Phase-resolved spectra of the millisecond pulsar SAX J1808.4-3658

Abstract

We analyze the RXTE observations of the April 1998 outburst of SAX J1808.4-3658. We show that the soft blackbody component of the X-ray spectrum is from the hotspot on the neutron star surface, while the hard component is from Comptonization of these blackbody photons in the plasma heated by the accretion shock. The disc cannot provide enough seed photons for Comptonization. It is truncated into an optically thin flow at ~20-40 R_g by some generic mechanisms common for black holes and neutron stars. Only below the truncation radius the flow is disrupted by the magnetic field and collimated onto the surface. The pulse profiles of both spectral components imply several constraints on the geometry of the system: the inclination angle, location and size of the hotspot and the shock.