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Date: Wed, 26 May 93 16:55:22 JST
From: Wim van Driel <lvandri@c1.mtk.nao.ac.jp>
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\begin{document}
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\begin{center}
{\Large \bf BVRI Photometry of SN1993J in M 81} \\
\smallskip
{\Large I. Period 01-30 April 1993} \\
\medskip
Wim {\sc van Driel}, Shigeomi {\sc Yoshida}, Yoshikazu {\sc Nakada},
Tsutomu {\sc Aoki}, \\
Takao {\sc Soyano}, Ken'ichi {\sc Tarusawa} and Takashi {\sc Ichikawa} \\
{\it Kiso Observatory {\footnote{ Kiso Observatory is an institution with
105-cm Schmidt telescope facilities open to outside users}, Institute of
Astronomy, University of Tokyo, Mitake, Kiso, Nagano 397-01, Japan } \\
\smallskip
{\rm Takuya {\sc Kakehashi}} \\
{\it Astronomical Institute, Tohoku University, Aoba, Sendai 980, Japan } \\
\smallskip
{\rm Ken'ichi {\sc Nomoto}} \\
{\it Department of Astronomy, University of Tokyo, Bunkyo-ku,
Tokyo 113, Japan } \\
{\rm and} \\
\smallskip
{\rm Ken'ichi {\sc Wakamatsu}} \\
{\it Physics Department, Gifu University, Yanagido, Gifu 501-11, Japan } \\
\smallskip
\rm Submitted to {\it P.A.S.J. (Letters)} \\
Received: 19 May 1993; Revised: 20 May \\
\end{center}
\sect{Abstract}
We present BVRI photometry of supernova SN1993J in Messier
81 for the period of 01--30 April 1993 obtained
with the 105-cm Schmidt telescope of Kiso
Observatory. The magnitudes were determined
through a comparison with two nearby reference stars.
The estimated accuracy of the data is $\pm$0.02 mag.
We also present an R-band pre-discovery image taken on March 25.6(UT),
2.7 days before the first detection,
from which we derive a (preliminary) upper limit of $\gsim$18 for SN1993J.
The supernova's light curve is unique, as it shows neither a typical
plateau nor a linear decline, which suggests that it constitutes a new
subclass of Type II supernova and that its progenitor was not a typical red
supergiant.
\smallskip
\noindent {\bf Key words:} Supernovae: general -
Supernovae: individual: SN1993J
\bigskip
\sect{1. Introduction}
Supernova SN1993J in the nearby spiral galaxy M 81 (= NGC 3031), which
was first detected on March 28.30 (UT) 1993, is the brightest Type II
supernova observable from the Northern hemisphere in this century.
The determination of its light curve is especially
important since an accurate distance to M 81, based on
Hubble Space Telescope observations, will be
available in the near future.
\sect{2. Observations and data reduction}
The 105-cm F/3.1 Kiso Schmidt telescope was equipped with a
single-chip CCD camera in the prime focus
which has an effective imaging area of
1000$\times$1018 pixels of \as{0}{75}$\times$\as{0}{75} each, giving a
field-of-view of \am{12}{5}$\times$\am{12}{7}
($\Delta\alpha\times\Delta\delta$).
Johnson B and V, and Cousins R and I-band
filters were used. Exposure times were short (6-120 sec) in order
to prevent image saturation.
On 18 nights in the period of 1--30 April 1993 a total of about 700
usable CCD images were made of M 81, see Table 2. The data reduction
was performed in a standard way with the IRAF software package,
using bias frames and dome flat fields.
Since most of the observations were made under non-photometric conditions
we determined the magnitude of the supernova by comparison with two
photometric reference stars: GSC 4383-0928 (hereafter star B) and
GSC 4383-0434 (star 1), adopting the magnitudes from Corwin (1993).
We measured magnitudes in the images with the APPHOT aperture photometry
package in IRAF, using circular apertures of 5 times the FWHM diameter of
the stellar images (typically 5$''$).
We did not apply any color terms to the magnitudes, as they
have not been determined accurately yet for the Kiso CCD observations, which
started only recently. However, a preliminary analysis of standard star
observations show them to be $\lsim$0.04 mag in all
bands.
\sect{3. Results}
The resulting magnitudes and colors are listed in Table 1.
These are mean values per night, derived from the number of frames per
band listed in the last column (nobs), and averages of the magnitudes
determined with respect to each of the two local standard stars (B and 1)
mentioned above. We estimate an accuracy of about $\pm$0.02 mag for the
Kiso data from a comparison of the values derived with respect to
the two standards.
The light curve in all four bands is shown in Figure 1, together with
photoelectric and CCD magnitudes obtained with other
telescopes, as compiled by T. Kato.
Our values agree quite well with the other magnitudes,
typically to within $\pm$0.03 mag of the average value.
The B-V, V-R and V-I colors of the supernova as function of time are shown
in Fig.2, together with B-V data compiled by T. Kato; the estimated error in the
Kiso data is about $\pm$0.03 mag.
\subsect{3.1. The March 25.6 pre-discovery image}
On March 25.60 (UT) 1993 we did not detect the supernova in a 5 min. R-band
exposure of M 81 made through a thin layer of clouds, as part of a supernova-search
program. This observation
is of interest since it was made 2.7 days before the first detection
on March 28.30 at m$\sim$13.7 (Neely 1993, unfiltered CCD), and 2.5 days
after the last observation of the quiescent precursor at
{\it m}$_{\rm R}$=19.9 by Prugniel (1993). We first estimated
(van Driel \& Wakamatsu 1993) an upper limit of about 17 mag from this
image, which we later changed to about 18 mag. We now find these upper limits too
conservative, however.
Since estimating an upper limit for the magnitude of the supernova is encumbered by
the bright disk emission, we made an unsharp masked image by subtracting a smoothed
image from the original, in order to remove the extended emission. This masked
image clearly shows numerous faint stars, but a reliable upper limit
(expected to lie in the range of $\sim$18--19.5 mag) can only be estimated from
it when we have determined magnitudes for the faint stars in the field.
\subsect{3.2. The unique light curve}
SN1993J was identified as a Type II supernova (SN II) from the very blue,
featureless continuum and a weak \Ha\ line (e.g., Taniguchi et al. 1993).
Its light curve reached the first maximum in less than about 4 days,
declined in about 5 days, rose to a second maximum in 10--15 days
depending on wavelength, and then declined again.
SNe II have been sub-classified into a plateau type (SN II-P) and a
linear-decline type (SN II-L) according to the light curve shape. A
comparison (see Fig. 3) of the SN1993J light curve with those
of two SNe II-P and two SNe II-L clearly indicates the unique behavior of
1993J (see also Patat et al. 1993 and reference therein),
which shows neither a clear plateau nor a monotonic decline. SN1993J is
also much redder than the others. Such differences imply that SN1993J
constitutes a new subclass of SNe II and that its progenitor was not a typical
K0-type massive red supergiant (Nomoto et al. 1993).
Obviously, further observations are necessary for a precise
classification of SN1993J. We intend to continue the BVRI photometry at
Kiso Observatory.
\medskip
We would like to thank Dr. T. Kato of Kyoto University for compiling
the continuous stream of photometric data and making them available to the
astronomical community.
\sect{References}
\begin{list}{}{\setlength{\leftmargin}{1cm}\setlength{\itemindent}{-1cm}
\setlength{\topsep}{0cm}\setlength{\itemsep}{-0.2cm}} \item
\vspace*{-0.6cm}
\item
Corwin H.C., 1993, IAU Circ. 5742
\item
Neely A., 1993, in IAU Circ. 5740
\item
Nomoto K., Suzuki T., Shigeyama T., Kumagai S., Yamaoka H., Saio
H., 1993, Nature, submitted
\item
Patat F., Barbon R., Cappellaro E., Turatto M., 1993, A\&AS 98, 443
\item
Prugniel P., 1993, in IAU Circ. 5742
\item
Taniguchi Y., Murayama T., Sato Y., Yadoumaru Y., Ohyama Y., Kosugi J.,
Yoshida M., Kurakami T., 1993, PASJ, in press
\item
van Driel W., Wakamatsu K., 1993, IAU Circ. 5737
\end{list}
\sect{Figure captions}
\begin{description}
\item
[\bf Figure 1.] The light curve of SN1993J in the B, V, R and I-band. Shown
are the mean Kiso data per night ($\Box$), as well as photoelectric
and CCD measurements ($\bullet$) taken from various sources, as compiled
by Dr. T. Kato of Kyoto University. Estimated uncertainty is $\pm$ 0.02
mag in the Kiso data. The magnitude scale shown is for the B-band data,
and the V, R and I data have been shifted by -0.3, -0.6 and
-0.9 mag, respectively, with respect to it.
\item
[\bf Figure 2.] Colors of SN1993J as function of time, determined from
the Kiso observations, plus B-V data compiled by Dr. T. Kato of Kyoto
University. The estimated error of the Kiso data is $\pm$ 0.03 mag.
Note that the zero points of the V-R and V-I data have been shifted with
respect to that of the B-V color (shown along the vertical scale) by
-0.4 and 0.4 mag, respectively, for the sake of clarity.
\item
[\bf Figure 3.] Comparison of the V-band light curve of SN1993J (all data
available) with
those of four other Type II supernovae: the plateau-type SNe 1988A and 1969L,
and the linear-descent types 1980K and 1979C.
The light curves have been shifted along the time axis, in order to let
the estimated onset of the explosion coincide in time. The curves (from
top to bottom) have been shifted in brightness by -0.3, -1.5, 0.5, 3.0 and
3.5 mag, respectively, in order to facilitate the comparison.
\end{description}
\begin{table}
\begin{tabular}{cccccllll}
\multicolumn{9}{c}{{\bf Table 1.} Photometry of SN1993J - 01--30 April 1993} \\
\medskip \\
\hline
Date (UT) & B & V & R & I & B--V & V--R & V--I & {\footnotesize nobs} \\
{\footnotesize (yyyy mm dd.ddd)} & {\footnotesize (mag)} &
{\footnotesize (mag)} & {\footnotesize (mag)} & {\footnotesize (mag)} &
{\footnotesize (mag)} & {\footnotesize (mag)} & {\footnotesize (mag)} & \\
\hline
1993 04 01.499 & 11.49 & 11.21 & 10.98 & 10.74 & 0.28 & 0.23 & 0.47 & 2 \\
1993 04 02.511 & 11.83 & 11.48 & 11.21 & 10.94 & 0.35 & 0.27 & 0.54 & 2 \\
1993 04 03.530 & 12.25 & 11.76 & 11.42 & 11.14 & 0.49 & 0.34 & 0.62 & 2 \\
1993 04 05.539 & 12.31 & 11.85 & 11.48 & 11.26 & 0.46 & 0.37 & 0.59 & 6 \\
1993 04 06.593 & 12.25 & 11.78 & 11.37 & 11.20 & 0.47 & 0.41 & 0.58 & 48 \\
1993 04 07.667 & 12.18 & 11.65 & 11.23 & 11.14 & 0.53 & 0.42 & 0.51 & 2 \\
1993 04 08.620 & 12.05 & 11.56 & 11.19 & 11.04 & 0.49 & 0.37 & 0.51 & 21 \\
1993 04 10.530 & 11.83 & 11.31 & 10.98 & 10.85 & 0.52 & 0.33 & 0.46 & 44 \\
1993 04 11.273 & 11.75 & 11.25 & 10.88 & 10.79 & 0.50 & 0.37 & 0.46 & 6 \\
1993 04 12.639 & 11.59 & 11.12 & 10.77 & 10.65 & 0.47 & 0.35 & 0.47 & 5 \\
1993 04 14.551 & 11.47 & 10.97 & 10.63 & 10.53 & 0.50 & 0.34 & 0.44 & 8 \\
1993 04 15.463 & 11.44 & 10.93 & 10.58 & 10.47 & 0.51 & 0.35 & 0.42 & 4 \\
1993 04 16.475 & 11.42 & 10.88 & 10.54 & 10.44 & 0.54 & 0.34 & 0.44 & 4 \\
1993 04 17.468 & 11.42 & 10.86 & 10.52 & 10.41 & 0.56 & 0.34 & 0.47 & 4 \\
1993 04 18.476 & 11.48 & 10.86 & 10.51 & 10.41 & 0.62 & 0.35 & 0.45 & 3 \\
1993 04 19.454 & 11.51 & 10.87 & 10.51 & 10.39 & 0.64 & 0.36 & 0.48 & 4 \\
1993 04 20.444 & 11.58 & 10.91 & 10.51 & 10.40 & 0.67 & 0.40 & 0.51 & 4 \\
1993 04 30.487 & 12.87 & 11.69 & 11.06 & 10.86 & 1.18 & 0.63 & 0.83 & 4 \\
\hline
\end{tabular}
\end{table}
\end{document}
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