SIMBAD references

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 blackbody, 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 2 σ upper limit to the frequency derivative of|ν{dot}|<1.3x10^–14 Hz/s. 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 ν{dot}=(-5.98±0.14)x10^–16 Hz/s. The inferred magnetic field strength is 1.5x10¹³ 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 noncondensed 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 ∼4 Myr. This is longer than the cooling and kinematic ages, as was found for similar objects, but at almost a factor 10, the discrepancy is more extreme. A puzzle raised by our measurement is that the implied rotational energy loss rate of ∼3x10³⁰ erg/s is orders of magnitude smaller than what was inferred from the Hα nebula surrounding the source.