to the burst. [ABSTRACT FROM AUTHOR]

r -mode oscillations in rotating neutron stars are a source of continuous gravitational radiation. We investigate the excitation of r -modes by the mechanical impact on the neutron star surface of stochastically accreted clumps of matter, assuming that the Chandrasekhar–Friedman–Schutz instability is not triggered. The star is idealized as a slowly rotating, unmagnetized, one-component fluid with a barotropic equation of state in Newtonian gravity. It is found that the r -mode amplitude depends weakly on the equation of state but sensitively on the rotation frequency |$\nu _{\rm s}$|⁠. The gravitational wave strain implicitly depends on the equation of state through the damping time-scale. The root-mean-square strain is |$h_{\rm rms} \approx 10^{-35} (\nu _{\rm s}/ 10\, {\rm Hz})^{2} (R_*/10\, {\rm km})^2 (\Delta t_{\rm acc}/1\, {\rm yr})^{1/2} (f_{\rm acc}/1\, {\rm kHz})^{-1/2} (\dot{M}/10^{-8}\text{M}_{\odot } \, \text{yr}^{-1}) (v/0.4c) (d/1\, {\rm kpc})^{-1}$|⁠ , which is comparable to the strain from g -, p -, and f -modes excited by stochastic accretion, where |$R_*$| is the radius of the star, |$\Delta t_{\rm acc}$| is the uninterrupted duration of an accretion episode, |$f_{\rm acc}$| is the mean clump impact frequency, |$\dot{M}$| is the accretion rate, v is the impact speed, and d is the distance of the star from the Earth. An observational test is proposed, based on the temporal autocorrelation function of the gravitational wave signal, to discern whether the Chandrasekhar–Friedman–Schutz instability switches on and coexists with impact-excited r -modes before or during a gravitational wave observation. [ABSTRACT FROM AUTHOR]

Most stars form in dense clusters within high-mass star-forming regions, where protoplanetary discs may be exposed to intense UV radiation from nearby massive stars. While previous studies have typically focussed on isolated sources in low-mass regions, recent observational campaigns have started to probe the chemistry of irradiated discs in unprecedented detail. Interpreting this data requires complex chemical models, yet few studies have examined these discs' chemistry, and none have incorporated the photoevaporative wind launched by external UV fields into their physical structure. In this study, we post-process radiation hydrodynamics simulations of externally irradiated protoplanetary discs using the thermochemical code dali , comparing models with and without the wind to assess its impact on disc chemistry. Results show that UV radiation is rapidly attenuated by the disc in both cases. However, thermal re-emission from the wind at longer wavelengths enhances disc heating, increasing the gas-phase abundances of some key volatiles. Synthetic line fluxes vary by orders of magnitude between wind and windless models, primarily due to emission from the wind itself rather than abundance variations within the disc. Our findings demonstrate that the photoevaporative wind significantly influences the physical and chemical structure, and observational characteristics, of externally irradiated discs. We conclude that incorporating the wind into chemical models is essential for accurately predicting chemical abundances, interpreting observations, and ultimately understanding planet formation in these common yet complex environments. [ABSTRACT FROM AUTHOR]

We present a detailed spectral study of an intermittent-AMXP Aql X-1 during the pulse-on and pulse-off stages by using the archival Rossi X-ray Timing Explorer (RXTE) data. We first perform temporal analysis by using Z |$_n^2$| technique in three different energy bands, 3.0–13.0, 13.0–23.0, and 23.0–33.0 keV, for the last 128 s time segment of the RXTE data including pulse-on region. We show that the pulse is the most significant in the softest band. We, then, show that the spectrum is represented the best via combination of absorbed blackbody, disc blackbody, and a Gaussian line. We modelled the last four segments of the data 30188-03-05-00 to better compare pulse-on and pulse-off stages. We found a vague residual in the spectral fit of the pulse-on segment between |$\sim$| 3.0 and 13.0 keV, which agrees with the result of temporal analysis. We show that the residual may be represented with an extra blackbody component with the temperature of 1.75 keV and the radius of 0.75 |$\pm$| 0.49 km. For deeper analysis, we performed phase-resolved spectroscopy to the last 128 s, pulse-on , segment. We obtain two separate spectra for the spin phase ranges of 0.75–0.25 (pulse-high) and 0.25–0.75 (pulse-low), and followed the same procedure. We display that the residual becomes more clear for pulse-high compared to the pulse-low. We report that the additional blackbody component, which models the residual, indicates a hotspot from the surface of the neutron star with the radius of 1.65 |$\pm$| 0.74 km whose temperature is 1.65 keV. [ABSTRACT FROM AUTHOR]

The Western Vardar ophiolite, a thrust sheet of oceanic crust and mantle obducted onto the Adriatic passive margin in the Late Jurassic, crops out along the entire Balkan Peninsula. The Mirdita Ophiolite forms the northern Albanian segment of this unit. In northeast Albania near Bajram Curri, a 200–700 m thick metamorphic sole is preserved at its base. The assembly of obducted mantle rocks and metamorphic sole constitutes a plate interface that formed during the intraoceanic subduction stage preceding obduction; we call this a fossil intraoceanic plate interface in this paper. This setting allows to study the interrelated tectonometamorphic evolution and rock-water interaction between the subducted and exhumed metamorphic sole and concomitant mantle wedge serpentinization in the overlying units. We combined detailed lithological and structural mapping with micro-scale analyses along this plate interface. Three tectonic units were distinguished. Mylonitic harzburgites overlie a tightly folded, tectonised subophiolitic mélange along a SE-dipping contact that defines the fossil intraoceanic plate interface. The tectonised subophiolitic mélange itself was separated into a structurally lower non-metamorphic broken formation and a higher metamorphic sole, separated by an isoclinally folded thrust. Within the metamorphic sole, the temperature and degree of deformation increase towards the structural top. Shear sense indicators in calcschists of the metamorphic sole show top to the west transport of the overriding units. All metamorphic sole lithologies were overprinted at lower greenschist-facies conditions, reflecting their exhumation from intraoceanic subduction. Corresponding microstructures indicate mineral growth at isotropic stresses, suggesting that deformation migrated into structurally lower, frontally accreted non-metamorphic units of the sub-ophiolitic mélange marking the start of obduction onto the passive Adriatic margin. Ongoing westward transport led to folding of the entire sub-ophiolitic succession. Harzburgites are more deformed towards the plate interface, forming a mylonitic fabric. There, harzburgites contain accessory Cr-rich spinel and the foliation is dissected by multiple generations of veins containing serpentine and magnetite. Vein density is highest along the plate interface and decreases up-section, suggesting that serpentinisation was triggered by devolatilisation reactions in the sediments of the metamorphic sole that were subducting below the harzburgites, and the upwards migration of volatiles into the overlying mantle wedge. [ABSTRACT FROM AUTHOR]

This review provides an observational perspective on the fundamental properties of super-Eddington accretion onto supermassive black holes in quasars. It begins by outlining the selection criteria, particularly focusing on optical and UV broad-line intensity ratios, used to identify a population of unobscured super-Eddington candidates. Several defining features place these candidates at the extreme end of the Population A in main sequence of quasars: among them are the highest observed singly-ionized iron emission, extreme outflow velocities in UV resonance lines, and unusually high metal abundances. These key properties reflect the coexistence of a virialized sub-system within the broad-line region alongside powerful outflows, with the observed gas enrichment likely driven by nuclear or circumnuclear star formation. The most compelling evidence for the occurrence of super-Eddington accretion onto supermassive black holes comes from recent observations of massive black holes at early cosmic epochs. These black holes require rapid growth rates that are only achievable through radiatively inefficient super-Eddington accretion. Furthermore, extreme Eddington ratios, close to or slightly exceeding unity, are consistent with the saturation of radiative output per unit mass predicted by accretion disk theory for super-Eddington accretion rates. The extreme properties of super-Eddington candidates suggest that these quasars could make them stable and well-defined cosmological distance indicators, leveraging the correlation between broad-line width and luminosity expected in virialized systems. Finally, several analogies with accretion processes around stellar-mass black holes, particularly in the high/soft state, are explored to provide additional insight into the mechanisms driving super-Eddington accretion. [ABSTRACT FROM AUTHOR]

We propose a scenario for mass evolution of massive black holes (MBHs) in galactic nuclei, to explain both the mass correlation of supermassive black holes (SMBHs) with bulges and the downsizing behavior of active galactic nuclei. Primordial gas structures that produce galactic bulges are supposed to be formed at |$z \sim 10$| and the core region, called the nuclear region (NR) here, is considered to be a place for an MBH to grow to an SMBH. The downsizing behavior requires the MBH to significantly increase its mass in a time |$\sim$| 1 Gyr. The rapid mass increase is discussed as being realized only when the MBH stays in a very high-density region such as the core of a molecular cloud throughout the period |$\sim$| 1 Gyr. According to these arguments, MBHs formed from population III stars born in mini-halos at |$z \sim 20$| –30 are excluded from the candidates for the seed black hole for an SMBH and only MBHs from population II stars born in the core of the central molecular cloud (CMC) in the NR are left as candidates. The MBHs in the dense core of the CMC started increasing in mass through mass accretion and the most massive black hole (MMBH) saw the most rapid evolution, possibly restraining the relatively slow evolutions of the less massive black holes. Dynamical interactions of the MMBH with ambient MCs induced the wandering motion and the further mass increase. However, when the MMBH mass exceeds a boundary mass, dynamical friction with field stars brakes the MMBH wandering and mass accretion. This scenario can semi-quantitatively reproduce both the downsizing behavior and the SMBH mass–bulge mass correlation with reasonable parameter values. [ABSTRACT FROM AUTHOR]

We performed shot analyses of X-ray and optical subsecond flares observed during the low/hard state of the 2018 outburst in MAXI J1820+070. Optical shots were less spread than X-ray shots. The amplitude of X-ray shots was highest at the onset of the outburst, and they faded at the transition to the intermediate state. The timescale of shots was ~0.2 s, and we detected abrupt spectral hardening synchronized with this steep flaring event. The time evolution of optical shots was not similar to that of X-ray shots. These results suggest that accreting gas blobs triggered a series of magnetic reconnections at the hot inner accretion flow in the vicinity of the black hole, which enhanced X-ray emission and generated flaring events. Rapid X-ray spectral hardening would be caused by this kind of magnetic activity. Also, synchrotron emission not only at the hot flow but also at the jet plasma would contribute to the optical rapid variability. We also found that the low/hard state exhibited six different phases in the hardness–intensity diagram and the correlation plot between the optical flux and the X-ray hardness. The amplitude and duration of X-ray shots varied in synchrony with these phases. This time variation may provide key information about the evolution of the hot flow, the low-temperature outer disk, and the jet-emitting plasma. [ABSTRACT FROM AUTHOR]

Binary star systems can accrete material originating from a circumbinary disc. Since it is common for the circumbinary disc to be tilted with respect to the binary orbital plane, we test whether the accretion dynamics can be a diagnostic for binary–disc misalignment. We present hydrodynamical simulations to model the accretion flow from a circumbinary disc around an eccentric binary with initial tilts ranging from |$0^\circ$| to |$180^\circ$| in increments of |$15^\circ$|⁠. Based on the initial tilt, the circumbinary disc will align towards three different configurations: prograde coplanar, polar, or retrograde coplanar. For discs with initial tilts evolving towards prograde coplanar alignment, the accretion rates on to the primary and secondary stars exhibit alternating preferential accretion. Circumbinary discs evolving towards polar alignment exhibit no alternating preferential accretion on to the binary unless the initial tilt is close to the critical tilt that sets the boundary between coplanar or polar alignment. Such cases cause strong disc warping, leading to disc breaking. The inner disc becomes eccentric, leading to alternating preferential accretion on to the binary. As the break propagates outward, the disc tilt damps towards a polar state and the disc eccentricity decreases. As the disc recircularizes, the accretion rate transitions back from alternating preferential accretion to non-alternating accretion. Lastly, no alternating preferential accretion exists for discs undergoing retrograde coplanar alignment. From the summary of the accretion rates from our suite of smoothed particle hydrodynamics simulations, it is evident that the accretion rate evolution can be affected by the initial tilt and subsequent evolution of the circumbinary disc. [ABSTRACT FROM AUTHOR]

We present optical monitoring of the neutron star low-mass X-ray binary Swift J1858.6–0814 during its 2018–2020 outburst and subsequent quiescence. We find that there was strong optical variability present throughout the entire outburst period covered by our monitoring, while the average flux remained steady. The optical spectral energy distribution is blue on most dates, consistent with emission from an accretion disc, interspersed by occasional red flares, likely due to optically thin synchrotron emission. We find that the fractional rms variability has comparable amplitudes in the radio and optical bands. This implies that the long-term variability is likely to be due to accretion changes, seen at optical wavelengths, that propagate into the jet, seen at radio frequencies. We find that the optical flux varies asymmetrically about the orbital period, peaking at phase |$\sim$| 0.7, with a modulation amplitude that is the same across all optical wavebands, suggesting that reprocessing off of the disc, companion star and ablated material is driving the phase dependence. The evidence of ablation found in X-ray binaries is vital in understanding the long-term evolution of neutron star X-ray binaries and how they evolve into (potentially isolated) millisecond pulsars. [ABSTRACT FROM AUTHOR]

Black hole X-ray binaries in their hard and hard-intermediate states display hard and soft time lags between broad-band noise variations (high-energy emission lagging low-energy and vice versa), which could be used to constrain the geometry of the disc and Comptonizing corona in these systems. Comptonization and reverberation lag models, which are based on light-travel delays, can imply coronae that are very large (hundreds to thousands of gravitational radii, |$R_{g}$|⁠) and in conflict with constraints from X-ray spectral modelling and polarimetry. Here, we show that the observed large and complex X-ray time lags can be explained by a model where fluctuations are generated in and propagate through the blackbody-emitting disc to a relatively compact (⁠|$\sim$| 10 |$R_{g}$|⁠) inner corona. The model naturally explains why the disc variations lead coronal variations with a Fourier-frequency dependent lag at frequencies |$\lt 1$| Hz, since longer variability time-scales originate from larger disc radii. The propagating fluctuations also modulate successively the coronal seed photons from the disc, heating of the corona via viscous dissipation and the resulting reverberation signal. The interplay of these different effects leads to the observed complex pattern of lag behaviour between disc and power-law emission and different power-law energy bands, the energy-dependence of power-spectral shape, and a strong dependence of spectral-timing properties on coronal geometry. The observed spectral-timing complexity is thus a natural consequence of the response of the disc-corona system to mass-accretion fluctuations propagating through the disc. [ABSTRACT FROM AUTHOR]

The Málaga coast, in the south of Spain, is a densely populated tourist destination where ports, marinas and coastal protection structures of various typologies (e.g., groins, breakwaters, revetments) and shapes (e.g., "Y", "L", etc., shaped groins) have been emplaced. Such structures have modified the long- and cross-shore sediment transport and produced changes in beach morphology and the evolution of nearby areas. To characterize the changes related to shore-normal structures, beach erosion/accretion areas close to coastal anthropic structures were measured using a sequence of aerial orthophotos between 1956 and 2019, and the potential littoral sediment transport for the two main littoral transport directions was determined by means of the CMS (Coastal Modeling System). Available data on wave propagation and coastal sediment transport reflect the complex dynamics of the study area, often characterized by the coexistence of opposing longshore transport directions. Accretion was observed on both sides of ports in all studied periods and groins and groups of groins presented mixed results that reflect the heterogeneity of the study area; in certain sectors where the wave regime is bidirectional, changes in the shoreline trend were observed during the study period. The study cases described in this paper emphasize the difficulties in finding clear spatial and temporal trends in the artificially induced erosion/accretion patterns recorded along a heavily modified shoreline. [ABSTRACT FROM AUTHOR]

X-ray reflection spectroscopy is a powerful tool to study the strong gravity region of black holes. The next generation of astrophysical X-ray missions promises to provide unprecedented high-quality data, which could permit us to get very precise measurements of the properties of the accretion flow and of the space–time geometry in the strong gravity region around these objects. In this work, we test the accuracy of the relativistic calculations of the reflection model relxill and of its extension to non-Kerr space–times relxill_nk in view of the next generation of X-ray missions. We simulate simultaneous observations with Athena /X-IFU and LAD of bright Galactic black holes with a precise and accurate ray-tracing code and we fit the simulated data with the latest versions of relline and relline_nk. While we always recover the correct input parameters, we find residuals in the fits when the emission from the inner part of the accretion disc is higher. Such residuals disappear if we increase the number of interpolation points on the disc in the integral of the transfer function. We also simulate full reflection spectra and find that the emission angle from the accretion disc should be treated properly in this case. [ABSTRACT FROM AUTHOR]

General relativistic corrections are calculated for the quadrupole moment of a magnetically confined mountain on an accreting neutron star. The hydromagnetic structure of the mountain satisfies the general relativistic Grad–Shafranov equation supplemented by the flux-freezing condition of ideal magnetohydrodynamics, as in previous calculations of the magnetic dipole moment. It is found that the ellipticity, and hence the gravitational wave strain, are up to 12% greater than in the analogous Newtonian system. The direct contribution of the magnetic field to the nonaxisymmetric component of the stress-energy tensor is shown to be negligible in accreting systems such as low-mass X-ray binaries. [ABSTRACT FROM AUTHOR]

In the inner regions of protoplanetary discs, ionization chemistry controls the fluid viscosity, and is thus key to understanding various accretion, outflow and planet formation processes. The ionization is driven by thermal and non-thermal processes in the gas phase, as well as by dust-gas interactions that lead to grain charging and ionic and thermionic emission from grain surfaces. The latter dust–gas interactions are moreover a strong function of the grain size distribution. However, analyses of chemical networks that include ionic/thermionic emission have so far only considered grains of a single size (or only approximately treated the effects of a size distribution), while analyses that include a distribution of grain sizes have ignored ionic/thermionic emission. Here, we (1) investigate a general chemical network, widely applicable in inner disc regions, that includes gas phase reactions, ionic and thermionic emission, and an arbitrary grain size distribution; (2) present a numerical method to solve this network in equilibrium; and (3) elucidate a general method to estimate the chemical time-scale. We show that (a) approximating a grain size distribution by an 'effective dust-to-gas ratio' (as done in previous work) can predict significantly inaccurate grain charges; and (b) grain charging significantly alters grain collisional time-scales in the inner disc. For conditions generally found in the inner disc, this work facilitates (i) calculation of fluid resistivities and viscosity; and (ii) inclusion of the effect of grain charging on grain fragmentation and coagulation (a critical effect that is often ignored). [ABSTRACT FROM AUTHOR]

Recent high angular resolution ALMA observations have revealed rich information about protoplanetary discs, including ubiquitous substructures and three-dimensional gas kinematics at different emission layers. One interpretation of these observations is embedded planets. Previous 3D planet–disc interaction studies are either based on viscous simulations or non-ideal magnetohydrodynamics (MHD) simulations with simple prescribed magnetic diffusivities. This study investigates the dynamics of gap formation in 3D non-ideal MHD discs using non-ideal MHD coefficients from the look-up table that is self-consistently calculated based on the thermochemical code. We find a concentration of the poloidal magnetic flux in the planet-opened gap (in agreement with previous work) and enhanced field-matter coupling due to gas depletion, which together enable efficient magnetic braking of the gap material, driving a fast accretion layer significantly displaced from the disc mid-plane. The fast accretion helps deplete the gap further and is expected to negatively impact the planet growth. It also affects the corotation torque by shrinking the region of horseshoe orbits on the trailing side of the planet. Together with the magnetically driven disc wind, the fast accretion layer generates a large, persistent meridional vortex in the gap, which breaks the mirror symmetry of gas kinematics between the top and bottom disc surfaces. Finally, by studying the kinematics at the emission surfaces, we discuss the implications of planets in realistic non-ideal MHD discs on kinematics observations. [ABSTRACT FROM AUTHOR]

IGR J17591–2342, a transient accretion-powered millisecond X-ray pulsar, was discovered during its 2018 outburst. Here, we present a timing and spectral analysis of the source using AstroSat data of the same outburst. From the timing analysis, we obtain updated values of binary orbital parameters, which reveal an average pulsar spin frequency of 527.425 6984(8) Hz. The pulse profiles can be fit well with four harmonically related sinusoidal components with fractional amplitudes of fundamental and second, third, and fourth harmonics as ~13 per cent, ~6 per cent, ~0.9 per cent, and ~0.2 per cent, respectively. The energy-dependent study of pulse profiles in the range of 3–20 keV shows that the fractional amplitude of both the fundamental and first overtone is consistent with being constant across the considered energy band. Besides, a decaying trend has been observed for both the fundamental and first overtone in the phase-delay versus energy relation, resulting in soft X-ray (2.8–3.3 keV) phase lags of |$\sim$| 0.05 and |$\sim$| 0.13 with respect to |$\le 15$|  keV photons, for the fundamental and first overtone, respectively. The combined spectra from the Large Area X-ray Proportional Counters and the Soft X-ray Telescope aboard AstroSat in the 1–18 keV range can be fit well with an absorbed model consisting of a Comptonization, a blackbody and a Gaussian emission-line component yielding as best-fitting parameters a blackbody seed photon temperature |$kT_{\rm bb}$| |$\sim 0.95 \pm 0.03$| keV, and an electron temperature |$kT_{\rm e}$| |$\sim 1.54 \pm 0.03$| keV. The spectral aspects suggest the scattering of photons from the accretion disc or the neutron star's surface. [ABSTRACT FROM AUTHOR]

The shoreline experiences significant changes due to human activities and natural processes, resulting in erosion and accretion. Studying the morphology of shorelines is crucial for identifying the causes and impacts of coastal processes along the coast, which helps protect the ecosystem and facilitate future development. This study comprehensively analyses shoreline changes along the Kerala coast from 2000 to 2024, revealing substantial shifts due to frequent cyclones and other natural events. Key findings include significant erosion at Shangumugam, Valliyathura, Punthura, and Edapadu beaches and notable accretion at Thumba, Kochuveli, Vizhinjam, Adimalathura, Pullavila, Karumkulam, and Poovar. The study employs a novel trend analysis approach to evaluate the localized impact of cyclonic events on various coastal locations. Trend analysis from 2000 to 2024, using high-resolution satellite images, identified consistent erosion patterns, particularly after Cyclone Ockhi in 2017 and Cyclone Tauktae in 2021, which caused significant, unrecovered erosion along the Vizhinjam coastline. Regression analysis validates the findings with a high correlation (R² = 0.872 against beach profile data and R² = 0.96 against field data). It is observed that the frequency of cyclonic activity has increased in recent times along the Kerala Coast. The study accentuates Kerala’s increasing vulnerability to natural disasters and the necessity of advanced monitoring techniques for effective coastal management. [ABSTRACT FROM AUTHOR]

Accretion and migration usually proceed concurrently for giant planet formation in the natal protoplanetary disks. Recent works indicate that the concurrent accretion onto a giant planet imposes significant impact on the planetary migration dynamics in the isothermal regime. In this work, we carry out a series of 2D global hydrodynamical simulations with Athena++ to explore the effect of thermodynamics on the concurrent accretion and migration processes of the planets in a self-consistent manner. The thermodynamics effect is modeled with a thermal relaxation timescale using a β -cooling prescription. Our results indicate that radiative cooling has a substantial effect on the accretion and migration processes of the planet. As cooling timescales increase, we observe a slight decrease in the planetary accretion rate, and a transition from the outward migrating into inward migration. This transition occurs approximately when the cooling timescale is comparable to the local dynamical timescale ( β ∼ 1 ), which is closely linked to the asymmetric structures from the circumplanetary disk (CPD) region. The asymmetric structures in the CPD region which appear with an efficient cooling provide a strong positive torque driving the planet migrate outward. However, such a positive torque is strongly suppressed, when the CPD structures tend to disappear with a relatively long cooling timescale ( β ≳ 10 ). Our findings may also be relevant to the dynamical evolution of accreting stellar-mass objects embedded in disks around active galactic nuclei. [ABSTRACT FROM AUTHOR]

Context. The advancement of the Event Horizon Telescope has enabled the study of relativistic jets in active galactic nuclei down to sub-parsec linear scales even at high redshift. Quasi-simultaneous multifrequency observations provide insights into the physical conditions in compact regions and allow accretion theories to be tested. Aims. Initially, we aimed to measure the magnetic field strength close to the central supermassive black hole in NRAO 530 (1730−130) by studying the frequency-dependent opacity of the jet matter, Faraday rotation, and the spectral index in the millimeter-radio bands. Methods. NRAO 530 was observed quasi-simultaneously at 15, 22, 43, 86, and 227 GHz at four different very long baseline interferometer (VLBI) networks. By means of imaging and model-fitting, we aligned the images, taken at different frequencies. We explored opacity along the jet and the distribution of the linearly polarized emission in it. Results. Our findings reveal that the jet of NRAO 530 at 86 and 227 GHz is transparent down to its origin, with 70 mJy emission detected at 227 GHz potentially originating from the accretion disk. The magnetic field strength near the black hole, estimated at 5rg, is 3 × 103 − 3 × 104 G (depending on the central black hole mass). These values represent some of the highest magnetic field strengths reported for active galaxies. We also report the first ever VLBI measurement of the Faraday rotation at 43−227 GHz, which reveals rotation measure values as high as −48 000 rad/m2, consistent with higher particle density and stronger magnetic fields at the jet's outset. The complex shape of the jet in NRAO 530 is in line with the expected behavior of a precessing jet, with a period estimated to be around 6 ± 4 years. [ABSTRACT FROM AUTHOR]

Outflows are critical components of many astrophysical systems, including accreting compact binaries and active galactic nuclei (AGN). These outflows can significantly affect a system's evolution and alter its observational appearance by reprocessing the radiation produced by the central engine. sirocco (Simulating Ionization and Radiation in Outflows Created by Compact Objects – or 'the code formerly known as python ') is a Sobolev-based Monte Carlo ionization and radiative transfer code. It is designed to simulate the spectra produced by any system with an azimuthally symmetric outflow, from spherical stellar winds to rotating, biconical accretion disc winds. Wind models can either be parametrized or imported, e.g. from hydrodynamical simulations. The radiation sources include an optically thick accretion disc and various central sources with flexible spectra and geometries. The code tracks the 'photon packets' produced by the sources in any given simulation as they traverse and interact with the wind. The code assumes radiative near-equilibrium, so the thermal and ionization state can be determined iteratively from these interactions. Once the physical properties in the wind have converged, sirocco can be used to generate synthetic spectra at a series of observer sightlines. Here, we describe the physical assumptions, operation, performance and limitations of the code. We validate it against tardis, cmfgen , and cloudy , finding good agreement, and present illustrative synthetic spectra from disc winds in cataclysmic variables, tidal disruption events, AGN, and X-ray binaries. sirocco is publicly available on GitHub , alongside its associated data, documentation and sample input files covering a wide range of astrophysical applications. [ABSTRACT FROM AUTHOR]

Extragalactic black hole X-ray binaries (BH-XRBs) are the most intriguing X-ray sources as some of them are 'home' to the most massive stellar-mass BHs ever found. In this work, we conduct a comprehensive study of three massive, eclipsing extragalactic BH-XRBs i.e. M33X-7, NGC300X-1, and IC10X-1 and using entire X-ray observations available from XMM–Newton and NuSTAR till date. Preliminary analysis using diskbb and power-law models shows that the sources have steep spectra and sub-Eddington luminosities (L < 0.69 L |$_{\mathrm{ Edd}}$|⁠), with major flux contribution from non-thermal component, resembling the relatively uncharted steep power-law state (SPL). To understand the accretion disc properties in this state, we explore alternate modelling scenario that reveals the presence of a 'hot' (⁠|$kT_{\mathrm{ in}}=1\!-\!2$|  keV) slim-disc (diskpbb) with radial temperature profile |$T(r)\propto r^{-p}$| (⁠|$p=0.5\!-\!0.66$|⁠), along with a cooler (⁠|$kT_{\mathrm{ in}}=0.1\!-\!0.2$|  keV) standard thermal disc (diskbb). We carry out the continuum-fitting method using relativistic slim-disc model (slimbh) and estimate the mass range of M33 X–7, NGC300X-1, and IC10X-1 is to be 9–15 M |$_{\odot }$|⁠ , 9–28 M |$_{\odot }$|⁠ , and 10–30 M |$_{\odot }$|⁠ , respectively. Further, eclipse periods are determined by modelling the light curve, using which we estimate the size of the eclipsing bodies. Modelling of the eclipse spectra revealed the complete obscuration of soft spectral component during eclipse, implying the emission of hard component from an extended accretion region. Based on our findings, we provide an inference on geometry of accretion disc in these wind-fed systems and compare their properties with the other two extragalactic BH-XRBs. [ABSTRACT FROM AUTHOR]

A fraction of the ultraluminous X-ray (ULX) sources are known to be accreting neutron stars as they show coherent X-ray pulsations with pulse periods ranging from |${\sim} 1{\!-\!}30$|  s. While initially thought to host intermediate-mass black holes, ULXs have since been recognized as a diverse class of objects, including ULX pulsars. These pulsars require models specifically tailored to account for their unique accretion physics, distinct from those used for Galactic black hole binaries. The X-ray spectra of all Galactic accreting X-ray pulsars (including sources in the Magellanic Clouds) are dominated by a high-energy cut-off power law and some of the sources show a soft excess, some emission lines, cyclotron absorption features, etc. In this work, we undertake a comprehensive analysis of the broad-band X-ray spectra of five ULX pulsars using simultaneous XMM – Newton and NuSTAR observations and show that their X-ray spectra can be effectively described by spectral models, similar to those used for the local accretion-powered X-ray pulsars. A soft excess is detected in all the sources which is also consistent with the local X-ray pulsars that have low absorption column density. We have marginal detection or low upper limit on the presence of the iron K-alpha emission line from these sources, which is a key difference of the ULX pulsars with the local accreting X-ray pulsars. We discuss the implication of this on the nature of the binary companion and the accretion mechanism in the ULX pulsars. [ABSTRACT FROM AUTHOR]

The moose (Monitoring Observations of SMC X-1's Excursions) program uses the Neutron Star Interior Composition Explorer Mission (NICER) to monitor the high-mass X-ray binary SMC X-1 during its superorbital period excursions. Here, we perform X-ray spectral analyses of 26 NICER observations of SMC X-1, taken at the tail-end of the excursion between 2021-04-01 and 2022-01-05. We use a single spectral model to fit spectra observed in high, intermediate, and low states, using a combination of a partial covering fraction model, a blackbody disc, and a power-law component. We find that the partial covering fraction varies significantly with the superorbital state during superorbital excursion. Our findings suggest that the low/high state in SMC X-1 is caused by a very high obscuration of the accretion disc. [ABSTRACT FROM AUTHOR]

Low-mass X-ray binaries with a neutron star as the primary object show a complex array of phenomenology during outbursts. The observed variability in X-ray emission primarily arises from changes in the innermost regions of the accretion disk, neutron star surface, and corona. In this work, we present the results of a comprehensive X-ray spectral and timing analysis of the neutron star transient MAXI J1807+132 during its 2023 outburst using data from the NICER observatory. The outburst is marked by a very rapid rise in the count rate by about a factor of 20 in a day. The source undergoes full state transitions and displays the hysteresis effect in the hardness and rms intensity diagrams. Spectral analysis with a three-component model is consistent with disk truncation during the hard states and reaching the last stable orbit during the intermediate and soft states. We discuss the different values of the last stable radius in the context of the possible distance of the source and magnetic field strength. The characteristic frequencies throughout the hard and intermediate states are found to be strongly correlated with the inner radius of the disk. Together with the spectral and fast variability properties, we attempt to trace the evolution of the size of the corona along the outburst. Following the main outburst, the source undergoes a high-amplitude reflare, wherein it shows a complex behavior with relatively high variability (10%), but low hardness. [ABSTRACT FROM AUTHOR]