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[vsnet-preprint 7] CH Cyg preprint
CH Cyg preprint
Dear Colleagues,
The following paper is accepted for publication as IBVS No. 4918.
The figure is available at:
http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/CH_Cyg/
Regards,
Taichi Kato
====
\documentstyle[twoside,epsf]{article}
\input{ibvs2.sty}
\begin{document}
\IBVShead{xxxx}{xx July 2000}
\IBVStitletl{Supersoft Source Activity as a Possible Interpretation of}{Temporary Fadings of CH Cyg}
\IBVSauth{Taichi~Kato$^1$}
\vskip 5mm
\IBVSinst{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp}
\IBVSobj{CH Cyg}
\IBVStyp{ZAND}
\IBVSkey{symbiotic binary}
\begintext
CH Cyg is a well-known, but enigmatic, symbiotic variable, which is known
to show complex activity (e.g. Miko{\l}ajewski et al. 1990). Superimposed
on the increasing symbiotic activity, sudden short-term fadings in optical
and ultraviolet have been observed. In order to explain this recurrent
feature, various models involving eclipses of the hot component have been
proposed (e.g. Hinkle et al. 1993; Skopal et al. 1996). However, the
apparent presence of fadings not strictly following the suggested ephemeris
suggests the possible presence of other mechanisms. Furthermore, the
asymmetry of fadings (rapid fading and more slower recovery) is also
difficult to explain by the eclipse model.
\vskip 3mm
Particularly noteworthy is the segment of recent light curve between
1998 and 2000 (upper panel of Figure 1). This figure clearly demonstrates
that fadings are not strictly periodic. Two distinct minima occurred
around HJD 2451360 and 2451690, whose interval was 330 d, while there
was no hint of a fading at around HJD 2451030. The light curve also
indicates the common feature of fadings: rapid decline followed by
slower brightening. Between these transient fadings, the system spends
most of time at bright state. All the above features of transient fadings
of CH Cyg are strikingly similar to quasi-periodic fadings (or low states)
of the peculiar binary V Sge (lower panel of Figure 1), in the recurrent
time of semi-periodic fadings, the relatively short duty cycle of faint
states, and in the depth.
\vskip 3mm
\IBVSfig{8cm}{fig1.ps}{Light curves of CH Cyg and V Sge, drawn from visual
observations reported to VSNET (http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/)}
V Sge has been recently recognized as a transient supersoft X-ray source
(SSXS), in which supersoft X-ray emission was only observed during its
low states (Greiner et al. 1998). The phenomenon is quite similar to
the Magellanic SSXS, RX J0513.9-6951 (Reinsch et al. 1996). The cause
of such recurrent fading episodes and the anti-correlation between
supersoft X-ray and optical light has not yet been well understood,
but Hachisu and Kato (1997) presents an interpretation by considering
the limit-cycle formation of optically thick wind, which can reproduce,
in particular, the asymmetric profile of fadings. The striking resemblance
of the CH Cyg light variation to that of V Sge raises a possibility that
fadings of CH Cyg may have been caused by transient SSXS phenomenon.
\vskip 3mm
The relatively common optical features of SSXSs are the presence of
strong He{\sc II} emission, and the appearance of jet features (for a recent
review, see G\"{a}nsicke et al. 2000). The He{\sc II} emission has been
usually regarded to be absent in CH Cyg, but Leedj\"arv et al. (1994)
detected the emergence of the He{\sc II} emission during the fading
episode. Furthermore, there are evidences of accompanying X-ray emission
(Leahy and Taylor 1987) and the jet ejection (Taylor et al. 1986)
during the fading occurring in 1984. These evidences seem to strengthen
the relationship to the transient SSXS phenomenon. One must note many
of these signatures were recorded during the dramatic episode starting
in 1984, which may be different in nature from the present milder fading
episodes. However, it would be noteworthy that these high-energy events
occurred during the optical low state, as in some transient SSXSs.
The author has searched the public library of ROSAT observations (1WGA)
in order to see the possible correlation between optical variation and the
soft X-ray emission. There was only one available observation in late 1992
March, when CH Cyg was observed as a relatively hard source. CH Cyg was
then in an extended low state, which is not comparable to the present high
state with recurring faint states. Since V Sge is a hard X-ray source
outside the transient SSXS phase (Greiner et al. 1998), it is not surprising
CH Cyg was recorded as a hard X-ray emitter at single-epoch observation.
The present suggested relation between CH Cyg and SSXSs may lead to
a unified understanding of various phenomenon in SSXSs and symbiotic
variables, particularly regarding the enigmatic high-velocity jet formation
in CH Cyg-type symbiotic variables. The present interpretation is only
one of possibilities, which requires further proofs from observations.
Particularly crucial tests would include X-ray observations during the
short fading episodes.
\vskip 3mm
The author is grateful to VSNET observers who reported crucial observations
of CH Cyg and V Sge.
\references
G\"{a}nsicke, B. T., van Teeseling, A, Beuermann, K., Reinsch, K.,
2000, New Astronomy Reviews, 44, 143
Greiner, J., Teeseling, A., 1998, A\&A, 339, L21
Hachisu, I., Kato, M., 1997, presented at IAU Symposium 188
Hinkle, K. H., Fekel, F. C., Johnson, D. S., Scharlach, W. W., 1993,
AJ, 105, 1074
Leahy, D. A., Taylor, A. R., 1987, A\&A, 176, 262
Leedj\"arv, L., Miko{\l}ajewski, M., Tomov, T., 1994, A\&A, 287, 543
Miko{\l}ajewski, M., Miko{\l}ajewska, J., Khudyakova, T., 1990, A\&A,
235, 219
Reinsch, K., van Teeseling, A., Beuermann, K., Abbott, T. M. C.,
1996, A\&A, 309, L11
Skopal, A., Bode, M. F., Lloyd, H. M., Tamura, S., 1996, A\&A, 308, L9
Taylor, A. R., Seaquist, E. R., Mattei, J. A., 1986, Nature, 359, 38
\end{document}
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