The Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X‐Ray Bursts

We present a comprehensive observational and theoretical analysis of theamplitudes and profiles of oscillations that occur during thermonuclear X-raybursts from weakly-magnetized neutron stars in low mass X-ray binaries. Oursample contains 59 oscillations from six sources observed with the Rossi X-rayTiming Explorer. The oscillations that we examined occurred primarily duringthe decaying portions of bursts, and lasted for several seconds each. We findthat the oscillations are as large as 15% during the declines of the bursts,and they appear and disappear due to intrinsic variations in their fractionalamplitudes. However, the maxima in the amplitudes are not related to theunderlying flux in the burst. We derive folded profiles for each oscillationtrain to study the pulse morphologies. The mean rms amplitudes of theoscillations are 5%, although the eclipsing source MXB 1659-298 routinelyproduces 10% oscillations in weak bursts. We also produce combined profilesfrom all of the oscillations from each source. Using these pulse profiles, weplace upper limits on the fractional amplitudes of harmonic and half-frequencysignals of 0.3% and 0.6%, respectively (95% confidence). We then compare thepulse morphologies to theoretical profiles from models with one or twoantipodal bright regions on the surface of a rotating neutron star. We findthat if one bright region is present on the star, it must either lie near therotational pole or cover nearly half the neutron star in order to be consistentwith the observed lack of harmonic signals. If an antipodal pattern is present,the hot regions must form very near the rotational equator. We discuss howthese geometric constraints challenge current models for the production ofbrightness variations on the surface of a neutron star. (abridged)Comment: 12 pages, including 8 figures. To appear in ApJ v 581 No 1, Dec 10 2002. Revised to include discussion of scattering on harmonic amplitude