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IAUC 7318: 1999em

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                                                  Circular No. 7318
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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SUPERNOVA 1999em IN NGC 1637
     D. W. Fox and W. H. G. Lewin, Massachusetts Institute of Technology,
report, on behalf of a large collaboration: "SN 1999em has been imaged twice
with the Chandra X-ray Observatory (ACIS-S).  On Nov. 1-2, 112 photons were
detected during a 32-ks observation; preliminary spectral fits indicate a
soft source spectrum (photon index approximately 1.8) with a 0.1-8.0-keV flux
of 2 x 10**-14 erg cm-2 s-1, corresponding to an unabsorbed source luminosity
of 2 x 10**38 erg/s at 7.8 Mpc (Sohn and Davidge 1998, A.J. 115, 130).  On
Nov. 11-12, 60 photons were detected during a 32-ks observation; assuming a
(relatively) constant spectrum, this corresponds to a source luminosity of
1 x 10**38 erg/s."
     C. K. Lacey, Naval Research Laboratory; R. A. Sramek, National Radiohttp://rsd-www.nrl.navy.mil/7214/weiler/sne-home.html.
Whether our upper limits imply such a low CSM density that the radio emission
is undetectable, or such a high CSM density that it is optically thick to
radio emission at this early epoch, remains to be determined through further
     Lewin adds: "The combination of x-ray detection and radio nondetection
unusual.  The only other known example is the type-IIP SN 1994W in NGC 4041,
which has never been detected in the radio (3 sigma upper limit <1.1 x
10**26 erg s-1 Hz-1 at 8.4 GHz on 1998 Feb. 10) but has a reported ROSAT HRI
detection of approximately 8 x 10**39 erg/s during 1997 Oct. 21-23 (Schlegel
1999, Ap.J. in press; astro-ph/9910425).  This may imply a significant
difference in the physical properties of the presupernova systems between the
more readily radio and x-ray detectable type-IIL supernovae, such as 1979C and
1980K, and the type-IIP progenitor systems.  We encourage acquisition of
additional optical spectra of SN 1999em while its x-ray emission is still

                      (C) Copyright 1999 CBAT
1999 November 26               (7318)              Brian G. Marsden

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