| Precision Timing of an Accreting Millisecond Pulsar Over Seven Years (2006) | |||||||
Abstract | |||||||
| We present a comprehensive timing study of the 2.5 millisecond X-ray pulsar SAX 1808.4-3658. Using data from the Rossi X-ray Timing Explorer, we construct a phase-connected pulse timing solution for each of four outbursts (1998, 2000, 2002, and 2005) and measure the spin evolution. This effort is complicated by substantial pulse profile variability, including a rapid, glitch-like profile change and a strong correlation between the pulse shape and the source luminosity. We find intervals of both spin-up and spin-down during the outbursts, suggesting that the pulsar is near its spin equilibrium. We also constrain the spin evolution between outbursts and discuss the resulting limits on gravitational radiation torques. Finally, we refine the binary orbital parameters and find with high significance that the orbital period is increasing. | |||||||
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