arXiv subjects

Galina Lipunova

Publications and source records attributed to Galina Lipunova.

12 recordsLinked to original sources

arXiv2026

Shock-free super-Eddington accretion onto magnetized neutron star

Strong magnetic fields allow accreting neutron stars to form magnetically controlled funnel flows. At sufficiently high accretion rates, such flows become radiation-pressure-supported. In the conventional picture, the magnetospheric flow contains a nearly free-falling transonic outer region, a radiation-mediated shock, and a subsonic accretion column. We investigate the regime in which the height of the column is expected to exceed the magnetospheric radius. In this case a self-consistent transonic solution with a standing shock is no longer possible, and the entire magnetospheric flow remains subsonic. We construct a semi-analytical hydrodynamical solution for a fully subsonic, radiation-supported magnetospheric flow in the low-Mach-number limit. The magnetic field is treated in the force-free approximation, while the flow structure is determined by mass conservation, energy conservation, and the momentum equation along the field lines. We test the solution using one-dimensional time-dependent simulations with the code HACol. The shock-free solution is highly advective. Its enthalpy profile is close to the adiabatic limit, while the energy density is regulated by leakage of mass and heat through the sides of the accretion channel. Most of the observable power is expected to be released by plasma leaving the magnetospheric flow above the neutron-star surface. Such systems should be associated with super-Eddington accretion discs and low pulse fractions due to visibility selection and scattering in the disc wind. We suggest that many non-pulsating ultraluminous X-ray sources containing neutron stars may accrete in this shock-free regime.

astro-ph.HE↗

arXiv2026

Variable opacity and accretion-column regimes of magnetized neutron stars

Radiation-supported accretion columns of magnetized neutron stars can operate in several distinct regimes. As the accretion rate increases, magnetospheric accretion may proceed through hot-spot emission, efficiently cooling radiative shocks, and advective sinking columns. The structure of the accretion flow is determined by the global parameters such as magnetic field strength and mass accretion rate, but is also affected by opacity variations in strong magnetic fields. A decrease of opacity for photons with energy below the cyclotron energy is able to substantially augment the classical critical accretion rate separating efficiently cooling solutions from advective solutions. We propose an approach to determine the critical accretion rate above which the Basko-Sunyaev sinking solution becomes unavoidable. We also use the Basko-Sunyaev sinking solution to estimate the plasma temperature and compare the characteristic photon energy with the local cyclotron energy. For a combination of moderate magnetic fields and high accretion rates, the characteristic photon energy in advective columns typically exceeds the cyclotron energy. In this case, the opacity is not reduced by the magnetic field, and tall accretion columns are formed. Strong magnetic-field systems may retain short, efficiently cooling accretion columns over a wide range of super-Eddington accretion rates. We identify a region in accretion rate and magnetic-field strength where both efficiently cooling and advective solutions may coexist. In this region, the column may switch or oscillate between the regimes and geometries. The typical time of such oscillations are close to the replenishment time of the column, or to the thermal time near its bottom. The quasi-periodic oscillations, observed in some super-Eddington objects in the 1-100 mHz frequency range, may be related to such relaxation cycles.

astro-ph.HE↗

arXiv2026

The low luminosity end of Galactic HMXBs with eROSITA: Establishing a luminosity floor for accreting BeXRBs

We present a first look at the Galactic population of heretofore known HMXBs as observed by SRG/eROSITA during its first four surveys. eROSITA's sensitivity of $\sim10^{-13}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$, translating to $10^{32}$-$10^{34},\mathrm{erg}\,\mathrm{s}^{-1}$ in luminosity for most known HMXBs in the Milky Way, has thus far never been reached by any wide-area survey instrument. We present the extended log N-log L distribution of known HMXBs reaching down to $10^{32}\,\mathrm{erg}\,\mathrm{s}^{-1}$ using eROSITA, and show the large scatter that can be induced by source intrinsic variability. We present sub-type resolved luminosity distributions, showing that the Supergiant X-ray binaries (SgXBs) and Be X-ray binaries (BeXRBs) occupy different parts of the overall distribution, and reanalyse RXTE/ASM data and MAXI for comparison to eROSITA. The luminosity regime uncovered by eROSITA allows a systematic study of the "transient" BeXRBs, which are typically below the detection threshold of monitors outside of outburst, and whose low luminosity behavior has been a longstanding question. Signatures of stable accretion at low luminosities have been observed with pointed instruments for a fraction of the overall sample, so far. With the eROSITA results, we posit that accretion outside of outburst is likely the norm, since a vast majority (> 80%) of BeXRBs are detected at luminosities at least an order of magnitude higher than expected for the most X-ray luminous Be stars. We discuss the observed luminosity in the context of cold disk accretion and the "propeller" mechanism. We highlight a small subpopulation of "isolated" Be-stars that reach luminosities comparable to the least luminous BeXRBs, hinting at the presence of compact object companions.

astro-ph.HE↗

arXiv2026

Real-time observations of the transition to the quiescent state in an accreting magnetised neutron star: No propeller required?

The final stages of outbursts in transient X-ray pulsars (XRPs), which are characterised by a significant decline in the mass accretion rate, provide valuable insight into the physics of the accretion disc and its interaction with the strong magnetic field of the neutron star (NS). In particular, the `propeller effect', or centrifugal inhibition of accretion, has been proposed as a key mechanism governing both the onset luminosity and the timescale of the rapid transition to the quiescent state. In addition, it offers an independent method for estimating the magnetic field strength of the NS. On the other hand, the decrease in the mass accretion rate itself is driven by processes occurring in the accretion flow at larger distances from the NS. Recovering the information encoded in the light curve therefore requires sensitive high-cadence X-ray monitoring capable of capturing the rapid and often unpredictable transition from the accreting regime to the quiescent regime. In this study, we present the results of the first comprehensive monitoring campaign that tracks the entire transition to quiescence in the transient XRP 4U 0115+63 utilising observations by the NICER X-ray telescope. We show that the observed behaviour can be explained by the thermal-viscous disc instability model (DIM), with the emission observed immediately after an outburst possibly arising from the ongoing accretion from the recombined (`cold') disc and the subsequent quiescent emission being produced by the cooling NS. We further applied this model to a larger sample of XRPs encompassing a broad range of physical parameters. Ultimately, our findings indicate that the temporal behaviour of XRPs, including the quiescent state, can be consistently explained within the DIM framework without requiring the propeller effect as the primary mechanism governing the observed transition.

astro-ph.HE↗

arXiv2025

Non-Stationary Discs and Instabilities

We review our current knowledge of thermal and viscous instabilities in accretion discs around compact objects. We begin with classical disc models based on analytic viscosity prescriptions, discussing physical uncertainties and exploring time-dependent solutions of disc evolution. We also review the ionization instability responsible for outbursting dwarf nova and X-ray binary systems, including some detailed comparisons between alpha-based models and the observed characteristics of these systems. We then review modern theoretical work based on ideas around angular momentum transport mediated by magnetic fields, focusing in particular on knowledge gained through local and global computer simulations of MHD processes in discs. We discuss how magnetohydrodynamics (MHD) may alter our understanding of outbursts in white dwarf and X-ray binary systems. Finally, we turn to the putative thermal/viscous instabilities that were predicted to exist in the inner, radiation pressure-dominated regions of black hole and neutron star discs, in apparent contradiction to the observed stability of the high/soft state in black hole X-ray binaries.

astro-ph.HE↗

arXiv2023

On the appearance of non-local MRI in Keplerian accretion discs

We revisit the modal analysis of small perturbations in Keplerian ideal gas flows leading to magneto-rotational instability (MRI) using the non-local approach. We consider the case of constant vertical background magnetic field, as well as the case of radially dependent background Alfvén velocity. In the case of constant Alfvén velocity, MRI modes are described by a Schrödinger-like differential equation with some effective potential including 'repulsive' ($1/r^2$) and 'attractive' ($-1/r^3$) terms. Taking into account the radial dependence of the background Alfvén speed leads to a qualitative change in the shape of the effective potential. It is shown that there are no stationary energy levels corresponding to unstable modes $ω^2 < 0$ in ``shallow'' potentials. In thin accretion disks, the wavelength of the disturbance $λ=2π/k_z$ is smaller than the half-thickness $h$ of the disk only in ``deep'' potentials. The limiting value of the background Alfvén speed $(c_A)_\mathrm{cr}$, above which the magnetorotational instability does not occur, is found. In thin accretion disks with low background Alfvén speed $c_A\ll (c_A)_\mathrm{cr}$, the increment of the magnetorotational instability $ω\approx -\sqrt{3}\mathrm{i}c_Ak_z$ is suppressed compared to the value obtained in the local perturbation analysis.

astro-ph.HE↗

arXiv2023

Simulating the shock dynamics of a neutron star accretion column

Accretion onto a highly-magnetised neutron star runs through a magnetospheric flow, where the plasma follows the magnetic field lines in the force-free regime. The flow entering the magnetosphere is accelerated by the gravity of the star and then abruptly decelerated in a shock located above the surface of the star. For large enough mass accretion rates, most of the radiation comes from the radiation-pressure-dominated region below the shock, known as accretion column. Though the one-dimensional, stationary structure of this flow has been studied for many years, its global dynamics was hardly ever considered before. Considering the time-dependent structure of an accretion column allows us to test the stability of the existing stationary analytic solution, as well as its possible variability modes, and check the validity of its boundary conditions. Using a conservative scheme, we perform one-dimensional time-dependent simulations of an ideal radiative MHD flow inside an aligned dipolar magnetosphere. Whenever thermal pressure locally exceeds magnetic pressure, the flow is assumed to lose mass. Position of the shock agrees well with the theoretical predictions below a limit likely associated with advection effects: if more than $2/3$ of the released power is advected with the flow, the analytic solution becomes self-inconsistent, and the column starts leaking at a finite height. Depending on the geometry, this breakdown may broaden the column, mass-load the field lines, and produce radiation-driven, mildly relativistic ejecta. Evolving towards the equilibrium position, the shock front experiences damped oscillations at a frequency close to the inverse sound propagation time.

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arXiv2021

Physical modeling of viscous disc evolution around magnetized neutron star. Aql X-1 2013 outburst decay

We present a model of a viscously evolving accretion disc around a magnetized neutron star. The model features the varying outer radius of the hot ionized part of the disc due to cooling and the varying inner radius of the disc due to interaction with the magnetosphere. It also includes hindering of accretion on the neutron star because of the centrifugal barrier and irradiation of the outer disc and companion star by X-rays from the neutron star and disc. When setting inner boundary conditions, we take into account that processes at the inner disc occur on a time scale much less than the viscous time scale of the whole disc. We consider three types of outflow from the disc inner edge: zero outflow, one based on MHD calculations, and a very efficient propeller mechanism. The light curves of an X-ray transient after the outburst peak can be calculated by a corresponding, publicly available code. We compare observed light curves of the 2013 burst of Aql X-1 in X-ray and optical bands with modeled ones. We find that the fast drop of the $0.3-10$ keV flux can be solely explained by a radial shrinking of the hot disc. At the same time, models with the neutron star magnetic field $>10^8$ G have better fits because the accretion efficiency behaviour emphasizes the 'knee' on the light curve. We also find that a plato emission can be produced by a disc-reservoir with stalled accretion.

astro-ph.HE↗

arXiv2019

The cosmic microwave background spectral modification due to thermal Comptonization in galaxy clusters. Analytical consideration

See comments. We calculate analytically an effect of the thermal Comptonization on the cosmic microwave background (CMB) photons when they propagate through galaxy clusters. We estimate a deformation of the CMB spectrum due to multiple Compton scattering of the photons off electrons of an optically-thin hot spherical plasma cloud. This new approach allows us to disentangle and determine the optical depth and the electron temperature of a galaxy cluster using the CMB observations.

astro-ph.CO↗

arXiv2019

Super-Eddington accretion discs with advection and outflows around magnetized neutron stars

We present a model for a super-Eddington accretion disc around a magnetized neutron star taking into account advection of heat and the mass loss by the wind. The model is semi-analytical and predicts radial profiles of all basic physical characteristics of the accretion disc. The magnetospheric radius is found as an eigenvalue of the problem. When the inner disc is in radiation-pressure-dominated regime but does not reach its local Eddington limit, advection is mild, and the radius of the magnetosphere depends weakly on the accretion rate. Once approaching the local Eddington limit, the disc becomes advection-dominated, and the scaling for the magnetospheric radius with the mass accretion rate is similar to the classical Alfven relation. Allowing for the mass loss in a wind leads to an increase of the magnetospheric radius. Our model may be applied to a large variety of magnetized neutron stars accreting close to or above their Eddington limits: ultra-luminous X-ray pulsars, Be/X-ray binaries in outbursts, and other systems. In the context of our model we discuss the observational properties of NGC 5907~X-1, the brightest ultra-luminous pulsar known so far, and NGC 300~ULX-1 which is apparently a Be/X-ray binary experiencing a very bright super-Eddington outburst.

astro-ph.HE↗

arXiv2017

Discovery of the neutron stars merger GW170817/GRB170817A and Binary Stellar Evolution

The Multimessenger discovery of the merger of two neutron stars on August 17, 2017, GW170817 / GRB170817A, accompanied by a gamma-ray burst and an optical kilonova, is a triumph of the ideas about the evolution of the baryon component in the Universe. Despite the current uniqueness of this observation, the obtained variety of experimental data makes it possible right now to draw important theoretical conclusions about the origin of the double neutron star, their merger, and the subsequent flare-up of the electromagnetic radiation. We present that the discovery of the merger at a distance of 40 Mpc is in full agreement with the very first calculations of the Scenario Machine (Lipunov et al. 1987). In modern terms, the predicted rate is ~ 10 000 Gpc-3.

astro-ph.HE↗

arXiv2007

Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources

We derive the luminosity-temperature relation for the super-critically accreting black holes (BHs) and compare it to the data on ultraluminous X-ray sources (ULXs). At super-Eddington accretion rates, an outflow forms within the spherization radius. We construct the accretion disc model accounting for the advection and the outflow, and compute characteristic disc temperatures. The bolometric luminosity exceeds the Eddington luminosity L_Edd by a logarithmic factor 1+0.6 ln mdot (where mdot is the accretion rate in Eddington units) and the wind kinetic luminosity is close to L_Edd. The apparent luminosity for the face-on observer is 2-7 times higher because of geometrical beaming. Such an observer has a direct view of the inner hot accretion disc, which has a peak temperature T_max of a few keV in stellar-mass BHs. The emitted spectrum extends as a power-law F_E ~ E**{-1} down to the temperature at the spherization radius T_sp ~ mdot**(-1/2) keV. We associate T_max with a few keV spectral components and T_sp with the soft, 0.1-0.2 keV components observed in ULXs. An edge-on observer sees only the soft emission from the extended envelope, with the photosphere radius exceeding the spherization radius by orders of magnitude. The dependence of the photosphere temperature on luminosity is consistent with that observed in the super-Eddington accreting BHs SS 433 and V4641 Sgr. Strong outflows combined with the large intrinsic X-ray luminosity of the central BH explain naturally the presence of the photoionized nebulae around ULXs. An excellent agreement between the model and the observational data strongly argues in favour of ULXs being super-critically accreting, stellar-mass BHs similar to SS 433, but viewed close to the symmetry axis.

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