A Deep Search for Pulsations from the Nearby Isolated Neutron Star Rx J1856.5−3754

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 (DDT) Chandra LETG/HRC-S observation completed on 2001 Oct 15. No pulsations were detected. We find a 99% confidence upper limit on the pulsed fraction of ∼ 4.4% for worst-case sinusoidal pulsations with frequency . 50Hz and frequency derivatives −5 × 10 ≤ ḟ ≤ 0Hz s. This lack of detection is consistent with a neutron star with R ≈ 6 km and M ≈ 1 M⊙ at a distance ∼ 60 pc, as determined by Pons et al. (2002). The strong gravitational field of such a star sufficiently deflects X-ray emission from the emitting regions so as to prevent significant modulation of the signal. Subject headings: stars: individual (RX J1856.5−3754) — stars: neutron — X-rays: stars