| Résumé | We report the detection, in archival ROSAT and ASCA observations, of X-ray emission from the direction of DA 495 (G65.7+1.2), a likely supernova remnant of uncertain classification but with similarities to the Crab Nebula. An unusual feature of the radio nebula is its annular morphology, with a flux minimum at the geometrical center. In the soft X-ray band, the ROSAT data reveal a compact source near the edge of the central radio "hole"; the hard X-ray morphology, at the limit of ASCA's angular resolution, is suggestive of extended emission coincident with the ROSAT source. The X-ray flux is roughly constant with time, and its spectrum is well described by a power law with photon index ~1.7. Taken together, this evidence suggests identification of the X-ray source with a magnetospherically active neutron star and its associated wind nebula. Timing analysis of the ASCA data yields only a weak upper bound on pulsations with periods >~30 ms. These results reveal for the first time the high-energy engine that powers the DA 495 radio nebula and strengthen its classification as a plerionic supernova remnant, one that may represent the poorly explored late evolutionary stages of Crab-like nebulae. |
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