Dynamics of the flows accreting onto a magnetized neutron star

Bykov, A. M.; Krassilchtchikov, A. M.

doi:10.1134/1.1738153

Cited by 25 publications

(31 citation statements)

References 27 publications

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“…However, radio (Gaensler et al 2011;Haverkorn & Spangler 2013), submillimeter (Zaroubi et al 2015) and neutral hydrogen (Heiles & Troland 2005; Kalberla & Kerp 2016) observations suggest that the magnetic field in the ISM is strongly non-Gaussian, spatially intermittent, and filamentary. Such an intermittent field is also expected theoretically, as a result of turbulent dynamo action (Wilkin et al 2007) and random shock compression (Bykov & Toptygin 1985, 1987Bykov 1988). The magnetic field generated by dynamo action in galaxy clusters is also likely to be intermittent (Ruzmaikin et al 1989;Subramanian et al 2006).…”

Section: Implementation Of Random Magnetic Fieldsmentioning

confidence: 54%

Relative distribution of cosmic rays and magnetic fields

Seta

Shukurov

Wood

et al. 2017

Monthly Notices of the Royal Astronomical Society

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Synchrotron radiation from cosmic rays is a key observational probe of the galactic magnetic field. Interpreting synchrotron emission data requires knowledge of the cosmic ray number density, which is often assumed to be in energy equipartition (or otherwise tightly correlated) with the magnetic field energy. However, there is no compelling observational or theoretical reason to expect such tight correlation to hold across all scales. We use test particle simulations, tracing the propagation of charged particles (protons) through a random magnetic field, to study the cosmic ray distribution at scales comparable to the correlation scale of the turbulent flow in the interstellar medium ( 100 pc in spiral galaxies). In these simulations, we find that there is no spatial correlation between the cosmic ray number density and the magnetic field energy density. In fact, their distributions are approximately statistically independent. We find that low-energy cosmic rays can become trapped between magnetic mirrors, whose location depends more on the structure of the field lines than on the field strength.

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Section: Implementation Of Random Magnetic Fieldsmentioning

confidence: 54%

Relative distribution of cosmic rays and magnetic fields

Seta

Shukurov

Wood

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…The electron temperature of the column was set to 5.8 keV for both accretion rates, which is close to the electron temperature obtained in numerical modelling (Bykov & Krasilshchikov 2004). The electron number density is a function ofṀ and the polar cap radius (Eq.…”

Section: Discussionmentioning

confidence: 99%

“…In the model with a collisionless shock, most of the energy is released near the NS surface where the continuum spectrum Comptonised by electrons between the NS surface and the shock is produced. Numerical simulations performed by Bykov & Krasilshchikov (2004) suggest that at a given accretion rate neither T e and n e change significantly downstream of the shock. The shape of the emerging spectrum depends on the energy exchange between photons and electrons (the dimensionless Comptonisation y-parameter, Kompaneets (1956)), which in the strong magnetic field is (Basko & Sunyaev 1975) …”

Section: Hardness Ratio -Luminosity Dependencementioning

confidence: 99%

“…This is likely to be the main stopping agent of the accretion flow when the photon luminosities are low enough for radiation pressure to be unimportant for the accretion flow breaking (but in the CRSF, where the optical depth is very high even at low luminosities, see Bykov & Krasilshchikov (2004)). This model has recently been used to describe the observed non-linear dependencies of the CRSF parameters on luminosity in another subcritical X-ray pulsar GX 304−1 (Rothschild et al 2017).…”

Section: Colissionless Shock Modelmentioning

confidence: 99%

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Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cepheus X-4

Vybornov

Klochkov

Gornostaev

et al. 2017

A&A

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Context. X-ray spectra of accreting pulsars are generally observed to vary with their X-ray luminosity. In particular, the hardness of the X-ray continuum is found to depend on luminosity. In a few sources, the correlation between the energy of the cyclotron resonance scattering feature (CRSF) and the luminosity is clear. Different types (signs) of the correlation are believed to reflect different accretion modes. Aims. We analyse two NuSTAR observations of the transient accreting pulsar Cep X-4 during its 2014 outburst. Our analysis is focused on a detailed investigation of the dependence of the CRSF energy and of the spectral hardness on X-ray luminosity, especially on short timescales. Methods. To investigate the spectral changes as a function of luminosity within each of the two observations, we used the intrinsic variability of the source on the timescale of individual pulse cycles (tens of seconds) , the so-called pulse-to-pulse variability. Results. We find that the NuSTAR spectrum of Cep X-4 contains two CRSFs: the fundamental line at~30 keV and its harmonic at~55 keV. We find for the first time that the energy of the fundamental CRSF increases and the continuum becomes harder with increasing X-ray luminosity not only between the two observations, that is, on the long timescale, but also within an individual observation, on the timescale of a few tens of seconds. We investigate these dependencies in detail including their non-linearity. We discuss a possible physical interpretation of the observed behaviour in the frame of a simple one-dimensional model of the polar emitting region with a collisionless shock formed in the infalling plasma near the neutron star surface. With this model, we are able to reproduce the observed variations of the continuum hardness ratio and of the CRSF energy with luminosity.

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“…However, if a collisionless shock forms (which is an assumption) then its height above the NS surface depends on the mass accretion rate: the lower the mass accretion rate, the higher the location of the collisionless shock. Numerical simulations show that the height of the shock can be ∼ 10 5 cm (Bykov & Krasil'shchikov 2004). Thus, if collisionless shocks form, one can expect the appearance of sharp dips due to eclipses of extended X-ray sources above the NS surface even in low luminosity (L < 10 37 erg s −1 ) XRPs.…”

Section: Sub-critical X-ray Pulsarsmentioning

confidence: 99%

On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass–radius relation

Mushtukov

Verhagen

Tsygankov

et al. 2017

Monthly Notices of the Royal Astronomical Society

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The luminosity of accreting magnetised neutron stars can largely exceed the Eddington value due to appearance of accretion columns. The height of the columns can be comparable to the neutron star radius. The columns produce the X-rays detected by the observer directly and illuminate the stellar surface, which reprocesses the X-rays and causes additional component of the observed flux. The geometry of the column and the illuminated part of the surface determines the radiation beaming. Curved spacetime affects the angular flux distribution. We construct a simple model of the beam patterns formed by direct and reflected flux from the column. We take into account the possibility of appearance of accretion columns, whose height is comparable to the neutron star radius. We argue that depending on the compactness of the star the flux from the column can be either strongly amplified due to gravitational lensing, or significantly reduced due to column eclipse by the star. The eclipses of high accretion columns result in specific features in pulse profiles. Their detection can put constraints on the neutron star radius. We speculate that column eclipses are observed in X-ray pulsar V 0332+53, leading us to the conclusion of large neutron star radius in this system (∼ 15 km if M ∼ 1.4M ). We point out that the beam pattern can be strongly affected by scattering in the accretion channel at high luminosity, which has to be taken into account in the models reproducing the pulse profiles.

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Dynamics of the flows accreting onto a magnetized neutron star

Cited by 25 publications

References 27 publications

Relative distribution of cosmic rays and magnetic fields

Relative distribution of cosmic rays and magnetic fields

Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cepheus X-4

On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass–radius relation

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