Discovery of 172-s Pulsations from a Be/X-ray Binary Candidate AX J0051.6-7311 in the SMC with ASCA

The results from three ASCA observations of AX J0051.6-7311 = RX J0051.9-7311 are reported. Coherent pulsations with a barycentric period of 172.40 +/- 0.03 s were discovered in the third observation, with an exceptionally long exposure time of ~177 ks. The X-ray spectrum was found to remain unchanged through these observations, with a photon index of ~0.9. Energy-resolved pulse profiles in the third observation reveal that the pulsations are mostly due to photons with an energy above ~2 keV. Archival data of ROSAT and Einstein indicate that AX J0051.6-7311 exhibits a flux variation having a factor ~> 20.