A Decade Of Timing An Accretion-Powered Millisecond Pulsar: The Continuing Spin Down And Orbital Evolution Of Sax J1808.4-3658

• Main Authors: Chakrabarty, Deepto (Contributor), Patruno, Alessandro (Author), Markwardt, Craig B. (Author), van der Klis, Michiel (Author), Wijnands, Rudy (Author), Morgan, Edward H (Author), Hartman, Jacob M., Ph. D. Massachusetts Institute of Technology (Author)
• Other Authors: Massachusetts Institute of Technology. Department of Physics (Contributor), MIT Kavli Institute for Astrophysics and Space Research (Contributor), Morgan, Edward H. (Contributor)
• Format: Article
• Language: English
• Published: IOP Publishing, 2013-01-08T15:35:06Z.
• Subjects: Article
• Online Access: Get fulltext

 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) × 10[superscript -18] Hz s[superscript -1], 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 = 1.5 × 10[superscript 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) × 10[superscript -12] s s[superscript -1], also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.