XMM-Newton discovery of 7 s pulsations in the isolated neutron star RX J1856.5-3754

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 Pdot<1.9x10^{-12} s/s (90% c.l.) on the period derivative. This implies a surface magnetic field smaller than 1.2x10^{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 ~1.2%, the smallest ever seen in an isolated X-ray pulsar.