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The Be/X-ray binary pulsar RX J0440.9+4431 went through a giant outburst in December 2022 with a peak flux of |$\sim$| 2.3 Crab in 15–50 keV. We studied the broad-band timing and spectral properties of RX J0440.9+4431 using four AstroSat observations, where the source transited between subcritical and supercritical accretion regimes. Pulsations were detected significantly above 100 keV. The pulse profiles were found to be highly luminosity- and energy-dependent. A significant evolution in the pulse profile shape near the peak of the outburst indicates a possible change in the accretion mode and beaming patterns of RX J0440.9+4431. The rms pulsed fraction was luminosity- and energy-dependent, with a concave-like feature around 20–30 keV. The depth of this feature varied with luminosity, indicating changes in the accretion column height and proportion of reflected photons. The broad-band continuum spectra were best fitted with a two-component Comptonization model with a blackbody component or a two-blackbody component model with a thermal Comptonization component. A quasi-periodic oscillation (QPO) at 60 mHz was detected at a luminosity of |$2.6 \times 10^{37}$|  erg s |$^{-1}$|⁠ , which evolved into 42 mHz at |$1.5 \times 10^{37}$|  erg s |$^{-1}$|⁠. The QPO rms were found to be energy dependent with an overall increasing trend with energy. For the first time, we found the QPO frequency varying with photon energy in an X-ray pulsar, which poses a challenge in explaining the QPO with current models such as the Keplarian and beat frequency model. Hence, more physically motivated models are required to understand the physical mechanism behind the mHz QPOs. [ABSTRACT FROM AUTHOR]

We analytically and numerically study the hydrodynamic propagation of a precessing jet in the context of tidal disruption events (TDEs) where the star's angular momentum is misaligned with the black hole spin. We assume that a geometrically thick accretion disc undergoes Lense–Thirring precession around the black hole spin axis and that the jet is aligned with the instantaneous disc angular momentum. At large spin-orbit misalignment angles |$\theta _{\it LS}$|⁠ , the duty cycle along a given angle that the jet sweeps across is much smaller than unity. The faster jet and slower disc wind alternately fill a given angular region, which leads to strong shock interactions between the two. We show that precessing jets can only break out of the wind confinement if |$\theta _{\it LS}$| is less than a few times the jet opening angle |$\theta _{\rm j}$|⁠. The very small event rate of observed jetted TDEs is then explained by the condition of double alignment : both the stellar angular momentum and the observer's line of sight are nearly aligned with the black hole spin. In most TDEs with |$\theta _{\it LS}\gg \theta _{\rm j}$|⁠ , the jets are initially choked by the disc wind and may only break out later when the disc eventually aligns itself with the spin axis due to the viscous damping of the precession. Such late-time jets may produce delayed radio rebrightening as seen in many optically bright TDEs. Our model is also applicable to jets associated with (stellar mass) black hole-neutron star mergers where the black hole's spin is misaligned with the orbital angular momentum. [ABSTRACT FROM AUTHOR]

We provided a comprehensive study of the properties of the black hole in the low-mass X-ray binary system 4U 1543-47, specifically focusing on the 2021 outburst (MJD 59380–59470). Using observations from the Insight - HXMT mission, we employed X-ray reflection fitting method and analysed spectral data to estimate key black hole parameters. Through our investigation redbased on 6 out of the 52 available observations, we estimated the spin parameter of the black hole to be |$0.902_{-0.053}^{+0.054}$| and the inclination angle of the accretion disc to be |$28.91_{-1.24}^{+1.82}$| ° (90 per cent confidence limits, statistical only), then we discussed the influence of high luminosity. Based on the relxill series models are not suitable for thick disc scenario, and in comparison with findings from other studies, we propose that our estimation of the spin value may be exaggerated. [ABSTRACT FROM AUTHOR]

Binary black hole (BBH) evolution in the discs of active galactic nuclei (AGNs) is a promising channel for gravitational wave (GW)-driven mergers. It is, however, unclear whether binaries interacting with the surrounding disc undergo orbital contraction or expansion. We develop a simple analytical model of accreting BBHs in AGN discs to follow the orbital evolution from the disc-dominated regime at large separations into the GW-driven regime at small separations (the coupled 'disc + GW'-driven evolution). We obtain that accreting binaries expand in thick discs with aspect ratio greater than a critical value (> h crit); whereas accreting binaries contract and eventually merge in thin discs (< h crit). Interestingly, accreting BBHs can experience faster mergers compared to non-accreting counterparts, with a non-monotonic dependence on the disc aspect ratio. The orbital contraction is usually coupled with eccentricity growth in the disc-dominated regime, which lead to accelerated inspirals in the GW-driven regime. We quantify the resulting BBH merger time-scales in AGN discs (τmerger ∼ 105–107 yr) and estimate the associated GW merger rates (⁠|$\mathcal {R} \sim (0.2 {\small --} 5) \, \text{Gpc}^{-3} \text{yr}^{-1}$|⁠). Overall, accreting binaries may efficiently contract and merge in thin discs, hence this particular BBH-in-AGN channel may provide a non-negligible contribution to the observed GW merger event rate. [ABSTRACT FROM AUTHOR]

We present a comprehensive timing analysis on the Be/X-ray binary pulsar RX J0440.9+4431 using observations from NICER and Insight - HXMT during the 2022–2023 outburst. The power density spectrum (PDS) of RX J0440.9+4431 exhibits typical aperiodic variability in X-ray flux across a wide frequency range. During a supercritical accretion state, we detect quasi-periodic oscillations (QPOs) at 0.2–0.5 Hz in the light curves of five pulses for RX J0440.9+4431. The observed QPOs manifest during flares, while the flares appear at the peaks of the pulse profiles on a time-scale of seconds and are primarily caused by an increase in hard photons. These flares can be explained by increased material ingestion in the accretion column at a fixed phase, primarily generating hard photons. Alternatively, an increase in accretion rate, independent of phase, may result in highly beamed hard photons within the accretion column, causing the flares. We argue the origin of QPOs to instabilities within the accretion flow. Additionally, we find that the break frequencies in the noise power spectra align well with |$\propto L_{\mathrm{x}}^{3 / 7}$| across three orders of magnitude in the luminosity, which points to a relatively strong magnetic field in RX J0440.9+4431, estimated to be ~1013 G. [ABSTRACT FROM AUTHOR]

At present, there is little doubt that accretion discs surrounding compact astrophysical objects such as black holes, white dwarfs, and neutron stars may have magnetic field structures. Thus, they explain the transfer of angular momentum between different parts of the disc and some other processes. There are various ways to explain the occurrence of these magnetic fields. In this paper, we study the possibility of generation of magnetic fields due to the Biermann battery mechanism. It is associated with radial flows of protons and electrons. Due to their different masses, they interact differently with the rotating medium, producing circular currents that generate magnetic fields. Previously, a similar process was studied for galactic discs and it was shown that the battery mechanism can generate initial magnetic fields in such objects. Here, we discuss the action of the Biermann battery for accretion disks. This requires solving an integral equation of the second kind, which arises if we take into account the self-interaction of the magnetic field. It is shown that co-rresponding fields are quite significant and can play an important role in the evolution of magnetic fields in discs. [ABSTRACT FROM AUTHOR]

We present new analytical solutions for the evolution of protoplanetary discs (PPDs) where magnetohydrodynamic (MHD) wind-driven processes dominate. Our study uses a 1D model which incorporates equations detailing angular momentum extraction by MHD winds and mass-loss rates. Our solutions demonstrate that the disc retains its initial state during the early phases; however, it rapidly evolves towards a self-similar state in the later stages of disc evolution. The total disc mass undergoes a continuous decline over time, with a particularly rapid reduction occurring beyond a certain critical time threshold. This gradual decrease in mass is influenced by the wind parameters and the initial surface density of the disc. In the MHD wind-dominated regime, we show that the disc's lifespan correlates positively with the magnetic lever arm up to a certain threshold, irrespective of the initial disc size. PPDs with a larger magnetic lever arm are found to maintain significantly higher total disc mass over extended periods compared to their counterparts. The mass ejection-to-accretion ratio increases in efficient wind scenarios and is further amplified by a steeper initial surface density profile. Our analysis also reveals varied evolutionary trajectories in the plane of accretion rate and total disc mass, influenced by magnetic parameters and initial disc size. In scenarios with efficient MHD winds, discs with bigger sizes have extended operation time for mechanisms governing planet formation. [ABSTRACT FROM AUTHOR]

We studied the polarization properties of Cygnus X–1 in both hard and soft spectral states with imaging X-ray polarimetry explorer (IXPE) observations. The polarization degree is |$\sim 4~{{\ \rm per\ cent}}$|⁠ , and |$\sim 2.6-2.8~{{\ \rm per\ cent}}$| in the hard and soft states, respectively. The polarization angle is observed along the jet axis and remains the same in both states. Energy-dependent analysis revealed an increasing polarization with the energy. The observed polarization rules out the lamp-post corona and prefers a conical corona if the coronal geometry remains the same in the hard and soft states. The disc is not detected in the hard state but is found in the soft state in the IXPE spectra. The spectral analysis revealed that the disc emission contributes |$\sim 20~{{\ \rm per\ cent}}$| of the total emission in the soft state. The scattering of the seed photons inside the corona is likely to reduce the polarization. This could explain the different polarization in different spectral states. Additionally, if the disc polarization is perpendicular to that of the corona, it could also reduce the polarization in the soft state. [ABSTRACT FROM AUTHOR]

We report a comprehensive spectro-polarimetric study of the black hole binary LMC X-3 using simultaneous Imaging X-ray Polarimetry Explorer (IXPE), NICER , and NuSTAR observations in 0.5–20 keV energy band. The broad-band energy spectrum (0.5–20 keV) with NICER and NuSTAR is well described by the disc emission of temperature ∼1.1 keV and a weak Comptonizing tail beyond ∼10 keV. This evidently suggests a disc-dominated spectral state of the source with disc contribution of ∼96  per cent. The lack of variability (rms ∼ 0.5  per cent) in the power spectrum further corroborates the high/soft nature of the source. A significant polarization degree (PD) of 3.04 ± 0.40  per cent (>7σ) at a polarization angle of −44.24° ± 3.77° (>7σ) is found in 2–8 keV energy range of IXPE. In addition, PD is seen to increase with energy up to ∼4.35 ± 0.98  per cent (>3σ) in 4–8 keV band. Further, we attempt to constrain the source spin (a *) using broad-band spectral modelling that indicates a weakly rotating 'hole' in LMC X-3 with |$a_{*}=0.273_{-0.012}^{+0.011}\,\text{to}\,0.295_{-0.021}^{+0.008}$| (90  per cent confidence). Based on the spectro-polarimetric results, we infer that the polarization in LMC X-3 is resulted possibly due to the combined effects of the direct and/or reflected emissions from a partially ionized disc atmosphere. Finally, we discuss the relevance of our findings. [ABSTRACT FROM AUTHOR]

RX J0440.9+4431, a Be/X-ray binary, had its brightest outburst in 2022 since its discovery, with a peak X-ray flux of 2.25 Crab (as recorded by Swift /BAT, 15–50 keV). We analyze the timing properties of this giant outburst using data from Insight -Hard X-ray Modulation Telescope and Neutron Star Interior Composition Exlporer , focusing on the evolution of the pulse profile and pulse fraction. We observe that when the luminosity reached around ~ |$3\times 10^{37}\ {\rm erg\ \rm s^{-1}}$|⁠ , a transition from double-peaked to single-peaked pulse profiles occurred across the energy range, with the peak of the low-energy profile aligning gradually with the peak of the high-energy profile. This change indicates a transition from subcritical to supercritical accretion. Additionally, we found a concave in the pulse fraction as a function of energy around 20–30 keV throughout the entire outburst period. Compared to the low luminosity, the concave becomes weaker in high luminosities, and overall, the pulse fraction is higher. We propose that this concave could be caused by the scattering of high-energy photons by the atmosphere of a neutron star, leading to a dilution of the pulse fraction. As the accretion reaches the supercritical state, the accretion column height increases, resulting in a larger direct component of strongly beamed X-ray flux, and an elevated pulse fraction. [ABSTRACT FROM AUTHOR]

We present a comprehensive spectrotemporal analysis of five ultraluminous X-ray sources (ULXs) with central object likely being a black hole, using archival XMM–Newton observations. These sources, namely NGC1313 X−1, NGC5408 X-1, NGC6946 X−1, M82 X−1, and IC342 X−1, reveal short-term variability with fractional variance of 1.42 |$-27.28~{{\ \rm per\ cent}}\, $| and exhibit Quasi-periodic Oscillations (QPOs) with frequency νQPO ∼ 8−667 mHz. Long-term evolution of ULXs energy spectra (0.3–10 keV; excluding M82 X−1) are described satisfactorily with a model combination that comprises a thermal Comptonization component (nthComp , yielding Γnth ∼ 1.48–2.65, kT e ∼ 1.62–3.76 keV, τ ∼ 8–20, y-par∼1.16–6.24) along with a standard disc component (diskbb , kT in ∼ 0.16–0.54 keV). We find that these ULXs generally demonstrate anticorrelation between disc luminosity and inner disc temperature as |$L_{\rm disc} \propto T_{\rm in}^\alpha$|⁠ , where α = −3.58 ± 0.04 for NGC1313 X−1 and IC342 X−1, α = −8.93 ± 0.11 for NGC6946 X−1, and α = −10.31 ± 0.10 for NGC5408 X−1. We also obtain a linear correlation between bolometric luminosity L bol and Γnth that indicates spectral softening of the sources when L bol increases. We observe that in presence of QPO, Comptonized seed photon fraction varies in between |$\sim 5-20~{{\ \rm per\ cent}}\, $|⁠ , while the Comptonized flux contribution (50 |$-90~{{\ \rm per\ cent}}\, $|⁠) dominates over disc flux. Utilizing νQPO and L bol, we constrain ULXs mass by varying their spin (a k) and accretion rate (⁠|$\dot{m}$|⁠). We find that NGC6946 X−1 and NGC5408 X−1 seem to accrete at sub-Eddington accretion rate provided their central sources are rapidly rotating, whereas IC342 X−1 and NGC1313 X−1 can accrete in sub/super-Eddington limit irrespective to their spin values. [ABSTRACT FROM AUTHOR]

We investigate the effects of the magnetic diffusivity on the tears mode instability during the process of magnetic reconnection in the accretion flow around Sgr A* via 2D simulation. It is believed that the magnetic diffusivity plays an important role during the magnetic reconnection, so the temperature-dependent diffusivity ηTD is applied in this work. For comparison, the case with constant diffusivity ηC is also studied. In our simulations, there are many plasmoids formed due to the magnetic reconnection, and these plasmoids consequently merge many times. It is found that the temperature-dependent diffusivity will cause more frequent merger of the plasmoids. Because of the turbulence of the current sheet, the temperature distribution is non-uniform, so at the secondary X-points with the different temperature, a lot of plasmoids form and merge to become larger plasmoids. Then the larger plasmoids merge to become a huge plasmoid. In the case of the constant magnetic diffusion, the plasmoid merge less frequently than in the case of the temperature-dependent diffusivity. The huge plasmoid forms and then moves up from the current sheet in both cases. In the case with the temperature-dependent diffusivity, the huge plasmoids oscillate and deform for a long time. This phenomenon is not obvious in the case of the constant diffusivity; in this case the huge plasmoids form and then move out from the upper boundary of the simulation area without oscillation and deformation. [ABSTRACT FROM AUTHOR]

MAXI J1816–195 is a newly discovered accreting millisecond X-ray pulsar that went outburst in 2022 June. Through timing analysis with Neutron star Interior Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations, we find a transient modulation at ~2.5 Hz during the decay period of MAXI J1816–195. The modulation is strongly correlated with a spectral hardening, and its fractional rms amplitude increases with energy. These results suggest that the modulation is likely to be produced in an unstable corona. In addition, the presence of the modulation during thermonuclear bursts indicates that it may originate from a disc-corona where the optical depth is likely the main factor affecting the modulation, rather than temperature. Moreover, we find significant reflection features in the spectra observed simultaneously by NICER and NuSTAR , including a relativistically broadened Fe-K line around 6–7 keV, and a Compton hump in the 10–30 keV energy band. The radius of the inner disc is constrained to be R in  = (1.04–1.23)  R ISCO based on reflection modeling of the broad-band spectra. Assuming that the inner disc is truncated at the magnetosphere radius, we estimate that the magnetic field strength is |$\le 4.67 \times 10^{8}\, \rm G$|⁠. [ABSTRACT FROM AUTHOR]

Hard X-rays from atoll sources either can be emitted from the compact corona or the boundary layer around the neutron star surface. For the first time, we report the lags of the order of a few hundred seconds between soft and hard energy bands observed in an atoll source 4U 1728-34 using the AstroSat LAXPC data. We fitted the spectra with various models and found that the source was in the soft banana state during the observations and the inner disc radius was found to be ∼16 km. Based on a thermal Comptonization model, the electron temperature was found to be low (∼3 keV) with an index of Γ = 1.82 , indicating that the corona was compact in nature. We noted that the boundary layer emission was relatively more than the disc emission component. Based on the reflection model, we noted that the inner disk front was at R in ∼ 1.45 R ISCO with an electron temperature ∼2.72 keV. Based on the observed lags, the coronal height was estimated to be around 3-20 km assuming that the disc is close to the ISCO. We noted that the detected lags are similar to the depletion time scale of the boundary layer. These results indicate that the hard X-ray emission can arrive from the corona or boundary layer and the observed lags are due to readjustment time scales of either or both the physical structures. [ABSTRACT FROM AUTHOR]

We examine the effect of thermal conduction on the low-angular momentum hot accretion flow (HAF) around non-rotating black holes accreting mass at very low rate. While doing so, we adopt the conductive heat flux in the saturated form, and solve the set of dynamical equations corresponding to a steady, axisymmetric, viscous, advective accretion flow using numerical methods. We study the dynamical and thermodynamical properties of accreting matter in terms of the input parameters, namely energy (ε0), angular momentum (ℓ0), viscosity parameter (α), and saturation constant (Φs) regulating the effect of thermal conduction. We find that Φs plays a pivotal role in deciding the transonic properties of the global accretion solutions. In general, when Φs is increased, the critical point (r c) is receded away from the black hole, and flow variables are altered particularly in the outer part of the disc. To quantify the physically acceptable range of Φs, we compare the global transonic solutions with the self-similar solutions, and observe that the maximum saturation constant (⁠|$\Phi ^{\rm max}_{\rm s}$|⁠) estimated from the global solutions exceeds the saturated thermal conduction limit (Φsc) derived from the self-similar formalism. Moreover, we calculate the correlation between α and |$\Phi ^{\rm max}_{\rm s}$| and find ample disagreement between global solutions and self-similar solutions. Further, using the global flow variables, we compute the Bernoulli parameter (Be) which remains positive all throughout the disc, although flow becomes loosely unbound for higher Φs. Finally, we indicate the relevance of this work in the astrophysical context in explaining the possibility of mass-loss/outflows from the unbound disc. [ABSTRACT FROM AUTHOR]

To study time-dependent phenomena of plasma surrounding a non-rotating black hole with a dipolar magnetic field, we have developed a fully set of 3 + 1 formalism of generalized general relativistic magnetohydrodynamic equations. The general relativistic phenomena, in particular, have been investigated with respect to the Ohm law. Magnetofluid is supposed to flow in three directions and forms a thick disc structure around the central black hole. All physical quantities of the system are functions of three variables: radial distance r , polar angle θ, and time t. The radial, meridional, and time behaviours of all these quantities have been investigated. It has been shown that the electrical conductivity of the fluid is not constant and may be both positive and negative depending on the values of some free parameters. The initially purely rotating non-magnetized plasma in the presence of an external magnetic field gives rise to an azimuthal current density and a charge density measured by the comoving observer. This current generates an electromagnetic field inside the disc which has both poloidal and toroidal components. The dipolar magnetic field lines of the central black hole is able to penetrate the plasma disc, due to the presence of a finite resistivity for the plasma. The accreting plasma pushes them outwards and makes them parallel to the rotation axis of the disc in the meridional plane. [ABSTRACT FROM AUTHOR]

We have performed a series of two-dimensional two-temperature general relativistic magnetohydrodynamic simulations of magnetized accretion flows initiated from tori with different sizes and poloidal magnetic loop polarities. In these two-temperature simulations, we trace the process of heating electrons through turbulence and reconnection, most of the time these electrons are trapped in plasmoids. We found that the accretion process strongly depends on the size of the magnetic loops. The accretion flows never reach the magnetically arrested (MAD) regime in small loop cases. Interaction between magnetic field with different polarities dissipates and decreases the efficiency of magnetorotational instability. The dependence on the wavelength of the loops places a lower limit on the loop size. In the large loop cases, after reaching a quasi-steady phase, a transition from standard and normal evolution flow to MAD flow is observed. The transition of the accretion state and the transition time depends on the initial loop wavelength. The formation of plasmoids strongly depends on the size of the magnetic loops. The frequent magnetic reconnection between the magnetic loops is responsible for the formation of most of the plasmoids. For some plasmoids, Kelvin–Helmholtz and tearing instabilities are coexisting, showing another channel of plasmoid formation. The simulations present that electrons in the plasmoids are well-heated up by turbulent and magnetic reconnection. Different properties of plasmoid formation in different magnetic field configurations provide new insights for the understanding of flaring activity and electron thermodynamics in Sagittarius A*. [ABSTRACT FROM AUTHOR]

We investigate the dynamics of clumps that coexisted with/in advection-dominated accretion flows (ADAFs) by considering thermal conductivity. Thermal conduction can be one of the effective factors in the energy transportation of ADAFs; hence it may indirectly affect the dynamics of clumps by means of a contact force between them and their host medium. We first study the ensemble of clumps by assuming them as collisionless particles, and secondly we find the orbital motion of these clouds as individuals. For both parts, clumps are subject to the gravity of the central object and a drag force. The strong coupling between clumps and ADAF leads to equality between the average treatment of the clumps and the dynamics of their background. By employing the collisionless Boltzmann equation, we calculate the velocity dispersion of the clumps, which turns out to be approximately one order of magnitude higher than the ADAF. In fact, involving drag force in such a system causes the angular momentum of the clumps to be transported outwards by the ADAF, and hence the clouds eventually will be captured at the tidal radius. The results show that the presence of thermal conduction increases the root of the averaged radial velocity square, and this, in turn, speeds up the process of capturing the clouds through the tidal force. In the end, we focus on a typical individual cloud; the spiral orbits appear only due to the toroidal component of friction force. The parametric study again proves that the operation of thermal conduction helps in decreasing the lifetime of clumps. [ABSTRACT FROM AUTHOR]

In this study, we have considered time dependent evolution of advection dominated accretion flow (ADAF) in the presence of the toroidal magnetic field and radial viscosity. We have used time-dependent self-similar solutions for solving the 1D MHD equations in the spherical coordinates in the equatorial plane () and we have neglected terms with any θ and φ dependence. While the azimuthal viscosity υ as the turbulence factor in transporting the angular momentum and α-prescription for kinematic coefficient of viscosity is used in the most previous studies, recent studies show the disc structure can also be affected by the radial viscosity υr. In this work, we have assumed that the ratio is a dimensionless parameter 𝜉. We use ξ and β variables as free parameters to consider the effects of magnetic field and radial viscosity. The solutions indicate a transonic point in the accretion flow. This point approaches to outward by increasing the magnetic field and radial viscosity. Also, by adding strength of the magnetic field, the radial-velocity of the disc decreases and the disc compresses. Also, the flow is sub-Keplerian at all radii. The 𝜉 parameter has the same behavior in the inner and intermediate regions of the flow but in the outward of the flow, by adding the 𝜉 parameter, accretion rate increases and hence, it is expected that the disc has a shorter lifetime with radial viscosity.

We report the cross-correlation function studies of a neutron star low-mass X-ray binary, a Z source GX 5-1, using Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) energy bands onboard AstroSat. For the first time, we report the lag between soft (0.8–2.0 keV, SXT) and hard X-ray energy bands (10–20 and 16–40 keV, LAXPC) in GX 5-1 and detected lags of the order of a few tens to hundreds of seconds in the horizontal branch. We interpreted them as the readjustment time-scale of the inner region of the accretion disc. We used various two components and three-component spectral models to unfold the spectra and observed the changes in soft and hard component fluxes that were exhibiting horizontal branch oscillation variations. It was observed that the bbody component assumed to be originating from the boundary layer over the NS and was also found to vary along with the HBO variation where lags were detected. We constrained the size of the comptonizing region of the order 15–55 km, assuming that lags were due to variation in the size of the corona. We noticed a similar size of the comptonizing region after employing other models and suggest that the overall size of corona must be of the order of a few tens of km to explain the lags, HBO variation, and respective spectral variations. In a case study, it was noted that the BL size increases as GX 5-1 vary from the top of the HB to the upper vertex. [ABSTRACT FROM AUTHOR]

In this paper, we explore the self-similarity time evolution of a hot accretion flow around a compact object in the presence of a toroidal magnetic field. We focus on a simplified model which is axisymmetric, rotating, unsteady viscous-resistive under an advection-dominated stage. In this work, we suppose that both the kinematic viscosity and the magnetic diffusivity to be a result of turbulence in the accretion flow. To describe such a flow, we apply magneto-hydrodynamics equations in spherical coordinates, (r , , φ) and adopt unsteady self-similar solutions. By neglecting the latitudinal dependence of the flow, we obtain a set of one-dimensional differential equations governing the accretion system. In this research, we encounter two parameters related to the magnetic field; one of them is, β , defined as the ratio of the magnetic pressure to the gas pressure and the other one, Γ 0 applied in the magnetic diffusivity definition. Our results show that β is a function of position, and increases towards outer layers. On the other hand, we examine different strength of magnetic field by choosing different value of β 0 which is the value of β at the inner edge of disc. We see that both β 0 and Γ 0 have positive effect on growing the radial infall velocity but density and gas pressure decrease by larger values of these parameters. Moreover, the rotational velocity and temperature of accreting material reduce considerably under the influence of a stronger magnetic field. We also focus on the behavior of the mass accretion rate appearing as a descending function of position. Finally, our solutions confirm that the radial trend of the physical quantities in a dynamical accretion flow is different from the ones in a steady flow. However, the effect of various parameters on the physical quantities in our model is qualitatively consistent with similar steady models. [ABSTRACT FROM AUTHOR]

We present a revised analysis of the photometric reverberation mapping campaign of the narrow-line Seyfert 1 galaxy PKS 0558 − 504 carried out with the Swift Observatory during 2008–2010. Previously, Gliozzi et al. (2013) found using the Discrete Correlation Function (DCF) method that the short-wavelength continuum variations lagged behind variations at longer wavelengths, the opposite of the trend expected for thermal reprocessing of X-rays by the accretion disc, and they interpreted their results as evidence against the reprocessing model. We carried out new DCF measurements that demonstrate that the inverted lag-wavelength relationship found by Gliozzi et al. resulted from their having interchanged the order of the driving and responding light curves when measuring the lags. To determine the inter-band lags and uncertainties more accurately, we carried out new measurements with four independent methods. These give consistent results showing time delays increasing as a function of wavelength, as expected for the disc reprocessing scenario. The slope of the re-analysed delay spectrum appears to be roughly compatible with the predicted τ ∝ λ4/3 relationship for reprocessing by an optically thick and geometrically thin accretion disc, although the data points exhibit a large scatter about the fitted power-law trend. [ABSTRACT FROM AUTHOR]

We study the relativistic, inviscid, advective accretion flow around the black holes and investigate a key feature of the accretion flow, namely the shock waves. We observe that the shock-induced accretion solutions are prevalent and such solutions are commonly obtained for a wide range of the flow parameters, such as energy (⁠|${\cal E}$|⁠) and angular momentum (λ), around the black holes of spin value 0 ≤ a k < 1. When the shock is dissipative in nature, a part of the accretion energy is released through the upper and lower surfaces of the disc at the location of the shock transition. We find that the maximum accretion energies that can be extracted at the dissipative shock (⁠|$\Delta {\cal E}^{\rm max}$|⁠) are |$\sim 1{{\ \rm per\ cent}}$| and |$\sim 4.4{{\ \rm per\ cent}}$| for Schwarzschild black holes (a k → 0) and Kerr black holes (a k → 1), respectively. Using |$\Delta {\cal E}^{\rm max}$|⁠ , we compute the loss of kinetic power (equivalently shock luminosity, L shock) that is enabled to comply with the energy budget for generating jets/outflows from the jet base (i.e. post-shock flow). We compare L shock with the observed core radio luminosity (LR) of black hole sources for a wide mass range spanning 10 orders of magnitude with sub-Eddington accretion rate and perceive that the present formalism seems to be potentially viable to account LR of 16 Galactic black hole X-ray binaries (BH-XRBs) and 2176 active galactic nuclei. We further aim to address the core radio luminosity of intermediate-mass black hole (IMBH) sources and indicate that the present model formalism perhaps adequate to explain core radio emission of IMBH sources in the sub-Eddington accretion limit. [ABSTRACT FROM AUTHOR]

Gas-rich dusty circumstellar discs observed around young stellar objects are believed to be the birthplace of planets and planetary systems. Recent observations revealed that large-scale horseshoe-like brightness asymmetries are present in dozens of transitional protoplanetary discs. Theoretical studies suggest that these brightness asymmetries bf could be caused by large-scale anticyclonic vortices triggered by the Rossby wave Instability (RWI), which can be excited at the edges of the accretionally inactive region, the dead zone edge. Since vortices may play a key role in planet formation, investigating the conditions of the onset of RWI and the long-term evolution of vortices is inevitable. The aim of our work was to explore the effect of disc geometry (the vertical thickness of the disc), viscosity, the width of the transition region at the dead zone edge, and the disc mass on the onset, lifetime, strength and evolution of vortices formed in the disc. We performed a parametric study assuming different properties for the disc and the viscosity transition by running 1980 2D hydrodynamic simulations in the locally isothermal assumption with disc self-gravity included. Our results revealed that long-lived, large-scale vortex formation favours a shallow surface density slope and low- or moderate-disc masses with Toomre Q ≲ 1  h -1, where h is the geometric aspect ratio of the disc. In general, in low viscosity models, stronger vortices form. However, rapid vortex decay and re-formation is more widespread in these discs. [ABSTRACT FROM AUTHOR]

Remnant accretion discs formed in compact object mergers are an important ingredient in the understanding of electromagnetic afterglows of multimessenger gravitational-wave events. Due to magnetically and neutrino-driven winds, a significant fraction of the disc mass will eventually become unbound and undergo r-process nucleosynthesis. While this process has been studied in some detail, previous studies have typically used approximate initial conditions for the accretion discs, or started from purely hydrodynamical simulations. In this work, we analyse the properties of accretion discs formed from near equal-mass black hole–neutron star mergers simulated in general-relativistic magnetohydrodynamics in dynamical spacetimes with an accurate microphysical description. The post-merger systems were evolved until |$120\, {\rm ms}$| for different finite-temperature equations of state and black hole spins. We present a detailed analysis of the fluid properties and of the magnetic-field topology. In particular, we provide analytic fits of the magnetic-field strength and specific entropy as a function of the rest-mass density, which can be used for the construction of equilibrium disc models. Finally, we evolve one of the systems for a total of |$350\, \rm ms$| after merger and study the prospect for eventual jet launching. While our simulations do not reach this stage, we find clear evidence of continued funnel magnetization and clearing, a prerequisite for any jet-launching mechanism. [ABSTRACT FROM AUTHOR]

X-ray flux from the inner hot region around central compact object in a binary system illuminates the upper surface of an accretion disc and it behaves like a corona. This region can be photoionized by the illuminating radiation, and thus can emit different emission lines. We study those line spectra in black hole X-ray binaries for different accretion flow parameters including its geometry. The varying range of model parameters captures maximum possible observational features. We also put light on the routinely observed Fe-line emission properties based on different model parameters, ionization rate, and Fe abundances. We find that the Fe-line equivalent width W E decreases with increasing disc accretion rate and increases with the column density of the illuminated gas. Our estimated line properties are in agreement with observational signatures. [ABSTRACT FROM AUTHOR]

The classical nova YZ Reticuli was discovered in 2020 July. Shortly after this, we commenced a sustained, highly time-sampled coverage of its subsequent rapid evolution with time-resolved spectroscopy from the Global Jet Watch observatories. Its H-alpha complex exhibited qualitatively different spectral signatures in the following weeks and months. We find that these H-alpha complexes are well described by the same five Gaussian emission components throughout the six months following eruption. These five components appear to constitute two pairs of lines, from jet outflows and an accretion disc, together with an additional central component. The correlated, symmetric patterns that these jet/accretion disc pairs exhibit suggest precession, probably in response to the large perturbation caused by the nova eruption. The jet and accretion disc signatures persist from the first 10 d after brightening – evidence that the accretion disc survived the disruption. We also compare another classical nova (V6568 Sgr) that erupted in 2020 July whose H-alpha complex can be described analogously, but with faster line-of-sight jet speeds exceeding 4000 km s−1. We suggest that classical novae with higher mass white dwarfs bridge the gap between recurrent novae and classical novae such as YZ Reticuli. [ABSTRACT FROM AUTHOR]

In this paper, we employ Machine Learning algorithms on multimission observations for the classification of accretion states of outbursting black hole X-ray binaries for the first time. Archival data from RXTE , Swift , MAXI , and AstroSat observatories are used to generate the hardness intensity diagrams (HIDs) for outbursts of the sources XTE J1859+226 (1999 outburst), GX 339−4 (2002, 2004, 2007, and 2010 outbursts), IGR J17091−3624 (2016 outburst), and MAXI J1535−571 (2017 outburst). Based on variation of X-ray flux, hardness ratios, presence of various types of quasi-periodic oscillations (QPOs), photon indices, and disc temperature, we apply clustering algorithms like K-Means clustering and Hierarchical clustering to classify the accretion states (clusters) of each outburst. As multiple parameters are involved in the classification process, we show that clustering algorithms club together the observations of similar characteristics more efficiently than the 'standard' method of classification. We also infer that K-Means clustering provides more reliable results than Hierarchical clustering. We demonstrate the importance of the classification based on machine learning by comparing it with results from 'standard' classification. [ABSTRACT FROM AUTHOR]

Time evolution of advection-dominated accretion flow (ADAF) around a rotating compact object is presented. The time-dependent equations of fluid including the Coriolis force along with the centrifugal and pressure gradient forces are derived. In this research, it is assumed that angular momentum transport is due to viscous turbulence and the α-prescription is used for the kinematic coefficient of viscosity. Moreover, the general relativistic effects are neglected. In order to solve the equations, we have used a self-similar solution. The solutions show that the behaviour of the physical quantities in a dynamical ADAF is different from that for a steady accretion flow. Our results indicate that the physical quantities are dependent of rotation parameter which is defined as the ratio of the intrinsic angular velocity of the central body to the angular velocity of disc. Also, the effect of rotation parameter on these quantities is different for co and counter-rotating flows. The solution shows that by increasing the rotation parameter a , inflow–outflow region approaches the central object for co-rotating flow and moves outwards for counter-rotating flow. We find that when flow is fully advection dominated (f → 1), the entire gas has positive Bernoulli function. Also, we suggest that the Bernoulli function becomes more positive when the effect of rotation on the structure of disc decreases. [ABSTRACT FROM AUTHOR]

We explore the accretion states of tidal disruption events (TDEs) using a sample of seven X-ray bright sources. To this end, we estimate the relative contribution of the disc and corona to the observed X-ray emission through spectral modelling, and assess the X-ray brightness (through αox, L |$_{2\ \rm keV}$|⁠ , and f Edd,X) as a function of the Eddington ratio. We report strong positive correlations between αox and f Edd,bol; f Edd,X and f Edd,UV; and an anticorrelation for L |$_{2\ \rm keV}$| and f Edd,UV. TDEs at high f Edd,bol have thermal dominated X-ray spectra and high (soft) αox, whereas those at low f Edd,bol show a significant power-law contribution and low (hard) αox. Similar to X-ray binaries and active galactic nuclei, the transition between X-ray spectral states occurs around f Edd,bol ≈ 0.03, although the uncertainty is large due to the small sample size. Our results suggest that X-ray surveys are more likely to discover TDEs at low f Edd,bol, whereas optical surveys are more sensitive to TDEs at high Eddington ratios. The X-ray and optical selected TDEs have different UV and X-ray properties, which should be taken into account when deriving rates, luminosity, and black hole mass functions. [ABSTRACT FROM AUTHOR]

We report on the phase-coherent timing analysis of the accreting millisecond X-ray pulsar IGR J17591–2342, using Neutron Star Interior Composition Explorer (NICER) data taken during the outburst of the source between 2018 August 15 and 2018 October 17. We obtain an updated orbital solution of the binary system. We investigate the evolution of the neutron star spin frequency during the outburst, reporting a refined estimate of the spin frequency and the first estimate of the spin frequency derivative (⁠|$\dot{\nu }\sim -7\times 10^{-14}$| Hz s−1), confirmed independently from the modelling of the fundamental frequency and its first harmonic. We further investigate the evolution of the X-ray pulse phases adopting a physical model that accounts for the accretion material torque as well as the magnetic threading of the accretion disc in regions where the Keplerian velocity is slower than the magnetosphere velocity. From this analysis we estimate the neutron star magnetic field B eq = 2.8(3) × 108 G. Finally, we investigate the pulse profile dependence on energy finding that the observed behaviour of the pulse fractional amplitude and lags as a function of energy is compatible with the down-scattering of hard X-ray photons in the disc or the neutron star surface. [ABSTRACT FROM AUTHOR]

We supplement the analytic solution obtained by [O. M. Matthews et al., Mon. Not. R. Astron. Soc.356, 66 (2005)] to investigate the steady-state structure of radiation pressure-dominated disc under the influence of a stellar magnetic field which deploys a torque. The solutions converge to the non-magnetic [N. I. Shakura and R. A. Sunyaev, Astron. Astrophys.24, 337 (1973)] form when magnetic field of the star tends to zero and also at large radii. Effects of varying the mass accretion rates and the spin period of a typical neutron star on the disc parameters are presented. We further report that the presence of a magnetic correction term k reduces the radial extent upto which radiation pressure and electron scattering continue to be the major source of pressure and opacity, respectively. We also report that magnetic effects amplify the viscous timescale several times in the inner disc. [ABSTRACT FROM AUTHOR]

It is believed that large-scale horseshoe-like brightness asymmetries found in dozens of transitional protoplanetary discs are caused by anticyclonic vortices. These vortices can play a key role in planet formation, as mm-sized dust – the building blocks of planets – can be accumulated inside them. Anticyclonic vortices are formed by the Rossby wave instability, which can be excited at the gap edges opened by a giant planet or at sharp viscosity transitions of accretionally inactive regions. It is known that vortices are prone to stretching and subsequent dissolution due to disc self-gravity for canonical disc masses in the isothermal approximation. To improve the hydrodynamic model of protoplanetary discs, we include the disc thermodynamics in our model. In this paper, we present our results on the evolution of the vortices formed at the outer edge of an accretionally inactive region (dead zone) assuming an ideal equation of state and taking PdV work, disc cooling in the β-approximation, and disc self-gravity into account. Thermodynamics affects the offset and the mode number (referring to the number of small vortices at the early phase) of the RWI excitation, as well as the strength, shape, and lifetime of the large-scale vortex formed through merging of the initial small vortices. We found that the inclusion of gas thermodynamics results in stronger, however decreased lifetime vortices. Our results suggest that a hypothetical vortex-aided planet formation scenario favours effectively cooling discs. [ABSTRACT FROM AUTHOR]