arXiv subjects

N. R. Ikhsanov

Publications and source records attributed to N. R. Ikhsanov.

At least 19 recordsLinked to original sources

arXiv2025

MAD-Accretion Scenario in High-Mass X-Ray Binaries with Neutron Stars

The wind-fed accretion of gas with an intrinsic magnetic field onto neutron stars in high-mass X-ray binaries (HMXBs) is discussed. The criteria for the implementation of quasi-spherical accretion in the system, the formation of a Keplerian accretion disk and a non-Keplerian magnetically arrested disk (MAD-accretion scenario) are formulated. It is shown that the period of the equilibrium rotation of a neutron star in the MAD-accretion scenario, in addition to the parameters of the binary system, is also determined by the velocity, temperature, and degree of magnetization of the stellar wind of its massive companion. Within the hypothesis of the equilibrium rotation, we have estimated the parameters of the stellar wind for the most fully-studied quasi-equilibrium accreting pulsars in the HMXBs corresponding to the criteria for the MAD-accretion scenario implementation. We show that the period of the equilibrium rotation of a neutron star in the system, all other things being equal, increases with the growth of the magnetization degree of the stellar wind component of its massive companion flowing out in the plane of the system's orbit. Therefore, the periods of accreting pulsars in systems with similar parameters can differ from each other by several orders of magnitude and, under favorable conditions, reach values of several thousand and even tens of thousands of seconds.

astro-ph.HE↗

arXiv2025

Age Estimation of the Radcliffe Wave from Open Star Clusters

Four samples of open star clusters (OSCs) with average ages of 5.2, 18.6, 40, and 61 Myr have been analyzed. The selection of these OSCs was carried out from a narrow region inclined to the galactic axis y at an angle of 25$^\circ$. The spectral analysis of the vertical positions and velocities of the selected clusters showed that the Radcliffe wave is associated with OSCs no older than 30 Myr. The following estimates of the Radcliffe wave characteristics were obtained for the OSCs with an average age of 5.2 Myr: $z_{max}=117\pm12$ pc with the wavelength $λ=4.55\pm0.14$ kpc, the vertical velocity disturbance amplitude $W_{max}=4.86\pm0.19$ km s$^{-1}$ with the wavelength $λ=1.74\pm0.08$ kpc. For the OSCs with an average age of 18.6 Myr, the estimates are as follows: $z_{max} = 54\pm5$ pc and $λ=6.30\pm0.12$ kpc, the vertical velocity disturbance amplitude $W_{max}=7.90\pm0.16$ km s$^{-1}$ and $λ=0.83\pm0.11$ kpc. The radial motion of the Radcliffe wave away from the galactic center has been confirmed. The velocity of such movement is 10 pc Myr$^{-1}$. In our opinion, the spatial distribution of OSCs younger than 30 Myr does not contradict the hypothesis of the association of the Radcliffe wave with the impact of shock waves from supernova explosions that arose on an extended front comparable in scale to the entire wave, that is, about 2 kpc in size.

astro-ph.GA↗

arXiv2021

Diffuse bands 9577 and 9633 -- relations to other interstellar features

We study, for the first time, the relations of two strong diffuse bands (DIBs) at 9633 and 9577~Å, commonly attributed to C$_{60}^+$, to other interstellar features seen in optical and UV spectra including H{\sc i}, Ca{\sc i}, Fe{\sc ii}, Na{\sc i}, Ti{\sc ii}, CN, CH, CH$^+$, and C$_2$ and DIBs 5780, 5797, 6196, 6269, 6284, and 6614. We analyzed 62 lines of sight where the stellar contamination by Mg{\sc ii} was corrected or found negligible for DIB 9633. Equivalent widths of DIB 9577 were measured in 62 lines of sight. Poor mutual correlation between the strengths of the above features and the major diffuse bands (5780 and 5797) as well as with other DIBs (with some exceptions) were revealed. The considered DIBs are also poorly correlated with the features of neutral hydrogen, molecular carbon, and those of simple interstellar radicals. Perhaps this phenomenon can be explained if the diffuse band 9577 is an unresolved blend of two or more interstellar features. There are indications that 9633 and 9577 diffuse bands are stronger in $σ$-type clouds, i.e. these features resemble the behavior of reasonably broad DIBs, which are strong in the lines of sight where the UV flux from the very hot nearby stars plays an important role.

astro-ph.GA↗

arXiv2017

Magnetic field and radial velocities of the star Chi Draconis A

We present high-resolution spectropolarimetric observations of the spectroscopic binary Chi Dra. Spectral lines in the spectrum of the main component Chi Dra A show variable Zeeman displacement, which confrms earlier suggestions about the presence of a weak magnetic field on the surface of this star. Within about 2 years of time base of our observations, the longitudinal component BL of the magnetic field exhibits variation from -11.5 +/- 2.5 G to +11.1 +/- 2.1 G with a period of about 23 days. Considering the rotational velocity of Chi DraA in the literature and that newly measured in this work, this variability may be explained by the stellar rotation under the assumption that the magnetic field is globally stable. Our new measurements of the radial velocities (RV) in high-resolution I-spectra of Chi Dra A refined the orbital parameters and reveal persistent deviations of RVs from the orbital curve. We suspect that these deviations may be due to the in uence of local magnetically generated spots, pulsations, or a Jupiter-size planet orbiting the system.

astro-ph.SR↗

arXiv2016

AR Scorpii is a new white dwarf in the ejector state

Marsh et al. (2016) have recently reported the discovery of a radio-pulsing white dwarf in the cataclysmic variable AR Sco. The period of pulsations which are also seen in the optical and UV is about 117 seconds. High intensity of pulsing radiation and non-thermal character of its spectrum leave little room for doubt that the white dwarf in AR Sco operates as a spin-powered pulsar and, therefore, is in the ejector state. We show that this system is very much resembling a well-known object AE Aqr. In both systems the compact components are spin-powered and have relatively strong surface magnetic field of order of 100-500 MG. They originated due to accretion spin-up in the previous epoch during which the magnetic field of the white dwarf had substantially evolved being initially buried by the accreted matter and recovered to its initial value after the spin-up phase had ended.

astro-ph.HE↗

arXiv2016

On the magnetic fields of Be/X-ray pulsars in the Small Magellanic Cloud

We explore the possibility to explain the properties of the Be/X-ray pulsars observed in the Small Magellanic Cloud within the magnetic levitation accretion scenario. This implies that their X-ray emission is powered by a wind-fed accretion onto a neutron star (NS) which captures matter from a magnetized stellar wind. The NS in this case is accreting matter from a non-keplerian magnetically levitating disc (ML-disc) which is surrounding its magnetosphere. This allows us to explain the observed periods of the pulsars in terms of spin equilibrium without the need of invoking dipole magnetic fields outside the usual range ~ 10^11- 10^13 G inferred from cyclotron features of Galactic high mass X-ray binaries. We find that the equilibrium period of a NS, under certain conditions, depends strongly on the magnetization of the stellar wind of its massive companion and, correspondingly, on the magnetic field of the massive companion itself. This may help to explain why similar NSs in binaries with similar properties rotate with different periods yielding a large scatter of periods of the accretion-powered pulsar observed in SMC and our galaxy.

astro-ph.HE↗

arXiv2016

Features of the Matter Flows in the Peculiar Cataclysmic Variable AE Aquarii

The structure of plasma flows in close binary systems in which one of the components is a rapidly rotating magnetic white dwarf is studied. The main example considered is the AE Aquarii system; the spin period of the white dwarf is about a factor of 1000 shorter than the orbital period, and the magnetic field on the white dwarf surface is of order of 50 MG. The mass transfer in this system was analyzed via numerical solution of the system of MHD equations. These computations show that the magnetic field of the white dwarf does not significantly influence the velocity field of the material in its Roche lobe in the case of laminar flow regime, so that the field does not hinder the formation of a transient disk (ring) surrounding the magnetosphere. However, the efficiency of the energy and angular momentum exchange between the white dwarf and the surrounding material increases considerably with the development of turbulent motions in the matter, resulting in its acceleration at the magnetospheric boundary and further escape from the system at a high rate. The time scales of the transition of the system between the laminar and turbulent modes are close to those of the AE\,Aqr transition between its quiet and active phases.

astro-ph.SR↗

arXiv2015

Period clustering of the anomalous X-ray pulsars

In this paper we address the question of why the observed periods of the Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are clustered in the range 2-12 s. We explore a possibility to answer this question assuming that AXPs and SGRs are the descendants of High Mass X-ray Binaries (HMXBs) which have been disintegrated in the core-collapse supernova explosion. The spin period of neutron stars in HMXBs evolves towards the equilibrium period, averaging around a few seconds. After the explosion of its massive companion, the neutron star turns out to be embedded into a dense gaseous envelope, the accretion from which leads to the formation of a residual magnetically-levitating (ML) disk. We show that the expected mass of a disk in this case is 10^-7 - 10^-8 M_sun which is sufficient to maintain the process of accretion at the rate 10^14 - 10^15 g/s over a time span of a few thousand years. During this period the star manifests itself as an isolated X-ray pulsar with a number of parameters resembling those of AXPs and SGRs. Period clustering of such pulsars can be provided if the lifetime of the residual disk does not exceed the spin-down timescale of the neutron star.

astro-ph.HE↗

arXiv2015

A scenario of the formation of isolated X-ray pulsars with anomalously long period

A scenario of the formation of isolated X-ray pulsars is discussed with an application to one of the best studied objects of this class 1E 161348-5055. This moderately luminous, 10^33 - 10^35 erg/s, pulsar with a relatively soft spectrum, kT ~ 0.6-0.8 keV, is associated with an isolated neutron star, which is located near the center of the young (~2000 yr) compact supernova remnant RCW 103 and rotates steadily (|dν/dt| < 2.6 x 10^-18 Hz/s) with the period of 6.7 hr. We show that in the current epoch the neutron star is in the accretor state. The parameters of the source emission can be explained in terms of the magnetic-levitation accretion scenario in which the star with the surface magnetic field of 10^12 G accretes material onto its surface from a non-Keplerian magnetic fossil disk at the rate 10^14 g/s. A neutron star could evolve to this state in a High-Mass X-ray Binary (HMXB), which had disintegrated during the supernova explosion powered by the core-collapse of its massive component. The life-time of an isolated X-ray pulsar formed this way can be as long as a few thousand years.

astro-ph.HE↗

arXiv2014

A new look at Anomalous X-ray Pulsars

We explore a possibility to explain the phenomenon of the Anomalous X-ray Pulsars (AXP) and Soft Gamma-ray Repeaters (SGR) within the scenario of fall-back magnetic accretion onto a young isolated neutron star. The X-ray emission of the pulsar in this case is originated due to accretion of matter onto the surface of the neutron star from the magnetic slab surrounding its magnetosphere. The expected spin-down rate of the neutron star within this approach is close to the observed value. We show that these neutron stars are relatively young and are going through a transition from the propeller state to the accretor state. The pulsars activity in the gamma-rays is connected with their relative youth and is provided by the energy stored in the non-equilibrium layer located in the crust of low-mass neutron stars. This energy can be released due to mixing of matter in the neutron star crust with super heavy nuclei approaching its surface and getting unstable. The nuclei fission in the low-density region initiates chain reactions leading to the nuclear explosion. The outbursts are likely to be triggered by an instability developing in the region where the matter accreted by the neutron star is accumulated at the magnetic pole regions.

astro-ph.HE↗

arXiv2014

Spin evolution of long-period X-ray pulsars

Spin evolution of X-ray pulsars in High Mass X-ray Binaries (HMXBs) is discussed under various assumptions about the geometry and physical parameters of the accretion flow. The torque applied to the neutron star from the accretion flow and equilibrium period of the pulsars are evaluated. We show that the observed spin evolution of the pulsars can be explained in terms of a scenario in which the neutron star accretes material from a magnetized stellar wind.

astro-ph.HE↗

arXiv2014

On the origin of the peculiar cataclysmic variable AE Aquarii

The nova-like variable AE Aquarii is a close binary system containing a red dwarf and a magnetized white dwarf rotating with the period of 33 seconds. A short spin period of the white dwarf is caused by an intensive mass exchange between the system components during a previous epoch. We show that a high rate of disk accretion onto the white dwarf surface resulted in temporary screening of its magnetic field and spin-up of the white dwarf to its present spin period. Transition of the white dwarf to the ejector state occurred at a final stage of the spin-up epoch after its magnetic field had emerged from the accreted plasma due to diffusion. In the frame of this scenario AE Aqr represents a missing link in the chain of Polars evolution and the white dwarf resembles a recycled pulsar.

astro-ph.HE↗

arXiv2013

A new look at the origin of the 6.67\,hr period X-ray pulsar 1E~161348-5055

The point X-ray source 1E 161348-5055 is observed to display pulsations with the period 6.67 hr and |\dot{P}| \leq 1.6 x 10^{-9} s/s. It is associated with the supernova remnant RCW\,103 and is widely believed to be a ~2000 yr old neutron star. Observations give no evidence for the star to be a member of a binary system. Nevertheless, it resembles an accretion-powered pulsar with the magnetospheric radius ~3000 km and the mass-accretion rate ~ 10^{14} g/s. This situation could be described in terms of accretion from a (residual) fossil disk established from the material falling back towards the star after its birth. However, current fall-back accretion scenarios encounter major difficulties explaining an extremely long spin period of the young neutron star. We show that the problems can be avoided if the accreting material is magnetized. The star in this case is surrounded by a fossil magnetic slab in which the material is confined by the magnetic field of the accretion flow itself. We find that the surface magnetic field of the neutron star within this scenario is ~10^{12} G and that a presence of > 10^{-7} M_\sun magnetic slab would be sufficient to explain the origin and current state of the pulsar.

astro-ph.HE↗

arXiv2012

On the spin-down mechanism of the X-ray pulsar 4U 2206+54

Observations of the X-ray pulsar 4U 2206+54, performed over a 15-year span, show its period, which is now 5555 \pm 9 s, to increase drastically. Such behavior of the pulsar can be hardly explained in the frame of traditional scenarios of spin evolution of compact stars. The observed spin-down rate of the neutron star in 4U 2206+54 proves to be in good agreement with the value expected within magnetic accretion scenario which takes into account that magnetic field of the accretion stream can under certain conditions affect its geometry and type of flow. The neutron star in this case accretes material from a dense gaseous slab with small angular momentum which is kept in equilibrium by the magnetic field of the flow itself. The magnetic accretion scenario can be realized in 4U 2206+54 provided the magnetic field strength on the surface of the optical counterpart to the neutron star is in excess of 70 G. The magnetic field strength on the surface of the neutron star in the frame of this scenario turns out to be 4 x 10^{12} G which is in accordance with estimates of this parameter from the analysis of X-ray spectra of the pulsar.

astro-ph.HE↗

arXiv2012

Magnetically controlled accretion onto a black hole

An accretion scenario in which the material captured by a black hole from its environment is assumed to be magnetized (β~ 1) is discussed. We show that the accretion picture in this case is strongly affected by the magnetic field of the flow itself. The accretion power within this Magnetically Controlled Accretion (MCA) scenario is converted predominantly into the magnetic energy of the accretion flow. The rapidly amplified field prevents the accretion flow from forming a homogeneous Keplerian disk. Instead, the flow is decelerated by its own magnetic field at a large distance (Shvartsman radius) from the black hole and switches into a non-Keplerian dense magnetized slab. The material in the slab is confined by the magnetic field and moves towards the black hole on the time scale of the magnetic field annihilation. The basic parameters of the slab are evaluated. Interchange instabilities in the slab may lead to a formation of Z-pinch type configuration of the magnetic field over the slab in which the accretion power can be converted into jets and high-energy radiation.

astro-ph.HE↗

arXiv2012

4U 1626-67 as seen by Suzaku before and after the 2008 torque reversal

Aims. The accretion-powered pulsar 4U 1626-67 experienced a new torque reversal at the beginning of 2008, after about 18 years of steadily spinning down. The main goal of the present work is to study this recent torque reversal that occurred in 2008 February. Methods. We present a spectral analysis of this source using two pointed observations performed by Suzaku in 2006 March and in 2010 September. Results. We confirm with Suzaku the presence of a strong emission-line complex centered on 1 keV, with the strongest line being the hydrogen-like Ne Ly-alpha at 1.025(3) keV. We were able to resolve this complex with up to seven emission lines. A dramatic increase of the intensity of the Ne Ly-alpha line after the 2008 torque reversal occurred, with the equivalent width of this line reaching almost the same value measured by ASCA in 1993. We also report on the detection of a cyclotron line feature centered at ~37 keV. In spite of the fact that an increase of the X-ray luminosity (0.5-100 keV) of a factor of ~2.8 occurred between these two observations, no significant change in the energy of the cyclotron line feature was observed. However, the intensity of the ~1 keV line complex increased by an overall factor of ~8. Conclusions. Our results favor a scenario in which the neutron star in 4U 1626-67 accretes material from a geometrically thin disk during both the spin-up and spin-down phases.

astro-ph.HE↗

arXiv2012

Signs of magnetic accretion in young Be/X-ray pulsar SXP 1062

The spin behaviour of the neutron star in the newly discovered young Be/X-ray long-period pulsar SXP 1062 is discussed. The star is observed to rotate with the period of 1062s, and spin-down at the rate ~ - 2.6 \times 10^{-12} Hz s^{-1}. I show that all of the conventional accretion scenarios encounter major difficulties explaining the rapid spin-down of the pulsar. These difficulties can be, however, avoided within the magnetic accretion scenario in which the neutron star is assumed to accrete from a magnetized wind. The spin-down rate of the pulsar can be explained within this scenario provided the surface magnetic field of the neutron star is B_* ~ 4 \times 10^{13} G. I show that the age of the pulsar in this case lies in the rage (2-4) \times 10^4 yr, which is consistent with observations. The spin evolution of the pulsar is briefly discussed.

astro-ph.HE↗

arXiv2012

AE Aquarii represents a new subclass of Cataclysmic Variables

We analyze properties of the unique nova-like star AE Aquarii identified with a close binary system containing a red dwarf and a very fast rotating magnetized white dwarf. It cannot be assigned to any of the three commonly adopted sub-classes of Cataclysmic Variables: Polars, Intermediate Polars, and Accreting non-magnetized White Dwarfs. Our study has shown that the white dwarf in AE Aqr is in the ejector state and its dipole magnetic moment is $μ~ 1.5 \times 10^{34} G cm^3$. It switched into this state due to intensive mass exchange between the system components during a previous epoch. A high rate of disk accretion onto the white dwarf surface resulted in temporary screening of its magnetic field and spin-up of the white dwarf to its present spin period. Transition of the white dwarf to the ejector state had occurred at a final stage of the spin-up epoch as its magnetic field emerged from the accreted plasma due to diffusion. In the frame of this scenario AE Aqr represents a missing link in the chain of Polars evolution and the white dwarf resembles a recycled pulsar.

astro-ph.HE↗