A decade of timing an accretion-powered millisecond pulsar: The continuing spin down and orbital evolution of SAX J1808.4-3658

TL;DR

This study analyzes a decade of observations of the accretion-powered millisecond pulsar SAX J1808.4-3658, revealing ongoing spin-down likely due to magnetic dipole torque and orbital period increase, with implications for neutron star evolution.

Contribution

It provides the most recent timing analysis of SAX J1808.4-3658, confirming persistent spin-down and orbital expansion over a decade, and discusses the underlying physical mechanisms.

Findings

The pulsar's spin frequency continues to decrease at a consistent rate.

The binary orbital period is increasing at a measurable rate.

Spin-down is mainly during quiescence, likely caused by magnetic dipole torque.

Abstract

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 it is most likely due to the magnetic dipole torque from a B = 1.5x10^8 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.