Electronic Telegram No. 828 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://cfa-www.harvard.edu/iau/cbat.html SUPERNOVA 1981K IN NGC 4258 S. Immler, Center for Research and Exploration in Space Science and Technology and Goddard Space Flight Center, NASA; and B. Li, Y. Yang, and A. Wilson, University of Maryland, College Park, report the discovery of x-ray emission from SN 1981K (cf. IAUC 3803) in archival Chandra X-Ray Observatory data. The supernova is detected at a 4.5-sigma level in merged 36500-s Chandra ACIS-S observations taken on 2000 Apr. 17 (Observation Identifier 350, PI Wilson) and 2001 May 28 (O.I. 1618) with an offset from the optical position of < 1 arcsec. The net count rate of (4.8 +/- 1.1) x 10**(-4) counts/s corresponds to an unabsorbed (0.2-10 keV) flux and luminosity of (1.9 +/- 0.4) x 10**(-15) erg/cm/cm/s and (1.2 +/- 0.3) x 10**(37) ergs/s, respectively, for an adopted thermal plasma spectrum with a temperature of kT = 1 keV, a Galactic foreground column density of 1.16 x 10**(20) (Dickey and Lockman 1990, ARAA 28, 215), and a distance of 7.2 Mpc (Herrnstein et al. 1999, Nature 40, 539). No statistically significant difference in the fluxes for the two individual epochs is found. No x-ray source is detected at the position of the supernova in previous Einstein (1978) and Rosat observations (1992-1994). Assuming an initial velocity of the outgoing supernova shock of 10000 km/s and that the x-ray luminosity is caused by interaction of the supernova shock with circumstellar material, a mass-loss rate of the progenitor star around 1.5 x 10**(-5) solar masses/year is required. The high mass-loss rate is consistent with those of red supergiants, which confirms that SN 1981K was a type-II supernova, as indicated by the detection at radio wavelengths (Van Dyk et al. 1992, Ap.J. 396, 195). NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2007 CBAT 2007 February 1 (CBET 828) Daniel W. E. Green