EDP Sciences
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Volume 419, Number 3, June I 2004
Page(s) 1077 - 1085
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20034129

A&A 419, 1077-1085 (2004)
DOI: 10.1051/0004-6361:20034129

A phase-dependent absorption line in the spectrum of the X-ray pulsar RX J0720.4-3125

F. Haberl, V. E. Zavlin, J. Trümper and V. Burwitz

Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
(Received 29 July 2003/ Accepted 16 March 2004 )

The XMM-Newton spectra of the isolated neutron star show deviations from a Planckian energy distribution below 400 eV, similar to the spectra of RBS1223, another long-period X-ray pulsar, as reported recently by Haberl et al. (2003). For a Gaussian-shaped absorption line we derive an energy of 271 eV and an equivalent width of -40 eV from the phase-averaged spectra of . We investigate the spectral variations seen in hardness ratios as function of pulse phase and find that they are best described by changes in the depth of the absorption line. The line equivalent width changes between -31 eV around intensity maximum of the pulse and -58 eV at the declining part of the pulse. Small variations ( <20 eV) of the line energy with pulse phase may still be caused by statistical fluctuations. On the other hand, the black-body temperature varies significantly by 2.5 eV (statistical 90% errors typically 0.7 eV) reaching the highest value at pulse maximum. One possible interpretation for the absorption line is cyclotron resonance scattering of protons in a magnetic field with $B \simeq 5\,\times\, 10^{13}$ G. This field strength is compatible with estimates inferred from recent spin down measurements of the pulsar.

Key words: stars: individual: -- stars: neutron -- stars: magnetic fields -- X-rays: stars

Offprint request: F. Haberl, fwh@mpe.mpg.de

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