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35-Day Evolution of the Her X-1 Pulse Profile: Evidence for a Resolved Inner Disk Occultation of the Neutron StarGinga and RXTE observations have allowed an unprecedented view of the recurrent systematic pulse shape changes associated with the 35-day cycle of Her X-1, a phenomena currently unique among the known accretion-powered pulsars. We present observations of the pulse shape evolution. An explanation for the pulse evolution in terms of a freely precessing neutron star is reviewed and shown to have several major difficulties in explaining the observed pulse evolution pattern. Instead, we propose a phenomenlogical model for the pulse evolution based upon an occultation of the pulse emitting region by the tilted, inner edge of a precessing accretion disk. The systematic and repeating pulse shape changes require a resolved occultation of the pulse emission region. The observed pulse profile motivates the need for a pulsar beam consisting of a composite coaxial pencil and fan beam but the observed evolution pattern requires the fan beam to be focused around the neutron star and beamed in the antipodal direction. The spectral hardness of the pencil beam component suggests an origin at the magnetic polar cap, with the relatively softer fan beam emission produced by backscattering from within the accretion column, qualitatively consistent with several theoretical models for X-ray emission from the accretion column of an accreting neutron star.
Document ID
20000013350
Acquisition Source
Marshall Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Scott, D. Matthew
(Universities Space Research Association Huntsville, AL United States)
Leahy, Denis A.
(Calgary Univ. Alberta Canada)
Wilson, Robert B.
(NASA Marshall Space Flight Center Huntsville, AL United States)
Date Acquired
August 19, 2013
Publication Date
January 1, 1999
Subject Category
Astronomy
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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