We report the detection using Advanced Satellite for Cosmology and Astrophysics (ASCA) data of a hard X-ray diffuse emission around the gamma-ray pulsar PSR 1055-52. The pulsar is a middle-aged radio pulsar (P = 0.197 s, tau =P/P dot ~5.3{x10}5 yr) with a pulse profile similar to that of the Crab pulsar and is one of the EGRET gamma-ray pulsars. In the soft X-ray band, ROSAT detected the pulsar as a point source indicating radiation from the neutron star. The ASCA hard band image (2--8 keV), however, showed that the diffuse emission around the pulsar dominates the pulsar itself. Morphology suggests a ring around the pulsar with a cavity in between, although the source is clumpy. The size of the suggested ring is 20' (5.8 Dkpc pc, where Dkpc denotes the distance in kiloparsecs) in diameter. The luminosity of the diffuse source is ~2.0 x 1032 D2_{{kpc}} ergs s-1, which corresponds to 6.8 x 10-3 of the rotation power of the pulsar. Spectral analysis has been performed for the two brightest clumps. The spectrum can be fitted by a Raymond-Smith thermal plasma model with temperatures of 9 keV and 3 keV, respectively, for the two clumps and by a power-law model with photon indices of 1.7 and 2.2. Since the thermal model gives a very high pressure, we prefer the power model. We suggest that the discovered X-ray emission, together with morphology is the synchrotron nebula excited by the pulsar wind of PSR 1055-52.
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