2009ApJ...702.1673H

The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretion-powered pulsar SAX J1808.4-3658 during 1998-2008. We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions. The spin frequency of the source continues to decrease at a rate of (-5.5±1.2)x10^–18 Hz/s, which is consistent with the previously determined spin derivative. The spin down occurs mostly during quiescence, and is most likely due to the magnetic dipole torque from a B = 1.5x10⁸ G dipolar field at the neutron star surface. We also find that the 2 hr binary orbital period is increasing at a rate of (3.80±0.06) x10^–12 s/s, also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.