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 2 \sigma upper limit to the frequency derivative of | \dot { \nu } | < 1.3 \times 10 ^ { -14 } { Hz s ^ { -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 \dot { \nu } = ( -5.98 \pm 0.14 ) \times 10 ^ { -16 } { Hz s ^ { -1 } } .
The inferred magnetic field strength is 1.5 \times 10 ^ { 13 } { 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 \sim 4 Myr .
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 \sim 3 \times 10 ^ { 30 } { erg s } ^ { -1 } is orders of magnitude smaller than what was inferred from the H \alpha nebula surrounding the source .