X-ray nebula around the gamma-ray pulsar PSR 1055-52

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) over dot approximate to 5.3 x 10(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 D-kpc pc, where D-kpc denotes the distance in kiloparsecs) in diameter. The luminosity of the diffuse source is approximate to 2.0 x 10(32) D-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.