[IAUC] CBET 828: 20070201 : SUPERNOVA 1981K IN NGC 4258

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                                                  Electronic Telegram No. 828
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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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).


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                         (C) Copyright 2007 CBAT
2007 February 1                  (CBET 828)               Daniel W. E. Green