X-ray timing of the accreting millisecond pulsar SAX J1808.4-3658

We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658, an X-ray transient with a recurrence time of =2 yr, using data from the Rossi X-ray Timing Explorer covering 4 transient outbursts (1998-2005). Substantial pulse shape variability, both stochastic and systematic, was observed during each outburst. Analysis of the systematic pulse shape changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar surface drifts longitudinally and a second hot spot may appear. The overall pulse shape variability limits the ability to measure spin frequency evolution within a given X-ray outburst (and calls previous zi measurements of this source into question), with typical upper limits of Jil < 2.5 x 10-14 Hz s-1 (2a). However, combining data from all the outbursts shows with high (6 a) significance that the pulsar is undergoing long-term spin down at a rate /i = (-5.6 ± 2.0) x 10-16 Hz s-1, with most of the spin evolution occurring during X-ray quiescence. We discuss the possible contributions of magnetic propeller torques, magnetic dipole radiation, and gravitational radiation to the measured spin down, setting an upper limit of B < 1.5 x 108 G for the pulsar's surface dipole magnetic field and Q < 4.4 x 1036 g cm2 for the mass quadrupole moment. We also measured an orbital period derivative of Porb = (3.5 + 0.2) x 10-12 s s-1 We identify a strong anti-correlation between the fractional amplitude of the harmonic (r2) and the X-ray flux (fx) in the persistent pulsations of four sources: SAX J1808.4-3658, IGR J00291+5934, and XTE J1751-305, XTE J1807-294. These sources exhibit a powerlaw relationship r2 x( fx7 with slopes ranging from y = -0.47 to -0.70. The three other accreting millisecond pulsars that we analyzed, XTE J0929-314, XTE J1814-338, and HETE J1900.1-2455, do not as fully explore a wide range of fluxes, but they too seem to obey a similar relation. We argue that these trends may be evidence of the recession of the accretion disk as the outbursts dim. We examine the energy dependence of the persistent pulsations and thermonuclear burst oscillations from SAX J1808.4-3658.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Physics.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2007

Author dc:creator

• Hartman, Jacob M., Ph. D. Massachusetts Institute of Technology

Advisor dc:contributor.advisor

• Deepto Chakrabarty.

• Physics.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/45413

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/45413