Thanks to the high counting statistics provided by a recent XMM-Newton observation of RX J1856.5–3754 we have discovered that this isolated neutron star pulsates at a period of 7.055 s. This confirms that RX J1856.5–3754 is similar in nature to the other six thermally emitting , nearby neutron stars discovered in soft X–rays with ROSAT .
The pulsations are detected at consistent periods in several XMM-Newton observations spanning from April 2002 to October 2006 , yielding an upper limit of \dot { P } < 1.9 \times 10 ^ { -12 } s s ^ { -1 } ( 90 % c.l . )
on the period derivative .
This implies a surface magnetic field smaller than 1.2 \times 10 ^ { 14 } G , under the usual assumption of vacuum dipole magnetic braking .
The pulse profile is nearly sinusoidal with a pulsed fraction in the 0.15-1.2 keV range of only \sim 1.2 % , the smallest ever seen in an isolated X-ray pulsar .