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Title: A New Two-fluid Radiation-hydrodynamical Model for X-Ray Pulsar Accretion Columns

Abstract

Previous research centered on the hydrodynamics in X-ray pulsar accretion columns has largely focused on the single-fluid model, in which the super-Eddington luminosity inside the column decelerates the flow to rest at the stellar surface. This type of model has been relatively successful in describing the overall properties of the accretion flows, but it does not account for the possible dynamical effect of the gas pressure. On the other hand, the most successful radiative transport models for pulsars generally do not include a rigorous treatment of the dynamical structure of the column, instead assuming an ad hoc velocity profile. In this paper, we explore the structure of X-ray pulsar accretion columns using a new, self-consistent, “two-fluid” model, which incorporates the dynamical effect of the gas and radiation pressures, the dipole variation of the magnetic field, the thermodynamic effect of all of the relevant coupling and cooling processes, and a rigorous set of physical boundary conditions. The model has six free parameters, which we vary in order to approximately fit the phase-averaged spectra in Her X-1, Cen X-3, and LMC X-4. In this paper, we focus on the dynamical results, which shed new light on the surface magnetic field strength, themore » inclination of the magnetic field axis relative to the rotation axis, the relative importance of gas and radiation pressures, and the radial variation of the ion, electron, and inverse-Compton temperatures. The results obtained for the X-ray spectra are presented in a separate paper.« less

Authors:
 [1];  [2];  [3]
  1. Department of Electrical and Computer Engineering, United States Naval Academy, Annapolis, MD (United States)
  2. Naval Research Laboratory (retired), Washington, DC (United States)
  3. Department of Physics and Astronomy, George Mason University, Fairfax, VA USA (United States)
Publication Date:
OSTI Identifier:
22663904
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; APPROXIMATIONS; BOUNDARY CONDITIONS; COSMIC X-RAY SOURCES; DIPOLES; HYDRODYNAMICS; INCLINATION; LUMINOSITY; MAGNETIC FIELDS; NEUTRONS; PULSARS; RADIATION PRESSURE; ROTATION; SIMULATION; STARS; SURFACES; THERMODYNAMICS; TRANSPORT THEORY; VELOCITY; X-RAY SPECTRA

Citation Formats

West, Brent F., Wolfram, Kenneth D., and Becker, Peter A., E-mail: bwest@usna.edu, E-mail: kswolfram@gmail.com, E-mail: pbecker@gmu.edu. A New Two-fluid Radiation-hydrodynamical Model for X-Ray Pulsar Accretion Columns. United States: N. p., 2017. Web. doi:10.3847/1538-4357/835/2/129.
West, Brent F., Wolfram, Kenneth D., & Becker, Peter A., E-mail: bwest@usna.edu, E-mail: kswolfram@gmail.com, E-mail: pbecker@gmu.edu. A New Two-fluid Radiation-hydrodynamical Model for X-Ray Pulsar Accretion Columns. United States. doi:10.3847/1538-4357/835/2/129.
West, Brent F., Wolfram, Kenneth D., and Becker, Peter A., E-mail: bwest@usna.edu, E-mail: kswolfram@gmail.com, E-mail: pbecker@gmu.edu. Wed . "A New Two-fluid Radiation-hydrodynamical Model for X-Ray Pulsar Accretion Columns". United States. doi:10.3847/1538-4357/835/2/129.
@article{osti_22663904,
title = {A New Two-fluid Radiation-hydrodynamical Model for X-Ray Pulsar Accretion Columns},
author = {West, Brent F. and Wolfram, Kenneth D. and Becker, Peter A., E-mail: bwest@usna.edu, E-mail: kswolfram@gmail.com, E-mail: pbecker@gmu.edu},
abstractNote = {Previous research centered on the hydrodynamics in X-ray pulsar accretion columns has largely focused on the single-fluid model, in which the super-Eddington luminosity inside the column decelerates the flow to rest at the stellar surface. This type of model has been relatively successful in describing the overall properties of the accretion flows, but it does not account for the possible dynamical effect of the gas pressure. On the other hand, the most successful radiative transport models for pulsars generally do not include a rigorous treatment of the dynamical structure of the column, instead assuming an ad hoc velocity profile. In this paper, we explore the structure of X-ray pulsar accretion columns using a new, self-consistent, “two-fluid” model, which incorporates the dynamical effect of the gas and radiation pressures, the dipole variation of the magnetic field, the thermodynamic effect of all of the relevant coupling and cooling processes, and a rigorous set of physical boundary conditions. The model has six free parameters, which we vary in order to approximately fit the phase-averaged spectra in Her X-1, Cen X-3, and LMC X-4. In this paper, we focus on the dynamical results, which shed new light on the surface magnetic field strength, the inclination of the magnetic field axis relative to the rotation axis, the relative importance of gas and radiation pressures, and the radial variation of the ion, electron, and inverse-Compton temperatures. The results obtained for the X-ray spectra are presented in a separate paper.},
doi = {10.3847/1538-4357/835/2/129},
journal = {Astrophysical Journal},
number = 2,
volume = 835,
place = {United States},
year = {Wed Feb 01 00:00:00 EST 2017},
month = {Wed Feb 01 00:00:00 EST 2017}
}
  • The availability of the unprecedented spectral resolution provided by modern X-ray observatories is opening up new areas for study involving the coupled formation of the continuum emission and the cyclotron absorption features in accretion-powered X-ray pulsar spectra. Previous research focusing on the dynamics and the associated formation of the observed spectra has largely been confined to the single-fluid model, in which the super-Eddington luminosity inside the column decelerates the flow to rest at the stellar surface, while the dynamical effect of gas pressure is ignored. In a companion paper, we have presented a detailed analysis of the hydrodynamic and thermodynamicmore » structure of the accretion column obtained using a new self-consistent model that includes the effects of both gas and radiation pressures. In this paper, we explore the formation of the associated X-ray spectra using a rigorous photon transport equation that is consistent with the hydrodynamic and thermodynamic structure of the column. We use the new model to obtain phase-averaged spectra and partially occulted spectra for Her X-1, Cen X-3, and LMC X-4. We also use the new model to constrain the emission geometry, and compare the resulting parameters with those obtained using previously published models. Our model sheds new light on the structure of the column, the relationship between the ionized gas and the photons, the competition between diffusive and advective transport, and the magnitude of the energy-averaged cyclotron scattering cross-section.« less
  • The process of plasma accretion onto a neutron star with a strong magnetic field is considered. It is assumed that a substantial part of the gravitational energy of the gas being accreted can be released as radiation in the accretion column at a distance of a few radii from the star. The theory is consistent with observations of the x-ray pulsar. Her X-1, and predicts that the pulsar would have a supplementary source of soft ..gamma.. rays at energies Eapprox. =150 keV.
  • We propose that the quiescent emission of anomalous X-ray pulsars/soft gamma-ray repeaters (AXPs/SGRs) is powered by accretion from a fallback disk, requiring magnetic dipole fields in the range 10{sup 12}-10{sup 13} G, and that the luminous hard tails of their X-ray spectra are produced by bulk-motion Comptonization in the radiative shock near the bottom of the accretion column. This radiation escapes as a fan beam, which is partly absorbed by the polar cap photosphere, heating it up to relatively high temperatures. The scattered component and the thermal emission from the polar cap form a polar beam. We test our modelmore » on the well-studied AXP 4U 0142+61, whose energy-dependent pulse profiles show double peaks, which we ascribe to the fan and polar beams. The temperature of the photosphere (kT {approx} 0.4 keV) is explained by the heating effect. The scattered part forms a hard component in the polar beam. We suggest that the observed high temperatures of the polar caps of AXPs/SGRs, compared with other young neutron stars, are due to the heating by the fan beam. Using beaming functions for the fan beam and the polar beam and taking gravitational bending into account, we fit the energy-dependent pulse profiles and obtain the inclination angle and the angle between the spin axis and the magnetic dipole axis, as well as the height of the radiative shock above the stellar surface. We do not explain the high-luminosity bursts, which may be produced by the classical magnetar mechanism operating in super-strong multipole fields.« less
  • We present a model for Cygnus X-1, involving an accretion disk around a black hole, which fits the observed X-ray spectrum from 8 to 500 keV. The hard component of the spectrum (E>8 keV) is produced by an optically thin inner portion of the disk in which the electrons are 10$sup 9$ K and the ions are 3-- 300 times hotter.
  • We propose a new line-formation mechanism which might operate in the strong magnetic field near the surface of an X-ray pulsar. The strong polarization dependence of the photon propagation in the accreted plasma has an important effect on the spectrum of the pulsar. Cyclotron lines can appear in emission because scattering from one polarization mode into the other increases the depth from which photons can escape.