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J. H. Swank (Goddard Space Flight Center), A. Baykal (Middle East Technical University), M. J. Stark (Lafayette College)
SAX J2103.5+4545 is a transient pulsar with a 358 s pulse period discovered with BeppoSAX in a 1997 outburst, and a 12.68 dy orbital period found with the Rossi X-Ray Timing Explorer in a 1999 outburst. In the latter outburst the source exhibited the spin behavior predicted for a transient accreting pulsar accreting through a disk. Over the course of a year, it spun up as the flux increased, came to equilibrium and finally spun down when the flux dropped. The rate of change of the frequency exhibited a one-to-one correlation with the flux. It could be fit to the model due to Ghosh & Lamb of angular momentum accretion onto a neutron star for a distance of 3.2 kpc and a magnetic field of 1.2 \times 1013 Gauss. The pulses kept the same shape and amplitude as the flux declined, as if the accretion continued to be through a disk, with the X-rays produced at the neutron star rather than the magnetosphere. The source had not entered a propeller phase. This is the first time a transient pulsar has been tracked into the spin-down state. The magnetic field obtained is the strongest that has been estimated for binary accreting pulsars.
The first 150 days of the outburst and the remaining 200 days revealed a notable difference. Initially there was a strong modulation of the flux with the binary period, while after flux dropped, none was measurable. We interpret this change in terms of a change in the character of the mass exchange onto the neutron star, from accretion from a slow wind to accretion from a fast low density wind. The energy spectra also are correlated with the changes during the orbit and the outburst.