Discovery of a 69 millisecond X-ray pulsar: A compact source in the vicinity of the supernova remnant RCW 103

We report a rare discovery of a fast (69 ms) pulsar using X-ray data acquired with the Advanced Satellite for Cosmology and Astrophysics (ASCA). The highly significant detection arises from the serendipitous ASCA X-ray source AXS J161730-505505, located near the young Galactic supernova remnant (SNR) RCW 103. The epoch-folded light curve displays a single asymmetric pulse profile with a pulse fraction of similar to 50%. Spectral fits to the high-energy (i.e., greater than 3.5 keV) source data using a simple absorbed power-law model, assuming the hydrogen column density to the SNR, gives a photon index gamma = 1.6(-0.3)(+0.2) and an unabsorbed flux of similar to 6.4 x 10(-12) ergs cm(-2) s(-1). The extracted source spectrum below 3.5 keV is contaminated by mirror-scattered soft thermal emission from the similar to 9' diameter RCW 103, whose projected center is located just similar to 7' away. If the pulsar is associated with the remnant, the implied neutron star velocity is at the high end of the pulsar velocity distribution for the distance estimates to the remnant but still plausible. We suggest that AXS J161730-505505 is likely a young rotation-powered pulsar with a characteristic spin-down age of similar to 8000 yr. The physical association of the pulsar with RCW 103 and its central source, 1E 161348-5055, remains intriguing.