DETECTION OF A HARD X-RAY PLERION IN THE CANDIDATE HISTORICAL REMNANTG11.2-0.3

We present the results of a 32 ks Advanced Satellite for Cosmology and Astrophysics (ASCA) observation of G11.2-0.3, which is by far the str ongest candidate for the remnant of the historical supernova SN 386. A center-brightened, hard, nonthermal X-ray source was found within the remnant, which we interpret to be plerionic emission due to an embedd ed pulsar. Our observations indicate that the remnant is a member of t he class of ''composite'' remnants, as was hinted by previous observat ions. The central emission is not pulsed down to a detection limit of similar to 10(33) ergs s(-1). It could be that the putative pulsar is not beamed in our direction. We also argue that the distance and surfa ce brightness of G11.2-0.3 imply that the remnant is young and very li kely to be the counterpart of the supernova observed by the Chinese in A.D. 386, making it the youngest known Galactic composite. The possib le similarity between the spectra of G11.2-0.3 and 3C 58, two of the y oungest plerions, is brought out. These spectra are observed to be qui te different from that of the Crab, which is very similar in age. Last , we remark that only after a sustained observational effort spanning two decades by various groups is the true nature of G11.2-0.3 at last clear. To us this has been a valuable lesson that highlights the impor tance of a detailed multiwavelength effort, with emphasis on high-freq uency radio and X-ray spectral observations for proper classification of remnants. The overabundance of pure shell remnants must be an artif act of the lack of such observations and the selection effects that wo rk against the discovery of plerions.