\documentstyle[twoside,epsf]{article}

\input{ibvs2.sty}

\begin{document}

\IBVShead{4767}{7 September 1999}

\IBVStitletl{CCD Photometry }{ of the 1998 December Outburst of AQ E\lowercase{ri}}

\IBVSauth{T.~Kato, K.~Matsumoto}

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

\SIMBADobj{AQ Eri}
\IBVStyp{ UGSU }
\IBVSkey{Dwarf novae - photometry}

\begintext

AQ Eri is an SU UMa-type dwarf nova (Kato 1991), whose superhump period was reported to be 0\fday06225. Thorstensen et al.\ (1996) yield a spectroscopic orbital period of 0\fday06093, which places AQ Eri among
short orbital period systems. The relatively short orbital period and
the relative low frequency of outbursts make AQ Eri a candidate for
an intermediate system between usual SU UMa-type dwarf novae and
WZ Sge-type stars (cf.\ Nogami et al.\ 1996 for the discussion of CT Hya).

The outburst was detected by Hers (1998) on 1998 December 18.332
at $m_{\rm v}=13.5$. The lack of detected superoutburst since 1995 and
the brightness of the outburst prompted our time-series CCD
observations.

The 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). The magnitudes of the variable was measured relative to GSC 4758.784 (Tycho $V=10.79$, $B-V=+0.62$), whose constancy was confirmed by comparison with GSC 4758.334 ($V=10.93$, $B-V=+1.24$). Table~1 summarizes the observations.

\IBVSfig{10cm}{4767-f1.ps}{Overall light curve of AQ Eri}

\vskip-11pt
Figure 1 illustrates the overall light curve of the present observation.
Nightly averaged relative magnitudes and estimates of errors are plotted for the last two nights. The light curve shows a steep decline characteristic of a normal outburst. The data of the first two nights, when the variable was in outburst, were analyzed, after subtracting the linear declining trend, using the Phase Dispersion Minimization (PDM) method (Stellingwerf 1978). The result gave no very convincing periodicity, but seemed to have a weak signal around the reported orbital period, and not around the superhump period.

\begin{table}
\begin{center}
\caption{Summary of observations}
\begin{tabular}{ccccc}
\hline
JD start$^a$ & JD end$^a$ & $N^b$ \\
\hline
51167.001 & 51167.130 & 247 \\
51167.970 & 51168.121 & 244 \\
51168.960 & 51169.238 & 635 \\
51169.923 & 51169.969 & 95 \\
51170.942 & 51170.944 & 5 \\
\hline
\end{tabular}

\normalsize
\begin{tabular}{l}
$^a$ ${\rm JD}-2400000$ \\
$^b$ Number of frames \\
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10cm}{4767-f2.ps}{Hump profile of AQ Eri}

\vskip-11pt
In order to more closely examine the potential orbital modulation, the data of the first two nights were phase-averaged using the reported period of 0.06093 d. The result is shown in Figure 2, which shows the existence of a double-wave modulation in one orbital cycle. The amplitude of the modulation was $\sim$0.10 mag. The data on December 21 (the third night) were analyzed in the same way, giving the upper limit of the orbital modulation of $\sim$0.05 mag, indicating that the modulation during outburst decayed as the system faded. It is noteworthy that similar doubly humped (orbital) modulations during early outburst have been observed in the early stage of WZ Sge-type outbursts, and are considered as one of defining characteristics of WZ Sge-type phenomenon (e.g.\ Matsumoto et al.\ 1998). The present outburst of AQ Eri may have been followed the similar course of WZ Sge-type outburst, but failed to trigger a superoutburst. More extensive studies of outbursts, of similar systems with infrequent outbursts, are thus encouraged.

\bigskip

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

Hers, J., 1998, {\it VSNET alert circulation}, No.\,2458 (also available from \\ http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-alert/msg02458.html).

Kato, T., 1991, {\it IBVS}, No.\,3671

Matsumoto, K., Nogami, D., Kato, T., Baba, H., 1998, {\it PASJ}, {\bf 50}, 405

Nogami, D., Kato, T., Hirata, R., 1996, {\it PASJ}, {\bf 48}, 607

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

Thorstensen, J. R., Patterson, J., Shambrook, A., Thomas, G., 1996, {\it PASP}, {\bf 108}, 73

\end{document}


