Data Centre for Astrophysics
Astronomy Department of the University of Geneva
ISDC Seminar

Wednesday, July 9, 2003 at 16:00

Juri Poutanen
University of Oulu, Finland

On the Nature of the X-ray Emission from Accreting Millisecond Pulsars

Abstract. We study the pulse profiles of the accreting X-ray millisecond pulsar SAX J1808.4-3658 at different energies. The observed variability can be explained if the emission is produced by Comptonization in a hot slab (radiative shock) of Thomson optical depth 0.3-1 at the neutron star surface. The emission patterns of the black body and the Comptonized radiation are different: a ``pencil"- and a ``fan"-like, respectively. We construct a detailed model of the X-ray production accounting for the Doppler boosting, relativistic aberration and gravitational light bending in the Schwarzschild spacetime. Our model reproduces well the pulse profiles at different energies simultaneously, corresponding phase lags, as well as the time- averaged spectrum. We constrain the compact star mass to be bounded between 1.2 and 1.6 solar masses. The radius is determined to be 8.5+/-0.5 km or 10.8+/-0.7 km for a 1.4 or 1.6 solar mass neutron star, respectively. This puts strong constraints on the neutron star equation of state.

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