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Title:
The Long-term Timing of Accreting Millisecond Pulsar SAX J1808.4-3658
Authors:
Hartman, Jacob M.; Chakrabarty, D.; Patruno, A.; Watts, A.
Affiliation:
AA(Naval Research Lab), AB(MIT), AC(University of Amsterdam, The Netherlands), AD(University of Amsterdam, The Netherlands)
Publication:
American Astronomical Society, HEAD meeting #10, #24.03
Publication Date:
03/2008
Origin:
AAS
Bibliographic Code:
2008HEAD...10.2403H

Abstract

We present a comprehensive timing study of the 2.5 ms pulsar SAX J1808.4-3658, a soft X-ray transient with a recurrence time of roughly 2 years. Of the ten known accreting millisecond pulsars (all transients), it is the only source in which pulsations have been observed throughout multiple outbursts. Using data from the Rossi X-ray Timing Explorer, we construct a phase-connected pulse timing solution for each of four observed outbursts during 1998-2005. During the outbursts, there is substantial pulse shape variability. These changes are due to red pulse shape noise as well as systematic profile changes correlated with the outburst stages. We did not detect significant frequency derivatives during the outbursts, with typical 95% upper limits of 2 x 10-14 Hz s-1. By combining the outbursts, we measure a long-term spin-down with better than 6 sigma confidence. The rate is (-5.6 ± 2.0) x 10-16 Hz s-1, with most of the change occurring between the outbursts. We discuss constraints on the contributions of magnetic dipole, accretion, and gravitational wave torques to the spin-down. We also find that the 2 hr binary orbital period is increasing at a rate of (3.5 ± 0.2) x 10-12 s s-1. Finally, we note that the magnitude of the energy-dependent pulse phase lags is strongly dependent on the flux of the source, which strongly constrains the mechanisms that have been proposed to explain these soft lags.
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