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\IBVShead{4760}{30 August 1999}

\IBVStitle{CCD Photometry of the 1999 Outburst of CG CM\lowercase{a}}

\IBVSauth{T.~Kato$^1$, K.~Matsumoto$^1$, R.~Stubbings$^2$}
\vskip 5mm

\IBVSinst{Dept.\ of Astronomy, Kyoto University, Kyoto 606-8502, Japan\\ \indent e-mail: tkato@kusastro.kyoto-u.ac.jp, katsura@kusastro.kyoto-u.ac.jp}

\IBVSinst{19 Greenland Drive, Drouin 3818, Victoria, Australia e-mail: stubbo@qedsystems.com.au}

\SIMBADobj{CG CMa}
\IBVStyp{ dwarf nova }
\IBVSkey{Dwarf novae - astrometry - photometry}

\begintext

CG CMa was discovered by Verlooy on Franklin-Adams plates taken in
1934. According to Duerbeck (1987), the maximum was reached on 1934
January 12. The object had been considered as a possible classical nova,
one of most distant in our Galaxy. However, Duerbeck (1987) identified
the possible quiescent counterpart, and discussed the possibility of
a dwarf nova, based on the outburst amplitude. The cataclysmic
classification, however, became less likely when Zwitter and Munari (1995)
took the spectrum of the suggested quiescent counterpart, yielding
shallow absorption lines resembling those of an isolated white dwarf.

The major breakthrough in understanding the nature of this object
was brought by visual monitoring by R. Stubbings. His second historical
outburst detection on 1999 February 22 (Stubbings 1999) provided a unique
opportunity in unveiling the enigmatic object. The reported magnitude
was $m_{\rm v}=13.7$ on February 22.494 UT\@. On February 21, the object
was invisible below mag 15.0.

The CCD observations were done using an unfiltered ST-7 camera attached to
the Meade 25-cm Schmidt-Cassegrain telescope. The exposure time was 30~s.
The images were dark-subtracted, flat-fielded, and analyzed using the Java$^{\rm TM}$-based PSF photometry package
developed by one of the authors (TK).

\begin{table}
\begin{center}
\caption{Nightly averaged magnitudes of CG CMa}
\begin{tabular}{ccccc}
\hline
JD start$^a$ & JD end$^a$ & mean mag$^b$ & error$^c$ & $N^d$ \\
\hline
51232.074 & 51232.146 & 2.964 & 0.014 & 141 \\
51232.897 & 51232.970 & 3.222 & 0.019 & 74 \\
51234.904 & 51235.160 & 3.574 & 0.010 & 622 \\
51236.935 & 51237.113 & 3.735 & 0.026 & 174 \\
51237.907 & 51238.124 & 3.872 & 0.015 & 486 \\
51238.912 & 51239.120 & 4.107 & 0.030 & 489 \\
51240.911 & 51241.124 & 4.221 & 0.028 & 506 \\
51242.080 & 51242.083 & 4.670 & 2.380 & 2 \\
51242.920 & 51243.061 & 4.139 & 0.119 & 159 \\
51243.910 & 51244.131 & 4.375 & 0.044 & 415 \\
51247.915 & 51247.919 & 5.186 & 0.732 & 10 \\
51248.917 & 51248.920 & 4.275 & 0.241 & 10 \\
51249.913 & 51249.917 & 4.088 & 0.238 & 9 \\
51253.917 & 51253.920 & 4.660 & 0.187 & 10 \\
51255.084 & 51255.091 & 5.044 & 0.816 & 18 \\
51260.926 & 51260.929 & 5.633 & 0.678 & 10 \\
\hline
\end{tabular}

\normalsize
\begin{tabular}{l}
$^a$ ${\rm JD}-2400000$ \\
$^b$ Magnitude relative to GSC 6523.3618 \\
$^c$ Standard error of nightly average \\
$^d$ Number of frames \\
\end{tabular}
\end{center}
\end{table}


\IBVSfig{10cm}{4760-f1.ps}{Overall light curve of CG CMa}

\vskip-11pt
Soon after the initial observation, we realized the outbursting object
is slightly offset from the suggested quiescent counterpart. Using eight
GSC 1.1 stars (mean residual 0\farcs 3), we obtained the coordinates
of 07\hr 04\mm 05\fsec 28, $-$23\deg 45\arcm 34\farcs 2 (J2000.0). Using the 1997 December 27 image taken at Ouda Station, we yielded the coordinates of the proposed quiescent counterparts as 07\hr 04\mm 05\fsec01, $-$23\deg 45\arcm 34\farcs 8 (J2000.0) on the same reference frame. The latter coordinates being within 1\arcs{} of the USNO A1.0 position, it is now evident the outbursting object is different from Duerbeck's (1987) candidate. The finding was confirmed by Henden (1999) and Duerbeck et al.~(1999), who actually resolved the two components. We, in the following analysis, consistently used the newly obtained coordinates of the outbursting object for locating the aperture and PSF, but because of the small separation of two components being always smaller than the seeing size, the obtained fluxes should be considered as the combined light. At the brightest epoch of CG CMa, the contribution of the fainter companion was estimated as 8 \%. The flux of the variable was measured relative to GSC 6523.3618 ($V=11.65$, $B-V=1.07$, Sumner and Henden 1999), whose constancy was confirmed by comparison with GSC 6523.1912. Table 1 summarizes the observations.

Figure 1 illustrates the overall light curve of the 1999 outburst of
CG CMa. We consider the JD 2451260.9 point as the representative brightness
of the companion (= quiescent brightness). The star initially declined at
a rate of 0.17 mag\,d$^{-1}$ for seven days, then the decline became slower.
Even after 22 days after the outburst maximum, the object remained
significantly brighter than quiescence. The outburst thus lasted more than
22 days. The overall light curve strongly resembles those of WZ Sge-type
dwarf novae (eg.\ AL Com, Nogami et al.\ 1997). A possible fading, although
the error was large due to the weather, at JD 2451247.9 (16 day past maximum) may correspond to a ``dip'' observed in AL Com (Nogami et al.\ 1997).

\IBVSfig{10cm}{4760-f2.ps}{Period analysis of CG CMa}

We searched for superhump modulations in the time-series data. There was no
marked oscillation for the first two nights, but the wave emerged on
February 25 (JD 2451235). The Phase Dispersion Minimization (PDM,
Stellingwerf 1978) analysis was applied for the February 25--28 data,
after removing the steadily declining trend. The resultant theta diagram
is shown in Figure 2. The best period was 0.0636 $\pm$ 0.0001 d.

The superhump profile folded by this period is shown in Figure 3. The profile is that of typical, well-developed superhump. The full amplitude is 0.15 mag.

From the typical appearance of superhumps, we conclude CG CMa is a short period SU UMa-type dwarf nova, rather than a classical nova. The long duration and the profile of the outburst particularly suggest a WZ Sge-type classification, which rarely show outbursts. The inferred large outburst amplitude, which is a characteristic of WZ Sge-type dwarf novae, does not contradict with the astrometric implications (Henden 1999).

\newpage

\IBVSfig{10cm}{4760-f3.ps}{Superhump profile of CG CMa}

This work is partly supported by the Grant-in-Aid for Scientific Research
(10740095) of the Japanese Ministry of Education, Science, Culture, and
Sports (TK). Part of this work is supported by a Research Fellowship of the
Japan Society for the Promotion of Science for Young Scientists (KM).

\references

Duerbeck, H. W., 1987, {\it Space Sci.\ Rev.}, {\bf 45}, 58

Duerbeck, H. W., Schmeer, P., Knapen, J. H., Pollacco, D., 1999, {\it IBVS}, No.\,4759

Henden, A., 1999, {\it VSNET chat circulation}, No.\,1694 (also available from\\ http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-chat/msg01694.html).

Nogami, D., Kato, T., Baba, H., Matsumoto, K., Arimoto, J., Tanabe, K., Ishikawa, K., 1997, {\it ApJ.}, {\bf 490}, 840

Stubbings, R., 1999, {\it VSNET alert circulation}, No.\,2666 (also available from\\ http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-alert/msg02666.html).

Stellingwerf, R. F., 1978, {\it ApJ}, {\bf 224}, 953

Sumner, B., Henden, A., 1999, {\it VSNET chat circulation}, No.\,1650 (also available from http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-chat/msg01650.html).

Zwitter, T., Munari, U., 1995, {\it Astron.\ Astrophys.\ Suppl.},
     {\bf 114}, 575

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