AAS 195th Meeting, January 2000
Session 39. Neutron Stars with Partners
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

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[39.09] Light Curves of Rapidly Rotating Neutron Stars

T.M. Braje, R.W. Romani (Stanford University)

We explore rapid-rotation effects on the thermal surface emission light curves of thermal millisecond pulsars. These rotational effects include Doppler boosts and aberration, general relativistic propagation delays in the strong gravity near the star and, for some cases, frame-dragging perturbations of the light path. With a fiducial model having hot polar caps at 106K and a 3ms spin period, we see large energy-dependent phase lags and asymmetries in the thermal pulses, dominated by the Doppler terms. This effect provides a simple explanation of the soft phase lags reported for post-burst millisecond oscillations in Low Mass X-ray Binaries (Cui, Morgan & Titarchuk ApJL 504, 27; Strohmayer et. al., ApJL 498, 135). Other GR terms may be significant at smaller periods.

With an approximate model for the retarded, strong gravity structure of a magnetic dipole magnetosphere, we also explore the effect of resonant cyclotron scattering on this thermal surface flux of short period pulsars. The scattered flux may be lensed by the star, producing sharp, energy-dependent features in the light curves which can be observed with excellent count statistics. The positions of these features allows one, in principle, to make precision probes of frame-dragging and time delays in strong gravity, since the scattered radiation takes predictable paths through the near-star region.

Doppler effects, at a minimum, must be included as improvements to the standard slowly-rotating (Schwarzschild) models of millisecond pulsars when high quality data from the next generation of space missions are collected. Strong gravitational field effects may in special cases be detectable, as well.

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