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Dear Colleagues,
The following article on AH Eri is accepted for publication as IBVS 5105.
The figures are available at:
http://ftp.kusastro.kyoto-u.ac.jp/vsnet/preprints/AH_Eri/
Regards,
Taichi Kato
===
\documentstyle[twoside,epsf]{article}
\input{ibvs2.sty}
\begin{document}
\IBVShead{xxxx}{xx May 2001}
\IBVStitletl{Time-resolved photometry of AH Eri in outburst}
\IBVSauth{Taichi~Kato$^1$, Daisaku~Nogami$^{2,1}$}
\vskip 5mm
\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp}
\IBVSinst{Hida Observatory, Kyoto University, Gifu 506-1314, Japan,
e-mail: nogami@kwasan.kyoto-u.ac.jp}
\IBVSobj{AH Eri}
\IBVStyp{UGSS}
\IBVSkey{dwarf nova, photometry}
\begintext
AH Eri is a dwarf nova which had been a candidate for a system
with a short orbital period (Szkody 1987). Szkody et al. (1989)
performed CCD photometry in quiescence, and found 0.1--0.3 mag
modulations with a period of 42$\pm$2 min. Szkody et al. (1989)
interpreted this period as the possible spin period of a magnetic
white dwarf, as in DQ Her systems. However, since the similar period
in AL Com, which Howell and Szkody (1988) originally attributed to the
spin period, later turned out to be the double-wave modulations of the
81.6-min orbital period (for an extensive review of the object,
see Nogami et al. 1997), a question was raised whether the reported
42-min periodicity in AH Eri actually reflects the spin period
or more related to the orbital period.
\vskip 3mm
The question remained unsettled until the discovery of the firm
orbital period of 5.74 hrs by Thorstensen (1997). Thorstensen (1997)
also argued against the spin-period interpretation of the 42$\pm$2 min
by Szkody et al. (1989), based on the low strength of He II emission lines,
which are usually strong in magnetic cataclysmic variables.
\vskip 3mm
An outburst of AH Eri was announced on 1997 February 28 (Hers 1997).
We performed time-resolved CCD photometry on 1997 March 1 in order to
test the presence of the claimed 42$\pm$2 min periodicity.
The observations were done using a CCD camera (Thomson TH~7882, 576 $\times$
384 pixels, on-chip 2 $\times$ 2 binning adopted) attached to the Cassegrain
focus of the 60 cm reflector (focal length=4.8 m) at Ouda Station, Kyoto
University (Ohtani et al. 1992). An interference filter was used which
had been designed to reproduce the Johnson {\it V} band. The exposure
time was 40 s. The frames were first corrected for standard de-biasing
and flat fielding, and were then processed by a microcomputer-based
aperture photometry package developed by one of the authors (TK).
The magnitudes were determined relative to GSC 5319.1471 ($V=12.18,
B-V=+0.63$), whose short-term constancy was confirmed using GSC 5319.1526
($V=12.56, B-V=+0.65$). The magnitudes are taken from Henden and
Honeycutt (1997). A total of 90 useful frames were obtained.
Barycentric corrections to observed times were applied before the following
analysis.
\IBVSfig{10cm}{fig1.ps}{Light curve of AH Eri on 1997 March 1}
\vskip 3mm
The light curve drawn from these observations is presented in Figure 1.
The light curve shows a slow fading, but there were no apparent periodic
variations. After removing the linear fading trend, we performed a period
analysis between 0.02 and 0.04 d using the Phase Dispersion Minimization
(PDM) method (Stellingwerf 1978). The analysis did not yield a significant
periodicity. Figure 2 shows the phase-averaged light curve
folded by the reported period of 42 min. The light curve suggests that
the 42-min period may have been marginally detected. But because of the
lack of a firm signal in period analysis, we adopt the observed full
amplitude (0.03 mag) at the supposed 42-min period as the upper limit
of this periodicity. The upper limit of 0.03 mag is 3 to 10 times
smaller than reported in Szkody et al. (1989). We conclude that the
claimed 42-min periodicity of AH Eri did not appear, or was markedly
reduced in amplitude, during its outburst.
\IBVSfig{10cm}{fig2.ps}{Light curve of AH Eri folded by a test period of
42 min}
The authors are grateful to VSNET members for providing observations,
and to J. Hers for promptly notifying the outburst.
\references
Henden, A. A., Honeycutt, R. K., 1997, PASP, 109, 441
Hers, J., 1997, {\it VSNET alert circulation}, No. 755 \\
(available from
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/alert0/msg00755.html).
Howell, S. B., Szkody, P., 1988, PASP, 100, 224
Nogami, D., Kato. T., Baba, H., Matsumoto, K., Arimoto, J., Tanabe, K.,
Ishikawa, K., 1997, ApJ, 490, 840
Ohtani, H., Uesugi, A., Tomita, Y., Yoshida,
M., Kosugi, G., Noumaru, J., Araya, S., Ohta, K.
1992, Memoirs of the Faculty of Science, Kyoto University,
Series A of Physics, Astrophysics, Geophysics and Chemistry,
38, 167
Stellingwerf, R. F., 1978, ApJ, 224, 953
Szkody, P., 1987, ApJS, 63, 685
Szkody, P., Howell, S. B., Mateo, M., Kreidl, T. J., 1989, PASP, 101, 899
Thorstensen, J. R., 1997, PASP, 109, 1241
\end{document}
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