Abstract
We present the results of a deep search for pulsations from the nearby isolated neutron star RX J1856.5-3754 using the 450 ks Director's Discretionary Time Chandra observation completed on 2001 Oct 15. No pulsations were detected. We find a 99% confidence upper limit on the pulsed fraction of ~4.5% for worst-case sinusoidal pulsations with frequency <~50 Hz and frequency derivatives -5x10^-10 <= f-dot <= 0 Hz/s. The non-detection of pulsations is most likely due to an unfavorable viewing angle or emitting geometry. Such emitting geometries are much more likely to occur for more ``compact'' neutron stars which show increased gravitational light-bending effects. In this case, the non-detection implies a radius/mass ratio for RX J1856.5-3754 of R/M <~10 km/Msun.
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