Journal Article

Pulse-phase-resolved spectroscopy of continuum and reflection in SAX J1808.4−3658

Tony Wilkinson, Alessandro Patruno, Anna Watts and Philip Uttley

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 410, issue 3, pages 1513-1520
Published in print January 2011 | ISSN: 0035-8711
Published online January 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17532.x
Pulse-phase-resolved spectroscopy of continuum and reflection in SAX J1808.4−3658

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We perform phase-resolved spectroscopy of the accreting millisecond pulsar, SAX J1808.4−3658, during the slow-decay phase of the 2002 outburst. Simple phenomenological fits to Rossi X-ray Timing Explorer Proportional Counter Array data reveal a pulsation in the iron line at the spin frequency of the neutron star. However, fitting more complex spectral models reveals a degeneracy between iron-line pulsations and changes in the underlying hotspot blackbody temperature with phase. By comparing with the variations in reflection continuum, which are much weaker than the iron-line variations, we infer that the iron line is not pulsed. The observed spectral variations can be explained by variations in blackbody temperature associated with rotational Doppler shifts at the neutron star surface. By allowing blackbody temperature to vary in this way, we also find a larger phase shift between the pulsations in the Comptonized and blackbody components than has been seen in previous work. The phase shift between the pulsation in the blackbody temperature and normalization is consistent with a simple model where the Doppler shift is maximized at the limb of the neutron star, ∼90° prior to maximization of the hotspot projected area.

Keywords: stars: neutron; X-rays: binaries; X-rays: individual: SAX J1808.4−3658

Journal Article.  5442 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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