2002ApJ...570L..75R

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 October 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 Hz s^–1≤f{dot}≤0 Hz s^–1. The nondetection 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 nondetection implies a radius-to-mass ratio for RX J1856.5-3754 of R/M≲10 km.M^–1_☉.