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 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 Ṗ < 1.9× 10 s s (90% c.l.) on the period derivative. This implies a surface magnetic field smaller than 1.2×10 G, under the usual assumption of vacuum dipole magnetic braking. The pulse profile is nearly sinusoidal with a pulsed fraction of only ∼1.5%, the smallest ever seen in an isolated X-ray pulsar. Subject headings: stars: individual (RX J1856.5-3754) – stars: neutron – X–rays: pulsars