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High-Energy Astrophysics American and Soviet Perspectives/Proceedings from the U.S.-U.S.S.R. Workshop on High-Energy Astrophysics (1991) / Chapter Skim
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THE CYCLOTRON ABSORPTION LINE AND ECLIPSE TRANSITION PHENOMENA OF 4U 1538-52
Pages 77-86

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From page 77...
... The pulse-phase dependence of the intensity and spectrum can be mimicked by a model of X-ray emission from thin accretion-heated slabs at the magnetic poles of a rotating neutron star with its magnetic dipole axis inclined at 45 from the rotation axis. The observations also yielded data on the eclipse transitions which demonstrate that the radial density function at the base of the supersonic wind of the O-Wpe supergiant pnma~y has the forth of an exponential like that which characterizes the density run in the similar region of the O-bpe superg~ant primary of Cen X-3.
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From page 78...
... And just recently clear evidence of first and second harmonic absorption lines at 20 and 40 keV have been found with Ginga in two gamma-ray bursts. Our Ginga observation of 4U 153~52 adds a third definite example of cyclotron absorption line formation in an X-ray binary pulsar under what appear to be specially favorable circumstances for analysis (Clark al.
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From page 79...
... 4U 1538-52. The absorption line at 20 keV is new and opens interestung possibilities for detailed comparisons between the observed phenomena and the results of recent theoretical treatments of the emissions of magnetized slabs and columns of accretion-heated plasmas at the poles of magnetized neutron stars.
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From page 80...
... 1 0 20 30 40 50 ENERGY (keV) FIGURE 2 Pulse height spectra and inferred inadent energy spectra of 4U 153~52 for eight intervals of pulse phase.
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From page 82...
... and pulse-phase resolved energy spectra (bottom panel) for a model X-ray pulsar with the radiation pattern illustrated in the top panel and the orientations of magnetic dipole axis and spin axis specified in the text.
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From page 83...
... in rocket UV spectroscopy, theories of the wind acceleration process developed around the idea of radiation pressure arising from scattering of light from the Doppler-shifting UV lines of the metals in the wind. This works well in the supersonic regime, and since the radiation intensity varies as 1/r2, the resulting acceleration leads to velocity cones with a characteristic rapid initial rise and then a slow approach to an asymptotic terminal velocity with values in the range of 1000 to 3000 Ems, as observed in the P Cygni profiles of UV absorption lines.
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From page 84...
... that the density run is exponential Such a scale height, if interpreted as the characteristic of an isothermal, hydrostatic atmosphere of a 19 solar mass star, corresponds to a temperature of 106 K Following Hearn's idea that there may be a coronal layer at the base of the wind, heated by some mysterious process, in which the initial acceleration takes place, we constructed an ad hoc model with a 106 K base corona governed by the usual hydrodynamic flow equation with a gravity reduced by half by radiation pressure, and terminating at 1.4 R* at the high temperature sonic point where we assumed the heating mechanism turns off, the temperature drops to 50,000 K, and radiation driving takes over in the cool but supersonic regime.
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From page 85...
... along the lines of sight to 4U 1538 52 plotted against orbital phase during one eclipse ingress and egress. The solid curves are least-squares fits of column density curves derived from three trial atmospheric density functions of the primary BO star plus constant terms before and after eclipse.
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From page 86...
... 86 AMERICAN AND SOVIET PERSPECTIVES with an exponential density run with a radiation-dr~ven supersonic regime. But what is needed now is theoretical attention to the facts of X-ray eclipse transitions and the direct information they provide about He mysterious subsonic acceleration phase of early star wind generation.
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Key Terms

  • radiation pressure
  • density function
  • absorption line
  • supersonic regime
  • eclipse transition

  • cyclotron absorption
  • column density
  • density run
  • radial density
  • eclipse transitions

  • hydrostatic function
  • coronal layer
  • dipole axis
  • isothermal hydrostatic
  • exponential density

  • column densities
  • energy spectra
  • absorption lines
  • rapid initial
  • mass loss

  • neutron stars
  • magnetic dipole
  • slow approach
  • density functions
  • subsonic acceleration

  • scale height
  • neutron star
  • asymptotic terminal
  • measured column
  • hybrid model


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