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Title:
A Decade of Timing an Accretion-powered Millisecond Pulsar: The Continuing Spin Down and Orbital Evolution of SAX J1808.4-3658
Authors:
Hartman, Jacob M.; Patruno, Alessandro; Chakrabarty, Deepto; Markwardt, Craig B.; Morgan, Edward H.; van der Klis, Michiel; Wijnands, Rudy
Affiliation:
AA(Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA ; National Research Council Research Associate.; ), AB(Astronomical Institute "Anton Pannekoek," University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, Netherlands ), AC(Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA ), AD(CRESST/Department of Astronomy, University of Maryland, College Park, MD 20742, USA ; Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA ; ), AE(Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA ), AF(Astronomical Institute "Anton Pannekoek," University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, Netherlands ), AG(Astronomical Institute "Anton Pannekoek," University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, Netherlands )
Publication:
The Astrophysical Journal, Volume 702, Issue 2, pp. 1673-1678 (2009). (ApJ Homepage)
Publication Date:
09/2009
Origin:
IOP
ApJ Keywords:
binaries: general, stars: individual: SAX J1808.4–3658, stars: neutron, stars: rotation, X-rays: binaries, X-rays: stars
DOI:
10.1088/0004-637X/702/2/1673
Bibliographic Code:
2009ApJ...702.1673H

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) × 10-18 Hz s-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 × 108 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) × 10-12 s s-1, also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.
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