Central Bureau for Astronomical Telegrams
IAUC 5768: 1993J
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Circular No. 5768
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM EASYLINK 62794505
MARSDEN@CFA or GREEN@CFA (.SPAN, .BITNET or .HARVARD.EDU)
SUPERNOVA 1993J IN NGC 3031
M. P. Rupen and R. A. Sramek, National Radio Astronomy
Observatory; S. D. Van Dyk and K. W. Weiler, Naval Research Laboratory;
and N. Panagia, Space Telescope Science Institute, communicate:
"Two separate observing sessions of SN 1993J, made on Apr. 13 at 6
cm (4.86 GHz) with the Very Large Array in 'B' configuration, yield
flux density values of 0.33 +/- 0.08 and 0.28 +/- 0.05 mJy. These
measurements confirm the possible detection of radio emission at 6
cm on that same day recently reported by Strom (IAUC 5762). Such a
value is also consistent with the model results reported by Panagia,
Van Dyk, and Weiler on IAUC 5762. From the model, SN 1993J is
expected to strengthen rapidly in this radio band and reach a peak of
about 30 mJy by May 20. Radio monitoring with the VLA is continuing
on a nearly daily basis at this and other wavelengths in order
to follow the supernova's rapidly changing behavior."
S. Radford, R. Neri, S. Guilloteau, and D. Downes, Institut de
Radio Astronomie Millimetrique, Grenoble, write: "We obtained
observations of SN 1993J at R.A. = 9h51m19s.03, Decl. = +69 15'26".5
(equinox 1950.0; +/- 0".5) with the IRAM interferometer. At 87 GHz,
the flux was 11 mJy on Apr. 9 and 11, 13.5 mJy on Apr. 12, and 12.5
mJy on Apr. 15. At 110 GHz, the flux was 10 mJy on Apr. 16 (all
values +/- 2 mJy). These millimeter fluxes are less than those
measured at 15 and 22 GHz at Cambridge and with the VLA, indicating
non-thermal emission with a spectral index of about -0.5 on Apr. 9.
Within the 3-mm band, the spectral index cannot be derived reliably
because of the flux uncertainties. For a uniform expansion at
10 000 km/s and a distance of 3 Mpc, the angular diameter of the
remnant is about 0".00008 by Apr. 15. Hence the brightness temperature
at 87 GHz is 3 x 10E8 K. The decrease in flux between cm and
mm wavelengths shows the foreground ionized plasma (presumably the
remnant of the pre-supernova circumstellar envelope) is optically
thin at mm wavelengths. The nonthermal radio spectrum is cut off
exponentially at lower frequencies by the opaque foreground plasma.
The lack of dramatic time variation in the 3-mm flux during Apr. 9-
16 is further evidence that the foreground plasma has become
transparent in the mm range."
Corrigendum. On IAUC 5743, line -4, for indentification
read identification
1993 April 20 (5768) Daniel W. E. Green
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