Discovery of 5.16 s pulsations from the isolated neutron star RBS 1223

The isolated neutron star candidate RBS 1223 was observed with the Advanced CCD Imaging Spectrometer aboard the Chandra   observatory on 2000 June 24. A timing analysis of the data yielded a periodic modulation with a period $P=5.1571696^{+1.57\times 10^{-4}}_{-1.36\times 10^{-4}}$ s. Using ROSAT  HRI archived observations we detected a period $P=5.1561274\pm 4.4\times 10^{-4}$ s and determined period derivative $\dot{P}=(0.7 {-} 2.0)\times 10^{-11}$  s ${\rm s^{-1}}$. The detection of this period and $\dot{P}$ indicates that RBS 1223 has a “characteristic” age of 6000-12 000 years and huge magnetic field at the surface ($B_{\rm dipole} \approx (1.7{-} 3.2) \times 10^{+14} {\rm G}$) typical for anomalous  pulsars (AXPs).