Timing the Nearby Isolated Neutron Star Rx J1856.5−3754

RX J1856.5−3754 is the X-ray brightest among the nearby isolated neutron stars. Its X-ray spectrum is thermal, and is reproduced remarkably well by a black-body, but its interpretation has remained puzzling. One reason is that the source did not exhibit pulsations, and hence a magnetic field strength—vital input to atmosphere models—could not be estimated. Recently, however, very weak pulsations were discovered. Here, we analyze these in detail, using all available data from the XMM-Newton and Chandra X-ray observatories. From frequency measurements, we set a 2σ upper limit to the frequency derivative of |ν̇|< 1.3×10−14 Hzs−1. Trying possible phase-connected timing solutions, we find that one solution is far more likely than the others, and we infer a most probable value of ν̇ = (−5.98± 0.14)× 10−16Hzs−1. The inferred magnetic field strength is 1.5× 1013 G, comparable to what was found for similar neutron stars. From models, the field seems too strong to be consistent with the absence of spectral features for non-condensed atmospheres. It is sufficiently strong, however, that the surface could be condensed, but only if it is consists of heavy elements like iron. Our measurements imply a characteristic age of ∼4Myr. This is longer than the cooling and kinematic ages, as was found for similar objects, but at almost a factor ten, the discrepancy is more extreme. A puzzle raised by our measurement is that the implied rotational energy loss rate of ∼3×1030 ergs−1 is orders of magnitude smaller than what was inferred from the Hα nebula surrounding the source. Subject headings: stars: individual: (RX J1856.5−3754) — stars: neutron — X-rays: stars