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 X-ray observatory on 2000 June 24. A timing analysis of the data yielded a periodic modulation with a period P = 5.1571696(-1.36x10-4)(+1.57x10-4) s. Using ROSAT HRI archived observations we detected a period P = 5.1561274 +/- 4.4 x 10(-4) s and determined period derivative The detection of this period derivative (P) over dot = (0.7-2.0) x 10(-11) s s(-1). The detection of this period and (P) over dot indicates that RBS 1223 has a "characteristic" age of 6000-12 000 years and huge magnetic field at the surface (B-dipole approximate to (1.7-3.2) x 10(+14) G) typical for anomalous X-ray pulsars (AXPs).