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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]

Recently, a perturbative calculation to the first post-Newtonian order has shown that the analytically worked out Lense–Thirring precession of the orbital angular momentum of a test particle following a circular path around a massive spinning primary is able to explain the measured features of the jet precession of the supermassive black hole at the centre of the giant elliptical galaxy M87. It is shown that also the hole's mass quadrupole moment |$Q_2$|⁠ , as given by the no-hair theorems, has a dynamical effect, which cannot be neglected, as, instead, done so far in the literature. New allowed regions for the hole's dimensionless spin parameter |$a^\ast$| and the effective radius |$r_0$| of the accretion disc, assumed tightly coupled with the jet, are obtained by including both the Lense–Thirring and the quadrupole effects in the dynamics of the effective test particle modelling the accretion disc. One obtains that, by numerically integrating the resulting averaged equations for the rates of change of the angles |$\eta$| and |$\phi$| characterizing the orientation of the orbital angular momentum with |$a^\ast = +0.98$| and |$r_0=14.1$| gravitational radii, it is possible to reproduce, both quantitatively and qualitatively, the time series for them recently measured with the Very Long Baseline Interferometry technique. Instead, the resulting time series produced with |$a^\ast = -0.95$| and |$r_0=16$| gravitational radii turn out to be out of phase with respect to the observationally determined ones, while maintaining the same amplitudes. [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]

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]

We analyse the variability of the parsec-scale jet directions in active galactic nuclei (AGNs). Our analysis involves 317 AGNs at frequencies ranging from 2 to 43 GHz, and is made possible by developing an automatic jet direction measurement procedure. We find strong significant variations in a one quarter of these AGNs; the effect is likely ubiquitous, and not detected in the rest due to a limited sensitivity and observations epoch coverage. Apparent jet rotation speeds range from 0.21 deg yr |$^{-1}$| at 2 GHz to 1.04 deg yr |$^{-1}$| at 43 GHz. This strong frequency dependence indicates that the variability cannot be explained by jet components propagating ballistically without acceleration: more complex jet shapes or motion patterns are required. Still, we demonstrate that the apparent direction changes are predominantly caused by the jet nozzle rotations, and not by individual components propagating transversely to the jet. In this work, we focus on variability scales much longer than the times of observations, that is |$\gtrsim 50$| yr. Using our measurements, we bound potential periods to less than 1000 yr in the source rest frame for 90 per cent AGNs in the sample. These time-scales constrain jet direction variation mechanisms, with the most likely explanations being the plasma instabilities, the precession caused by the accretion disc with density |$\sim r^{-1}$|⁠ , and the orbital motion of binary systems. [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]

We present mid-infrared (MIR) spectral-timing measurements of the prototypical Galactic microquasar GRS 1915+105. The source was observed with the Mid-Infrared Instrument (MIRI) onboard JWST in June 2023 at an MIR luminosity |$L_{\rm MIR}$|   |$\approx$|  10 |$^{36}$|  erg s |$^{-1}$| exceeding past infrared levels by about a factor of 10. In contrast, the X-ray flux is much fainter than the historical average, in the source's now-persistent 'obscured' state. The MIRI low-resolution spectrum shows a plethora of emission lines, the strongest of which are consistent with recombination in the hydrogen Pfund (Pf) series and higher. Low-amplitude (⁠|$\sim$| 1 per cent) but highly significant peak-to-peak photometric variability is found on time-scales of |$\sim$| 1000 s. The brightest Pf (6–5) emission line lags the continuum. Though difficult to constrain accurately, this lag is commensurate with light-travel time-scales across the outer accretion disc or with expected recombination time-scales inferred from emission-line diagnostics. Using the emission line as a bolometric indicator suggests a moderate (⁠|$\sim$| 5–30 per cent Eddington) intrinsic accretion rate. Multiwavelength monitoring shows that JWST caught the source close in time to unprecedentedly bright MIR and radio long-term flaring. Assuming a thermal bremsstrahlung origin for the MIRI continuum suggests an unsustainably high mass-loss rate during this time unless the wind remains bound, though other possible origins cannot be ruled out. Polycyclic aromatic hydrocarbon features previously detected with Spitzer are now less clear in the MIRI data, arguing for possible destruction of dust in the interim. These results provide a preview of new parameter space for exploring MIR spectral timing in X-ray binaries and other variable cosmic sources on rapid time-scales. [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]

It is generally thought that active galactic nucleus (AGN) optical variability is produced, at least in part, by reprocessing of central X-rays by a surrounding accretion disc, resulting in wavelength-dependent lags between bands. Any good model of AGN optical variability should explain not only these lags, but also the overall pattern of variability as quantified by the power spectral density (PSD). Here, we present |$\sim$| daily g ′-band monitoring of the low-mass AGN NGC 4395 over 3 yr. Together with previous Transiting Exoplanet Survey Satellite (TESS) and Gran Telescopio Canarias (GTC)/HiPERCAM observations, we produce an optical PSD covering an unprecedented frequency range of ∼seven decades allowing excellent determination of PSD parameters. The PSD is well fitted by a bending power law with low-frequency slope |$\alpha _{L} = 1.0 \pm 0.2$|⁠ , high-frequency slope |$2.1^{+0.2}_{-0.4}$|⁠ , and bend time-scale |$3.0^{+6.6}_{-1.7}\,$|  d. This time-scale is close to that derived previously from a damped random walk (DRW) model fitted to just the TESS observations, although |$\alpha _{L}$| is too steep to be consistent with a DRW. We compare the observed PSD with one made from light curves synthesized assuming reprocessing of X-rays, as observed by XMM–Newton and Swift , in a disc defined by the observed lags. The simulated PSD is also well described by a bending power law but with a bend two decades higher in frequency. We conclude that the large-amplitude optical variations seen on long time-scales are not due to disc reprocessing but require a second source of variability whose origin is unknown but could be propagating disc accretion rate variations. [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]

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]

We present the results of a new X-ray to near-IR photometric and spectroscopic monitoring of the changing look active galactic nucleus NGC 2617 carried out from March 2022 to March 2024. We found significant variability at all wavelengths and in the intensities and profiles of the broad Balmer lines. Reverberation mapping was carried out for three observing seasons during 2021–2024. We obtained time delays of |$\sim$| 4 d for the response of the H  |${\beta }$| line to optical continuum variations. The X-ray variations correlate well with the UV and optical, with a few days lag for longer wavelengths. The K band lagged the B band by |$\sim$| 15 d during the last three seasons, which is significantly shorter than the delays reported previously by the 2016 and 2017–2019 campaigns. Near-IR variability arises from two different emission regions: the outer part of the accretion disc and a more distant dust component. The |$HK$| -band variability is governed primarily by dust. The H  |${\beta }$| /H  |${\alpha }$| ratio variations (for broad components only) correlate with the X-ray and UV fluxes. The spectral type changed from type 1.8 to type 1.9 during 2023 October–2024 January and then a more rapid change to type 1.5 occurred in 2024 February. We interpret these changes as a combination of two factors: changes in the accretion rate as a dominant cause but also the sublimation or recovery of dust along the line of sight. [ABSTRACT FROM AUTHOR]

The observations of comet C/2020 T2 (Palomar) were carried out using the 3.4 mm wavelength microwave band before perihelion from 2021 January 22 to July 5 and after perihelion on 2021 July 13. During this period, the comet was located at a heliocentric distance of between |$r_\text{h}$| = 2.840 and 2.055 au. The consecutive long-term monitoring of outgassing of C/2020 T2 was conducted with Purple Mountain Observatory (PMO) 13.7 m telescope and the Institut de RadioAstronomie Millimétrique (IRAM) 30 m telescope in the atmospheric radio window. The hyperfine triplet components of hydrogen cyanide (HCN (1−0) F = 0−1, F = 2−1, and F = 1−1) of the J = 1−0 vibrational ground-state transitions, as the primary tracer of molecular gas, were unambiguously identified in this comet C/2020 T2. Combining all data, we derived the positive signal of line width corresponds to the coma expansion velocity |$v_{\rm exp}$| from |$\sim$| 0.2 to |$\sim$| 0.4 km s |$^{-1}$|⁠. The mean gas molecular production rates of HCN were derived |$Q_{\rm HCN}$| = (2.92 |$\pm$| 0.51) |$\times$| 10 |$^{25}$| molecules s |$^{-1}$| at PMO 13.7 m, and |$Q_{\rm HCN}$| = (6.26 |$\pm$| 1.55) |$\times$| 10 |$^{25}$| molecules s |$^{-1}$| at IRAM 30 m, respectively. Overall, studying the gas composition of the long-period comet revealed abundant information about the missing link between interstellar molecular clouds and the outer regions of a distant protoplanetary accretion disc, and the relationship between amorphous ice sublimation mechanism and heliocentric distance. [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]

A new platform has been developed for the 1-MA COBRA generator to investigate the physical processes affecting the formation, collimation, and stability of high-speed outflows in magnetically driven laboratory plasma jets. Such experiments serve as diagnostically accessible surrogates for astrophysical jets under the assumption that the underlying dynamics are scale invariant. In contrast to previous current driven high energy density laboratory jet experiments that use radial/conical wire arrays or foils, the platform described here uses azimuthally symmetric gas-puff injection. This avoids the ablation phase from a solid target, allowing the jets to develop earlier and be driven longer without depleting their mass source and disrupting. A permanent magnet provides an initial poloidal magnetic field, which links the two concentric electrodes and mimics the boundary conditions of a star-accretion disk system. Extended magnetohydrodynamic effects can be assessed using a polarity convolute, which allows for reversal of the electrode bias. The resulting plasma jets exhibit remarkable stability, persisting for hundreds of nanoseconds and achieving aspect ratios ≳ 30 : 1. [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]

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]

Quasiperiodic eruptions (QPEs) are high-amplitude, soft X-ray bursts recurring every few hours, associated with supermassive black holes. Many interpretations for QPEs were proposed since their recent discovery in 2019, including extreme mass ratio inspirals and accretion disk instabilities. But, as of today, their nature still remains debated. We perform the first high-resolution X-ray spectral study of a QPE source using the Reflection Grating Spectrometers' gratings on board XMM-Newton, leveraging nearly 2 Ms of exposure on GSN 069, the first discovered source of this class. We resolve several absorption and emission lines including a strong line pair near the N vii rest-frame energy, resembling the P-Cygni profile. We apply photoionization spectral models and identify the absorption lines as an outflow blueshifted by 1700−2900 km s−1, with a column density of about 1022 cm−2 and an ionization parameter log (ξ /erg cm s−1) of 3.9−4.6. The emission lines are instead redshifted by up to 2900 km s−1, and likely originate from the same outflow that imprints the absorption features, and covers the full 4 π sky from the point of view of GSN 069. The column density and ionization are comparable to the outflows detected in some tidal disruption events, but this outflow is significantly faster and has a strong emission component. The outflow is more highly ionized when the system is in the phase during which QPEs are present, and from the limits, we derive on its location, we conclude that the outflow is connected to the recent complex, transient activity of GSN 069, which began around 2010. [ABSTRACT FROM AUTHOR]

The neutron star low-mass X-ray binary, EXO 0748−676, recently returned to outburst after a |$\sim\!\! 16$| yr-long quiescence. Since its return, there has been a global effort to capture the previously unseen rise of the source and to understand its somewhat early return to outburst, as it is typical for a source to spend longer in quiescence than in outburst. Here, we report on the simultaneous optical and X-ray detection of a type I X-ray burst, captured by XMM–Newton during a director's discretionary time observation on 2024 June 30. The data show three X-ray eclipses consistent with the known ephemeris and one type I X-ray burst at 60492.309 MJD. The X-ray burst is reprocessed into the optical band and captured by XMM–Newton's Optical Monitor during a 4399 s exposure with the B filter in image |$+$| fast mode. We determine that the optical peak lags the X-ray peak by |$4.46 \pm 1.71$| s. The optical and X-ray rise times are similar, but the optical decay time-scale is shorter than the X-ray decay time-scale. The reprocessing site is likely within a few light-seconds of the X-ray emitting region, so the companion star, accretion disc, and ablated material are all plausible. [ABSTRACT FROM AUTHOR]

Accretion of protoplanetary discs (PPDs) could be driven by magnetohydrodynamic disc winds rather than turbulent viscosity. With a dynamical prescription for angular momentum transport induced by disc winds, we perform 2D simulations of PPDs to systematically investigate the rate and direction of planet migration in a windy disc. We find that the the strength of disc winds influences the corotation region similarly to the 'desaturation' effect of viscosity. The magnitude and direction of torque depend sensitively on the hierarchy between the radial advection time-scale across the horseshoe due to disc wind |$\tau _{\rm dw}$|⁠ , the horseshoe libration time-scale |$\tau _{\rm lib}$| and U-turn time-scale |$\tau _{\rm U-turn}$|⁠. Initially, as wind strength increases and the advection time-scale shortens, a non-linear horseshoe drag emerges when |$\tau _{\rm dw} \lesssim \tau _{\rm lib}$|⁠ , which tends to drive strong outward migration. Subsequently, the drag becomes linear and planets typically still migrate inward when |$\tau _{\rm dw} \lesssim \tau _{\rm U-turn} \sim \tau _{\rm lib}h$|⁠ , where h is the disc aspect ratio. For a planet with mass ratio of |${\sim} 10^{-5}$|⁠ , the zone of outward migration sandwiched between inner and outer inward migration zones corresponds to |$\sim$| 10–100 au in a PPD with accretion rates between |$10^{-8}$| and |$10^{-7}\, \mathrm{ M}_\odot \text{yr}^{-1}$|⁠. [ABSTRACT FROM AUTHOR]

This review article summarizes two decades of laboratory research aimed at understanding the dynamics of accretion disks, with particular emphasis on magnetohydrodynamic experiments involving liquid metals and plasmas. First, the Taylor–Couette experiments demonstrated the generation of magnetorotational instability (MRI) in liquid metals, and highlighted how this instability is critically influenced by boundary conditions and the geometry of the applied magnetic field. These experiments also highlight the nonlinear transition to turbulence in accretion disks, and their link with other MHD instabilities in centrifugally-stable flows. A complementary approach, involving laboratory experiments with volumetric fluid driving rather than rotating boundaries, enables a quantitative study of angular momentum transport by Keplerian turbulence. Collectively, these various laboratory studies offer new constraints on the theoretical models designed to explain the dynamics of accretion disks. This is particularly true with regard to the role of Keplerian turbulence in protoplanetary disks, where recent observations from the ALMA telescope have considerably revised previously expected values of the magnitude of the turbulent fluctuations. Finally, the paper discusses outstanding questions and future prospects in laboratory modeling of accretion disks.

We study the variability of the thermal (accretion disc) and non-thermal (jet) emission of thirteen flat spectrum radio quasars in the optical and near infrared (OIR) regime using light curves spanning years with an average sampling of three observations per week. We fit a combination of a blackbody and a power-law function to the OIR data, in the blazar rest frame, to extract the corresponding thermal (disc) and non-thermal (jet) components from the total flux. We carry out this analysis for the entire duration of the light curves to obtain the variation of the disc and jet components over years. Reliability of our fits have been affirmed by successfully retrieving accurate parameters by employing our method to simulated data and by comparing our results with published disc luminosity obtained by other methods for a few well-observed blazars. In blazars, the thermal (disc) emission is difficult to extract because the relativistically beamed radiation of the jet dominates at all wavelengths. By employing this method, the disc emission in blazars may be estimated directly from photometric data at OIR bands instead of indirect methods, such as, inferring it from the emission line luminosities. We find that the variability of the disc and jet emission obtained by the above method are strongly correlated in most cases. [ABSTRACT FROM AUTHOR]

VY Scl binaries are a sub-class of cataclysmic variable (CV) which show extended low states, but do not show outbursts which are seen in other classes of CV. To better determine how often these systems spend in low states and to resolve the state transitions we have analysed Zwicky Transient Facility (ZTF) data on eight systems and Transiting Exoplanet Survey Satellite (TESS) data on six systems. Half of the sample spent most of the time in a high state; three show a broad range and one spends roughly half the time transitioning between high and low states. Using the ZTF data, we explore the colour variation as a function of brightness. In KR Aur, we identify a series of repeating outburst events whose brightness appears to increase over time. Using TESS data, we searched for periods other than the orbital. In LN UMa, we find evidence for a peak whose period varies between 3 and 6 d. We outline the current models which aim to explain the observed properties of VY Scl systems which includes disc irradiation and a white dwarf having a significant magnetic field. [ABSTRACT FROM AUTHOR]

'Z' type neutron star low-mass X-ray binaries typically show a 'Z'-like three-branched track in their hardness intensity diagram. However, a few such 'Z' sources show an additional branch known as the extended flaring branch (EFB). EFB has been poorly studied, and its origin is not known. It is thought to be an extension of the flaring branch (FB) or associated with Fe K  |$\alpha$| complex or an additional continuum due to the radiative recombination continuum (RRC) process. Using AstroSat observations, we have detected the EFB from two 'Z' sources, GX 340+0 and GX 5–1, and performed a broad-band spectral analysis in the 0.5–22 keV energy range. During EFB, both sources show the presence of a significant RRC component with absorption edges at |$7.91^{+0.16}_{-0.15}$| and |$8.10^{+0.16}_{-0.17}$|  keV, respectively along with blackbody radiation and thermal Comptonization. No signature of RRC was detected during the FB, which is adjoint to the EFB. No Fe K  |$\alpha$| complex is detected. Interestingly, inside EFB dips of GX 5-1, for the first time, we have detected flaring events of 30–60 s, which can be modelled with a single blackbody radiation. During the FB to EFB transition, an increase in the blackbody radius by a factor of 1.5–2 is observed in both sources. Our analysis strongly suggests that EFB is not an extension of FB or caused by the Fe K  |$\alpha$| complex. Rather, it is caused by a sudden expansion of the hot, thermalized boundary layer and subsequent rapid cooling. [ABSTRACT FROM AUTHOR]

Many accretion discs have been found to be distorted: either warped due to a misalignment in the system, or non-circular as a result of orbital eccentricity or tidal deformation by a binary companion. Warped, eccentric, and tidally distorted discs are not in vertical hydrostatic equilibrium, and thus exhibit vertical oscillations in the direction perpendicular to the disc, a phenomenon that is absent in circular and flat discs. In extreme cases, this vertical motion is manifested as a vertical 'bouncing' of the gas, potentially leading to shocks and heating, as observed in recent global numerical simulations. In this paper, we isolate the mechanics of vertical disc oscillations by means of quasi-2D and fully 3D hydrodynamic local (shearing-box) models. To determine the numerical and physical dissipation mechanisms at work during an oscillation we start by investigating unforced oscillations, examining the effect of initial oscillation amplitude, as well as resolution, boundary conditions, and vertical box size on the dissipation and energetics of the oscillations. We then drive the oscillations by introducing a time-dependent gravitational potential. A key result is that even a purely vertically oscillating disc is (parametrically) unstable to developing inertial waves, as we confirm through a linear stability analysis. The most important of these has the character of a bending wave, whose radial wavelength depends on the frequency of the vertical oscillation. The nonlinear phase of the instability exhibits shocks, which dampen the oscillations, although energy can also flow from the bending wave back to the vertical oscillation. [ABSTRACT FROM AUTHOR]

The transient Galactic black hole candidate Swift J1727.8-1613 went through an outburst for the very first time in 2023 August and lasted for almost 6 months. We study the timing and spectral properties of this source using publicly available archival Insight-HXMT data for the first 10 observation IDs that last from MJD 60181 to 60198 with a total of 92 exposures for each of the three energy bands. We have detected quasi-periodic oscillations (QPOs) in a frequency range of 0.21 ± 0.01–1.86 ± 0.01 Hz by fitting the power density spectrum. Based on the model-fitted parameters and properties of the QPOs, we classify them as type C in nature. We also conclude that the origin of the QPOs could be the shock instabilities in the transonic advective accretion flows around black holes. The spectral analysis was performed using simultaneous data from the three onboard instruments LE, ME, and HE of Insight-HXMT in the broad energy band of 2−150 keV. To achieve the best fit, spectral fitting required a combination of models, e.g., interstellar absorption, power-law, multicolor disk–blackbody continuum, Gaussian emission/absorption, and reflection by neutral material. From the spectral properties, we found that the source was in an intermediate state at the start of the analysis period and was transitioning to the softer states. The inner edge of the accretion disk moved inward in progressive days following the spectral nature. We found that the source has a high inclination of 78°−86°. The hydrogen column density from the model fitting varied in the range of (0.12 ± 0.02−0.39 ± 0.08) × 1022 cm−2. [ABSTRACT FROM AUTHOR]

Multi-epoch Chandra and XMM–Newton observations of the symbiotic system R Aquarii (R Aqr) spanning 22 yr are analysed by means of a reflection model produced by an accretion disc. This methodology helps dissecting the contribution from different components in the X-ray spectra of R Aqr: the soft emission from the jet and extended emission, the heavily extinguished plasma component of the boundary layer and the reflection contribution, which naturally includes the 6.4 keV Fe fluorescent line. The evolution with time of the different components is studied for epochs between September 2000 and December 2022, and it is found that the fluxes of the boundary layer and that of the reflecting component increase as the stellar components in R Aqr approach periastron passage, a similar behaviour is exhibited by the shocked plasma produced by the precessing jet. Using publicly available optical and UV data we are able to study the evolution of the mass-accretion rate |$\dot{M}_\mathrm{acc}$| and the wind accretion efficiency |$\eta$| during periastron. These exhibit a small degree of variability with median values of |$\dot{M}_\mathrm{acc}$| =7.3 |$\times 10^{-10}$|  M |$_\odot$|  yr |$^{-1}$| and |$\eta$| =7 |$\times 10^{-3}$|⁠. We compare our estimations with predictions from a modified Bondi–Hoyle–Lyttleton accretion scenario. [ABSTRACT FROM AUTHOR]

Changing-look active galactic nuclei (CLAGNs) are characterized by extreme variations in line emission over short time-scales, mostly affecting broad H  |$\beta$| lines. While a few hundred CLAGNs are known, a complete sample of turn-on CLAGNs is still elusive. Here, we present a search for turn-on CLAGNs in a complete sample of galaxies, using archival spectra and recent light curves. We obtained light curves from the Asteroid Terrestrial Impact Last Alert System for 16 232 emission-line galaxies, including both star-forming and active galaxies, at |$z< 0.1$| with spectra from the Six-degree Field Galaxy Survey (6dFGS). We first establish typical variability behaviour for different active galactic nucleus (AGN) types, as recorded between 2001 and 2009, and then select outliers from the bulk behaviour as CLAGN candidates. We obtain new spectra for the candidates and identify 12 new turn-on CLAGNs (appearing broad H  |$\beta$| line) and 19 new turn-off CLAGNs (disappearing broad H  |$\beta$| line). We may have missed AGNs that changed and reverted their state over the 15–20 yr since 6dFGS spectra were taken, and thus our CLAGN rates of 1.7 per cent for turn-on and 9.6 per cent for turn-off are lower limits. The turn-on rate is naturally much lower as the type 1.9/2 sample is dominated by obscured AGNs due to orientation, which is not expected to change. However, the number of turn-on (27) and turn-off (24) CLAGNs we find are similar, suggesting that our parent AGN sample is reasonably complete in our search volume at |$z < 0.1$|⁠. [ABSTRACT FROM AUTHOR]

MAXI J1803-298 is a transient black hole candidate discovered in 2021 May during an outburst that lasted several months. Multiple X-ray observations reveal recurring "dipping" intervals in several of its light curves, particularly during the hard/intermediate states, with a typical recurrence period of ∼7 hr. We report analysis of four NuSTAR observations of the source, supplemented with NICER data where available, over the duration of the outburst evolution covering the hard, intermediate, and the soft states. Reflection spectroscopy reveals the black hole to be rapidly spinning (a * = 0.990 ± 0.001) with a near edge-on viewing angle (i = 70° ± 1°). Additionally, we show that the light-curve dips are caused by photoelectric absorption from a moderately ionized absorber whose origin is not fully understood, although it is likely linked to material from the companion star impacting the outer edges of the accretion disk. We further detect absorption lines in some of the spectra, potentially associated with Fe xxv and Fe xxvi, indicative of disk winds with moderate to extreme velocities. During the intermediate state and just before transitioning into the soft state, the source showed a sudden flux increase, which we found to be dominated by soft disk photons and consistent with the filling of the inner accretion disk, at the onset of state transition. In the soft state, we show that models of disk self-irradiation provide a better fit and a preferred explanation to the broadband reflection spectrum, consistent with previous studies of other accreting sources. [ABSTRACT FROM AUTHOR]

Although ultraluminous X-ray pulsars (ULXPs) are believed to be powered by super-Eddington accretion onto a magnetized neutron star (NS), the detailed structures of the inflow–outflow and magnetic fields are still not well understood. We perform general relativistic radiation magnetohydrodynamics (GR-RMHD) simulations of super-Eddington accretion flows onto a magnetized NS with dipole and/or quadrupole magnetic fields. Our results show that an accretion disk and optically thick outflows form outside the magnetospheric radius, while inflows aligned with magnetic field lines appear inside. When the dipole field is more prominent than the quadrupole field at the magnetospheric radius, accretion columns form near the magnetic poles, whereas a quadrupole magnetic field stronger than the dipole field results in the formation of a belt-like accretion flow near the equatorial plane. The NS spins up as the angular momentum of the accreting gas is converted into the angular momentum of the electromagnetic field, which then flows into the NS. Even if an accretion column forms near one of the magnetic poles, the observed luminosity is almost the same on both the side with the accretion column and the side without it, because the radiation energy is transported to both sides through scattering. Our model suggests that galactic ULXP Swift J0243.6+6124 has a quadrupole magnetic field of 2 × 1013 G and a dipole magnetic field of less than 4 × 1012 G. [ABSTRACT FROM AUTHOR]

Accreting X-ray pulsars, located in X-ray binaries, are neutron stars with magnetic fields as strong as B ∼ 10 12 – 13 G. This review offers a concise overview of the accretion and radiation processes of X-ray pulsars and summarizes their rich observational features, particularly focusing on complex and variable temporal phenomena, spectral properties, and evolution, the new window for X-ray polarimetry and multi-wavelength advances. We also briefly discuss other related systems, i.e., gamma-ray binaries and pulsating ultraluminous X-ray sources. [ABSTRACT FROM AUTHOR]

This is a tutorial on the strong gravity effects (motion of massive and massless particles in a curved spacetime, evaluation of redshift factors, estimate of physical quantities in different reference frames, etc.) necessary to calculate the electromagnetic spectra of geometrically thin and optically thick accretion disks around black holes. The presentation is intentionally pedagogical, and most calculations are reported step by step. In the disk–corona model, the spectrum of a source has three components: a thermal component from the disk, a Comptonized component from the corona, and a reflection component from the disk. This tutorial reviews only the strong gravity effects, which can be decoupled from the physical processes involving the interaction between matter and radiation. The formulas presented here are valid for stationary, axisymmetric, asymptotically flat, circular spacetimes, so they can be potentially used for a large class of black hole solutions. [ABSTRACT FROM AUTHOR]

Supermassive black holes (SMBHs) are observed in active galactic nuclei interacting with their environments, where chaotical, discontinuous accretion episodes may leave matter remnants orbiting the central attractor in the form of sequences of orbiting toroidal structures, with strongly different features as different rotation orientations with respect to the central Kerr BH. Such ringed structures can be characterized by peculiar internal dynamics, where co-rotating and counter-rotating accretion stages can be mixed and distinguished by tori interaction, drying–feeding processes, screening effects, and inter-disk jet emission. A ringed accretion disk (RAD) is a full general relativistic model of a cluster of toroidal disks, an aggregate of axi-symmetric co-rotating and counter-rotating disks orbiting in the equatorial plane of a single central Kerr SMBH. In this work, we discuss the time evolution of a ringed disk. Our analysis is a detailed numerical study of the evolving RAD properties formed by relativistic thin disks, using a thin disk model and solving a diffusion-like evolution equation for an RAD in the Kerr spacetime, adopting an initial wavy (ringed) density profile. The RAD reaches a single-disk phase, building accretion to the inner edge regulated by the inner edge boundary conditions. The mass flux, the radial drift, and the disk mass of the ringed disk are evaluated and compared to each of its disk components. During early inter-disk interaction, the ring components spread, destroying the internal ringed structure and quickly forming a single disk with timescales governed by ring viscosity prescriptions. Different viscosities and boundary conditions have been tested. We propose that a system of viscously spreading accretion rings can originate as a product of tidal disruption of a multiple stellar system that comes too close to an SMBH. [ABSTRACT FROM AUTHOR]

Young stellar objects (YSOs) are protostars that exhibit bipolar outflows fed by accretion disks. Theories of the transition between disk and outflow often involve a complex magnetic field structure thought to be created by the disk coiling field lines at the jet base; however, due to limited resolution, these theories cannot be confirmed with observation and thus may benefit from laboratory astrophysics studies. We create a dynamically similar laboratory system by driving a $\sim$ 1 MA current pulse with a 200 ns rise through a $\approx$ 2 mm-tall Al cylindrical wire array mounted to a three-dimensional (3-D)-printed, stainless steel scaffolding. This system creates a plasma that converges on the centre axis and ejects cm-scale bipolar outflows. Depending on the chosen 3-D-printed load path, the system may be designed to push the ablated plasma flow radially inwards or off-axis to make rotation. In this paper, we present results from the simplest iteration of the load which generates radially converging streams that launch non-rotating jets. The temperature, velocity and density of the radial inflows and axial outflows are characterized using interferometry, gated optical and ultraviolet imaging, and Thomson scattering diagnostics. We show that experimental measurements of the Reynolds number and sonic Mach number in three different stages of the experiment scale favourably to the observed properties of YSO jets with $Re\sim 10^5\unicode{x2013}10^9$ and $M\sim 1\unicode{x2013}10$ , while our magnetic Reynolds number of $Re_M\sim 1\unicode{x2013}15$ indicates that the magnetic field diffuses out of our plasma over multiple hydrodynamical time scales. We compare our results with 3-D numerical simulations in the PERSEUS extended magnetohydrodynamics code. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]