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Institut für Astronomie und Astrophysik

Abteilung Astronomie

Sand 1, D-72076 Tübingen, Germany
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Dissertation Mathias Kunz

Hexe-Beobachtungen des Röntgenpulsars HER X-1

The accreting X-ray pulsar Her X-1 was observed in hard X-rays (E-20 keV) by MIR-HEXE. In this thesis, temporal and spectral characteristics of data obtained in 1987/88 are analysed.
The source was detected during the MainOn state of five 35d-cycles. Pulse periods and pulse profiles are established for these epochs. The secular Spin Up of Her X-1 with short term fluctuations is confirmed.
A decomposition of the MainOn pulse profile into two symmetrical components yields an offset of delta ~ 20° from the antipodal position, where identical emission characteristics of the hot spots are assumed and general relativistic effects are considered. The emission characteristic of Her X-1 has pencil and fan beam components, with the former dominating the source emission at hard X-rays. The emission from the two hot spots dominates the main pulse and the interpulse, respectively.
The pulse phase averaged spectrum in 20-110 keV is represented by a power law (alpha=0.93 fixed) with exponential turnover (kT-10.7(6) keV) and a narrow cyclotron resonance line. The line is clearly in absorption, with centroid at E_c=33.7(1.3) keV and FWHM<10 keV. By a critical review of experimental conditions it is shown, that the historical measurements are compatible with a constant surface magnetic field strength B=3.03(7)x10^12 Gaus and a constant plasma temperature kT = 10.9(4) kcV.
Pulse phase resolved spectroscopy shows, that the continuum shape at E>20 keV varies significantly across the pulse. Assuming a constant plasma temperature kT, the photon index variation alpha(phi) at lower energies can be reproduced both in amplitude and in its symmetry with respect to the decline of the main pulse, suggesting that the continuum spectrum of Her X-1 above 1 keV is determined by a unified elementary process.
The cyclotron line centroid varies in phase with the continuum intensity I_0. The magnitude of the centroid variation requires deviations from the dipole geometry. With the current observations, it is not possible to determine whether the non-antipodal emission regions or field inhomogeneities are responsible for this effect. The line deficit F_G also varies in phase with the continuum intensity I_0, where F_G proportional I_0^x with x=1.5(2). As the scattering cross section of continuum photons with plasma electrons is polarisation dependent, the relative line deficit F_G / I_0 of an unpolarised ensemble of continuum photons is most prominent in the direction of the magnetic field, favoring a pencil beam emission characteristic at hard X-rays.

Dissertation (23,63 Mb PDF file including figures)

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