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Date: Thu, 8 Apr 93 17:28:07 CDT
From: gav@astro.as.utexas.edu (Gerard de Vaucouleurs)
Subject: SN1993J, Note2 from G.V.
Hi!
Here are a few results from a preliminary analysis of the V light curve.
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{\bf SN1993J: Note no. 2 (4/8/93)}
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Including the pre-discovery CCD observation by A. Neely on March 28.3
= JD 074.80 (13.8 corrected to 13.2 for zero point), a cubic fit of the
visual light curve reduced to the TB (SSC) V scale [*A = 11.4, *B = 11.9,
*C = 14.0], with 70 data points (67 after 2$\sigma$ rejection of 3 residuals
larger than 0.5 mag) from March 28.30 to April 7.55, gives\\
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$v = 32.403 - 6.9561 x + 0.70548 x^{2} - 0.022395 x^{3}$\\
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where x = JD - 2449070.0.\\ \par
Extrapolation to m $\simeq$ 20.0 suggests an onset date of JD 072.3 = March
25.8 with an initial slope dm/dt $\simeq$ + 3 mag/day.
The onset date is consistent with the non-detection (R $>$ 17.0) on March
25.6 by van Driel and with v $>$ 14.0 on March 26.9 (x = 3.4, v = 16.0) by
Pujol and Ripero.\\
The visual maximum remains unchanged at v = 10.5 $\pm$ 0.1 on or about
JD 078.2 = March 31.7 (previous estimate JD 077.7).\\
The negative photographic observation by Merlin (m $>$ 16.0 on March 27.91
= JD 074.41) - when the calculated magnitude was v = 13.5 - remains a puzzle.
The limiting magnitude of the original film should be re-determined using
the SSC comparison stars.\\
The 'unfiltered' CCD observations are in general agreement with the visual
magnitudes but with a larger scatter ($\sigma$ = 0.27 mag). As previously
reported, however, the V-band CCD observations, corrected for zero point,
depart from the visual curve by up to 0.5 mag during the early decline phase.
There is better agreement now. \\
The reality or otherwise of the suspected pre-maximum 'jump' from
v $\simeq$ 11.2 on March 29 to 10.4 on March 30 depends critically on
the (mainly unreported) magnitudes adopted on these two dates for the
comparison stars. Given the standard deviation of 0.2 mag of the visual
observations from the cubic fit above, the observations of March 29 and
30 are all well within $\pm 2\sigma$ of the mean curve. These early
observations need to be carefully re-examined by the original observers
and re-reduced to the scale of the T-B Atlas (or to a revised scale still
to come).\\
The B-V color has been increasing at a steady (linear) rate of + 0.015
mag/day since JD 078.0 March 31.5. The two initial observations by Unger
on JD 077, suggesting a much more rapid initial reddening should be checked.
I am not aware of any previous supernova having shown such a discontinuity
in its rate of reddening.\\
Grateful acknowlegement is made to Dr. T. Kato for his many communications
and valuable 'photometric history' of SN1993J. \\
G. de Vaucouleurs (gav@astro.as.utexas.edu)
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