2002ApJ...569..903B

The 1999 November outburst of the transient pulsar SAX J2103.5+4545 was monitored with the large area detectors of the Rossi X-Ray Timing Explorer until the pulsar faded after a year. The 358 s pulsar was spun up for 150 days, at which point the flux dropped quickly by a factor of ~7, the frequency saturated, and, as the flux continued to decline, a weak spin-down began. The pulses remained strong during the decay and the spin-up/flux correlation can be fitted to the Ghosh and Lamb derivations for the spin-up caused by accretion from a thin, pressure-dominated disk, for a distance of ~3.2 kpc and a surface magnetic field of ~1.2x10¹³ G. During the bright spin-up part of the outburst, the flux was subject to strong orbital modulation, peaking ~3 days after periastron of the eccentric 12.68 day orbit, while during the faint part, there was little orbital modulation. The X-ray spectra were typical of accreting pulsars, describable by a cutoff power law, with an emission line near the 6.4 keV of Kα fluorescence from cool iron. The equivalent width of this emission did not share the orbital modulation, but nearly doubled during the faint phase, despite little change in the column density. The outburst could have been caused by an episode of increased wind from a Be star, such that a small accretion disk was formed during each periastron passage. A change in the wind and disk structure apparently occurred after 5 months such that the accretion rate was no longer modulated or the diffusion time was longer. The distance estimate implies that the X-ray luminosity observed was between 1x10³⁶ and 6x10³⁴ ergs.s^–1, with a small but definite correlation with the intrinsic power-law spectral index.