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, τ= P / ≈ 5.3 × 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 ≈ 2.0 × 10 32 D ergs s -1 , which corresponds to 6.8 × 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.