8th HEAD Meeting, 8-11 September, 2004
Session 25 X-ray Binaries and White Dwarfs
Poster, Friday, September 10, 2004, 9:00am-10:00pm

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[25.11] Why is it Difficult to Find Millisecond X-Ray Pulsars?

M. I. Krauss, D. Chakrabarty (MIT)

Most neutron stars in low-mass X-ray binaries are believed to have millisecond spin periods, but it has proven surprisingly difficult to detect persistent millisecond pulsations in these systems---fewer than 10 percent of neutron stars in LMXBs are persistent pulsars. At least two explanations for this have been posited: that most neutron stars are too weakly magnetized to channel the accretion flow onto their magnetic poles (e.g., Cumming et al. 2001), or that most LMXBs contain a scattering region of high enough optical depth to suppress pulsed emission (e.g., Titarchuk et al. 2002). Both of these effects should have measurable consequences in the X-ray spectra of LMXBs. We present here a systematic comparison of the X-ray spectra of neutron star LMXBs with and without millisecond pulsations using archival data from the Rossi X-Ray Timing Explorer. In particular, we take care to compare only sources in similar spectral states as determined by their position on an X-ray color-color diagram. We use these results to infer physical similarities and differences between the millisecond X-ray pulsars and their non-pulsing cousins.

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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.