  The November 1999 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
fit to the Ghosh and Lamb derivations for the spin-up caused by accretion from
a thin, pressure-dominated disk, for a distance 3.2 kpc and a surface magnetic
field 1.2 10^{13} Gauss. 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 cut-off power-law, with an emission line near the 6.4 keV of K
alpha 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 is 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 the X-ray luminosity observed was between 1 10^{36} ergs
s^{-1} and 6 \times 10^{34} ergs s^{-1}, with a small but definite correlation
of the intrinsic power-law spectral index.

Astrophysics