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The November 1999 outburst of the transient pulsar SAX J2103.5+4545 wasmonitored with the large area detectors of the Rossi X-Ray Timing Exploreruntil 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 frequencysaturated and, as the flux continued to decline, a weak spin-down began. Thepulses remained strong during the decay and the spin-up/flux correlation can befit to the Ghosh and Lamb derivations for the spin-up caused by accretion froma thin, pressure-dominated disk, for a distance 3.2 kpc and a surface magneticfield 1.2 10^{13} Gauss. During the bright spin-up part of the outburst, theflux was subject to strong orbital modulation, peaking 3 days after periastronof the eccentric 12.68 day orbit, while during the faint part, there was littleorbital 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 Kalpha fluorescence from cool iron. The equivalent width of this emission didnot share the orbital modulation, but nearly doubled during the faint phase,despite little change in the column density. The outburst could have beencaused by an episode of increased wind from a Be star, such that a smallaccretion disk is formed during each periastron passage. A change in the windand disk structure apparently occurred after 5 months such that the accretionrate was no longer modulated or the diffusion time was longer. The distanceestimate implies the X-ray luminosity observed was between 1 10^{36} ergss^{-1} and 6 \times 10^{34} ergs s^{-1}, with a small but definite correlationof the intrinsic power-law spectral index.Comment: Accepted for publication in Astrophysical Journa

X‐Ray Spectra and Pulse Frequency Changes in SAX J2103.5+4545