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\input ibvs2.sty

\begin{document}
\IBVShead{5101}{5 June 2001}

\IBVStitletl{First Spectroscopy of the Dwarf Nova KX A\lowercase{ql}:}{ a Possible New SU UM\lowercase{a} System}

\IBVSauth{Tappert, C.$^{\rm 1,2}$; Mennickent, R.E.$^1$}

\IBVSinst{Grupo de Astronom\'{\i}a, Facultad de F\'{\i}sica y Matem\'aticas,
Universidad de Concepci\'on, Casilla 160-C, \\ \indent Concepci\'on, Chile,
e-mail: claus@gemini.cfm.udec.cl, rmennick@stars.cfm.udec.cl}

\IBVSinst{Dipartimento di Astronomia, Universit\`a di Padova, Vicolo 
dell'Osservatorio 2, I-35122 Padova, Italy \\}

\SIMBADobjAliasJ2000{KX Aql}{HV 5428}{19 33 53.69 +14 17 47.3}
\IBVStyp{ CV }
\IBVSkey{spectroscopy}
\IBVSabs{We present the quiescence spectrum of the dwarf nova KX Aql. The}
\IBVSabs{most prominent spectral features are the extremely strong Balmer}
\IBVSabs{emission lines, indicative of a dwarf nova at a low mass-transfer}
\IBVSabs{rate. The absence of secondary absorption and a large outburst}
\IBVSabs{amplitude suggests KX Aql as a member of the SU UMa subclass.}

\begintext 

KX Aql (HV 5428) is a poorly studied cataclysmic variable star. The
Downes et al.\ (2001) catalogue reports a dwarf nova nature with a
rather long cycle length (300 days) and a photographic magnitude range
12.5--17.5, although the VSNET\footnote{\enskip
http://www.kusastro.kyoto-u.ac.jp/vsnet/gcvs/AQLKX.html} and
VSOLJ\footnote{\enskip
http://www.kusastro.kyoto-u.ac.jp/vsnet/etc/drawvsolj.cgi?text=AQLKX}
light curves occasionally show the star fainter than 18 mag. To our
knowledge, the system has not been examined spectroscopically yet. In
this note we confirm the dwarf nova classification, and provide for the
first time a detailed description of the optical spectrum.

\IBVSfig{10cm}{5101-f1.ps}{Flux calibrated (in 10$^{-16}$
erg\,cm$^{-2}$s$^{-1}$\AA$^{-1}$, top) and continuum normalized
(bottom) spectra of KX Aql}

\begin{table}
\begin{center}
\caption{Properties of the emission lines. Column 1 gives the wavelength 
determined by a Gaussian fit, column 2 the equivalent width, column 3 the 
Gaussian FWHM, column 4 the integrated line flux, column 5 and 6 the line
identification and the corresponding rest wavelength, respectively. The flux is
in units of 10$^{-16}$ erg\,cm$^{-2}$\,s$^{-1}$. All other values are in {\AA}\@.
Colons mark uncertain values
\medskip}
\label{spprop_tab}
\begin{tabular}{r l c c l c l}
\hline\noalign{\smallskip}
\multicolumn{1}{c}{(1)} & \multicolumn{1}{c}{(2)} & (3) & (4) & 
\multicolumn{1}{c}{(5)} & (6) & \multicolumn{1}{c}{(7)}  \\
\multicolumn{1}{c}{$\lambda$} & \multicolumn{1}{c}{$W_{\lambda}$} & FWHM & 
$F_{\lambda}$ & identification & $\lambda_0$ & remarks \\
\hline\noalign{\smallskip}
3898  & $-$47: & 20  & 271: & H$\zeta$             & 3889 & [1] \\
3943  & $-$45: & 11  & 246: & CaII K               & 3934 & [1] \\
3979  & $-$54: & 21  & 289: & H$\varepsilon$ + CaII H & 3970 & [1] \\
4109  & $-$62  & 24  & 293  & H$\delta$            & 4102 & \\
4191  & $-$9   & 35  & 31   & FeII                 & 4179 & uncertain \\
4244  & $-$7   & 20  & 21   & FeII                 & 4233 & uncertain \\
4293  & $-$12  & 13  & 16   &                      &      & ID? \\
4347  & $-$111 & 25  & 264  & H$\gamma$            & 4341 & \\
4424  & $-$4   & 20  & 10   &                      &      & ID? \\
4479  & $-$15  & 23  & 33   & HeI                  & 4472 & \\
4867  & $-$159 & 26  & 265  & H$\beta$             & 4861 & \\
4928  & $-$13  & 23  & 19   & HeI                  & 4922 & \\
4971  & $-$4   & 30  & 5    &                      &      & ID? \\
5028  & $-$13  & 39  & 24   & HeI                  & 5016 & \\
5176  & $-$13  & 30  & 22   & FeII                 & 5169 & \\
5882  & $-$60  & 33  & 61   & HeI                  & 5876 & \\
6566  & $-$320 & 30  & 282  & H$\alpha$            & 6563 & \\
6681  & $-$32  & 41  & 25   & HeI                  & 6678 & \\
7068  & $-$25  & 42  & 24   & HeI                  & 7065 & \\
8515  & $-$76  & 91  & 57   & CaII/Pa blend        &      & \\
8665  & $-$41  & 62  & 25   & Pa13/CaII            & 8665/8662 & \\
8757  & $-$37  & 87  & 17   & Pa12                 & 8751 & \\
8881: & $-$23: & 40: & 10:  & Pa11                 & 8863 & [2] \\
\hline
\multicolumn{7}{l}{\normalsize [1] lines are strongly blended} \\
\multicolumn{7}{l}{\normalsize [2] line distorted due to absorption feature 
(CCD error)} \\
\end{tabular}
\end{center}
\end{table}

\vskip-11pt
We obtained a 900-s integration time spectrum of KX Aql on May 30, 2000 
(JD 2451694) at the La Silla Observatory using DFOSC at the Danish 1.54-m 
telescope. Grism \#15 combined with a slit width of 2$^{\prime\prime}$ yielded 
a wavelength range of $\sim 3800$--9100 {\AA} and a spectral resolution of 15
{\AA}\@. The spectrum was corrected for bias and flat fields, as well as 
calibrated in wavelength and flux using standard IRAF\footnote{\enskip IRAF is 
distributed by the National Optical Astronomy Observatories.} routines.

The spectrum of KX Aql is presented in Fig.~1. It shows typical 
dwarf nova features, with strong emission lines of the Balmer and HeI series. 
Additionally, CaII H (hidden in H$\varepsilon$) and K emission is present at the 
blue, and the CaII triplet (blended with the Paschen series) at the red end of 
the spectrum. Table~1 contains all identified emission lines and
their basic quantities. Note especially the extraordinary strength of the 
H$\alpha$ line ($W_{{\rm H}\alpha} > 300$~{\AA}), and the absence of 
highly ionized lines like HeII\@. The spectrum furthermore shows no absorption 
features which could be assigned to the secondary star. A few emission lines 
remained unidentified due to the low resolution of our data, two other were 
tentatively assigned to FeII, but a final conclusion has to await 
high-resolution spectroscopy.

The normalized spectrum in the lower part of Fig.~1 was computed by
dividing the calibrated spectrum through a spline fit to the continuum. It 
emphasizes the strong Balmer decrement, suggesting an origin in an optically 
thin accretion disc. This, and the strong emission lines in general, indicate 
that the system was in quiescence during our observations. We folded the 
calibrated spectrum with Bessell (1990) filtercurves in order to extract 
spectrophotometric magnitudes, obtaining
\begin{displaymath}
V = 18.4,~~B-V = -0.5,~~V-R = 0.2.
\end{displaymath}
The recorded seeing during the observations was of the order 1\farcs5. On the
basis of previous experiences with the instrumental setup at the 1.54 
Danish, we thus expect the calibration to be better than $\sim 0\fmm15$ in $V$ 
considering the slit width of 2\arcs. This confirms the quiescent 
state of KX Aql during the observation. It furthermore means that, if the 
maximum $V$ magnitude is not too far from the listed photographic value of 12.5
(Downes et al., 2001), the system shows long-term variations with 
$\Delta V \ge 6$ mag.

The optically thin disc, suggested by the properties of the emission lines,
points to a state of low accretion rate, and the disc luminosity, i.e.\ its
continuum emission, can be expected to be rather low. The absence of 
late-type absorption features (e.g., NaI or TiO) therefore indicates a faint
secondary star. This, together with the probable large outburst amplitude and 
the long recurrence time, suggests that KX Aql is a member of the SU UMa star 
subclass of cataclysmic variables, i.e.\ a dwarf nova below the period gap with
a secondary star less massive than $\sim 0.3~M_{\odot}$. The observation of a 
superoutburst, or the determination of the orbital period, should be the 
definitive probe of this prediction. 

\references

Bessell, M.S., 1990, {\it PASP}, {\bf 102}, 1181
%\BIBCODE{1990PASP..102.1181B}

Downes, R.A., Webbink, R.F., Shara, M.M., Ritter, H., Kolb, U., Duerbeck, H.W.,
2001, {\it PASP}, in press

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