\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}
\IBVShead{4978}{31 October 2000}

\IBVStitle{BVRI observations of KT Persei in outburst}

\IBVSauth{Spogli, Corrado$^1$; Fiorucci, Massimo$^1$; Raimondo, Gabriella$^2$}

\IBVSinst{Osservatorio Astronomico, Universit\`a di Perugia, Via A. Pascoli
I-06100 Perugia, Italy}
\IBVSinst{Teramo Astronomical Observatory, Collurania, Teramo, Italy}

\SIMBADobj{KT Per}
\IBVStyp{ CWA }
\IBVSkey{photometry}
\IBVSabs{In this paper we report BVRI observations of the dwarf nova}
\IBVSabs{KT Per during an outburst.}
\IBVSabs{We briefly discuss how this behaviour can be in agreement with disk}
\IBVSabs{instability models.}

\begintext

KT Persei is a well known dwarf nova (DN) of the Z Cam subtype,
characterized by an orbital period of 0.1627 days and a typical time
interval between two subsequent outbursts of 26 days (Ritter \& Kolb
1998). The secondary has a spectral type of M$3.3\pm1$ and an apparent
brightness of $V=17.9$ (Thorstensen \& Ringwald 1997).

The observations were taken at the Astronomical Observatory of
Collurania--Teramo with the 0.72-m Ritchey--Chretien reflector,
equipped with a Tektronix 512 CCD camera and $B$, $V$ (Johnson), $R_c$,
$I_c$ (Cousins) filters. The photometric techniques used have already
been described by Spogli et al.\ (2000a and 2000b). We calibrated some
of the comparison stars in the finding chart reported by Misselt
(1996). The standard magnitudes are reported in Table~1. Considering
the standard deviation, our data are in agreement with the measurements
carried out by Misselt (1996), but show small systematic differences
with respect to the data published by Henden \& Honeycutt (1997). In
any case differences are always within three standard deviations.
Moreover we have included the calibration for the $I_c$ filter.

\begin{table}[h]
\caption{$BVR_cI_c$ magnitudes of the selected comparison stars}
\begin{center}
\begin{tabular}{ccccc}
\hline
No. & $B$ & $V$ & $R_c$ & $I_c$ \\
\hline
 1 & 13.09${}\pm{}$0.04 & 12.56${}\pm{}$0.04 & 12.22${}\pm{}$0.04 & 11.86${}\pm{}$0.04\\
 4 & 16.52${}\pm{}$0.06 & 15.16${}\pm{}$0.04 & 14.35${}\pm{}$0.04 & 13.50${}\pm{}$0.05\\
 5 & 15.35${}\pm{}$0.05 & 14.26${}\pm{}$0.04 & 13.64${}\pm{}$0.04 & 13.06${}\pm{}$0.05\\
 7 & 13.93${}\pm{}$0.04 & 13.27${}\pm{}$0.04 & 12.84${}\pm{}$0.04 & 12.41${}\pm{}$0.04\\
 8 & 16.53${}\pm{}$0.06 & 15.68${}\pm{}$0.04 & 15.18${}\pm{}$0.05 & 14.64${}\pm{}$0.05\\
\hline
\end{tabular}
\end{center}
\end{table}

We observed KT Per in August-September 1998 during all the outburst,
from the rise to the maximum to the subsequent decline. In Table~2 we
report the $BVR_cI_c$ magnitudes while the $V$ light curve can be found
in Figure~1. The observed maximum was $V \simeq 12.5$, a value which
is fainter than other outbursts observed in this source, but that can
be considered quite typical. This outburst follows a particular faint
state ($V \simeq 15.6$ in October 27th) that confirms the evidence that
sometimes KT Per becomes fainter than the minimum of $V=15\fmm4$ listed
by many catalogues (Thorstensen \& Ringwald 1997). The differences
between the minimum and the maximum are: $\Delta B \simeq 3.3$,
$\Delta V \simeq 3.0$, $\Delta R_c \simeq 2.7$ and $\Delta I_c \simeq
2.0$.

\IBVSfig{10cm}{4978-f1.ps}{$V$ light curve of KT Per}

\IBVSfig{10cm}{4978-f2.ps}{Colour index variations of KT Per during the outburst}

\IBVSfig{14cm}{4978-f3.ps}{Spectral slope changes in KT Per during the
rise (left panel) and the decline (right panel). The date (JD 2451000\,+)
is reported near the fluxes}

\begin{table}
\caption{$BVR_cI_c$ magnitudes of KT Per}
\begin{center}
\begin{tabular}{ccccc}
\hline
JD & \lower6pt\hbox{$B$} & \lower6pt\hbox{$V$} & \lower6pt\hbox{$R_c$} & \lower6pt\hbox{$I_c$} \\[-4pt]
(2451000\,+) & & & & \\
\hline
 52.4194&16.01${}\pm{}$0.05&15.61${}\pm{}$0.02&15.03${}\pm{}$0.02&14.21${}\pm{}$0.01\\
 56.5715&15.69${}\pm{}$0.03&15.25${}\pm{}$0.01&14.72${}\pm{}$0.02&13.94${}\pm{}$0.01\\
 57.5769&13.78${}\pm{}$0.03&13.47${}\pm{}$0.02&13.20${}\pm{}$0.02&12.90${}\pm{}$0.01\\
 57.6315&13.50${}\pm{}$0.03&13.22${}\pm{}$0.01&12.98${}\pm{}$0.02&12.72${}\pm{}$0.01\\
 58.5457&12.73${}\pm{}$0.02&12.54${}\pm{}$0.01&12.33${}\pm{}$0.02&12.16${}\pm{}$0.01\\
 58.6273&12.74${}\pm{}$0.04&12.58${}\pm{}$0.02&12.38${}\pm{}$0.03&12.18${}\pm{}$0.01\\
 59.5499&13.08${}\pm{}$0.02&12.93${}\pm{}$0.01&12.69${}\pm{}$0.02&12.47${}\pm{}$0.01\\
 59.6311&13.12${}\pm{}$0.04&12.98${}\pm{}$0.05&12.75${}\pm{}$0.04&12.51${}\pm{}$0.01\\
 60.4197&13.68${}\pm{}$0.03&13.53${}\pm{}$0.05&13.27${}\pm{}$0.04&12.94${}\pm{}$0.03\\
 63.4232&15.87${}\pm{}$0.03&15.40${}\pm{}$0.01&14.88${}\pm{}$0.02&14.18${}\pm{}$0.02\\
 67.3987&15.65${}\pm{}$0.03&15.21${}\pm{}$0.03&14.89${}\pm{}$0.02&14.20${}\pm{}$0.02\\
\hline
\end{tabular}
\end{center}
\end{table}

Figure 2 shows almost the entire loop of the $B-V$ colour index as a
function of magnitude for KT Per. Obviously during the maximum the DN
is blue ($B-V \simeq 0.2$) and during the minimum redder ($B-V \simeq
0.4$), but it is important to remark that for the first part of the
decline the system remains blue or becomes bluer. This behaviour can
be well explained with disk instability models as described in Spogli
et al.\ (2000b). Our data show a large loop in the colour-magnitude
diagram for KT Per, therefore constrain theoretical models and suggest
that the predicted outside-in outburst of disk instability models are
in agreement with the observations.

$BVR_cI_c$ observations of dwarf novae allow to evaluate the optical 
spectral behaviour and, therefore, they can be used as a test to
compare theoretical models of accretion disk emission. In particular 
they can be used to verify the often quoted theoretical flux distribution
of a stationary (infinitely) large accretion disk whose
surface elements radiate as black body spectra ($F(\nu)\propto\nu^{1/3}$,
see, e.g., Warner 1995).

To study the behaviour of the optical continuum during the outburst, we
converted the $BVR_cI_c$ magnitudes in fluxes using the conversion
factors reported by Bessell (1979). We corrected our observations for
interstellar reddening adopting the value $E_{B-V}=0.2$ reported by La
Dous (1989), then we have $A_V \simeq 0.6$. For a correction of the
fluxes in the $B, R_c$ and $I_c$ bands we used the interpolation
formula of Cardelli, Clayton \& Mathis (1989).

Using the flux values so obtained we noted that at minimum the spectral
distribution is dominated by the emission of the secondary star, while
at maximum the spectral distribution follows a power law
($F(\nu)\propto\nu^{\alpha}$) with $\alpha\simeq0.6$. Figure~3 shows
the spectral slope changes in KT Per in logarithmic scales. During the
decline the high frequency flux remains strong, a behaviour that may be
consistent with a strong irradiation of the hot inner part of the disk
while the outer part is already cooling.

These data confirm that at least for a sample of DNe (KT Per, SY Cnc,
DX And, V660 Her, AL Com, V503 Cyg, see Spogli et al.\ 1993, 1998,
2000a) it is not possible to approximate their emission during outburst
by a sum over all contributions of surface elements of an infinitely
large steady-state disk, which radiate like black bodies.


\references

Bessell, H.S., 1979, {\it PASP}, {\bf 91}, 589 
% \BIBCODE{1979PASP...91...589B}

Cardelli, J.A., Clayton, G.C., Mathis, J.S., 1989, {\it ApJ}, {\bf 345}, 245
% \BIBCODE{1989ApJ....345..245C}

Henden, A.A., Honeycutt, R.K., 1997, {\it PASP}, {\bf 109}, 441 
% \BIBCODE{1997PASP...109...441H}

La Dous, C., 1989, {\it A\&A}, {\bf 211}, 131
% \BIBCODE{1989A&A...211..131L}

Misselt, K.A., 1996, {\it PASP}, {\bf 108}, 146 
% \BIBCODE{1996PASP...108...146M}

Ritter, H., Kolb, U., 1998, {\it A\&AS}, {\bf 129}, 83
% \BIBCODE{1998A&AS..129...83R}

Spogli, C., Fiorucci, M., Tosti, G., 1993, {\it IBVS}, No.\ 3949 
% \BIBCODE{1993IBVS..3949....1S}

Spogli, C., Fiorucci, M., Tosti, G., 1998, {\it A\&AS}, {\bf 130}, 485 
% \BIBCODE{1998A&AS..130..485S}

Spogli, C., Fiorucci, M., Raimondo, G., 2000a, {\it IBVS}, No.\ 4944
% \BIBCODE{2000IBVS.4944....1S}

Spogli, C., Fiorucci, M., Raimondo, G., 2000b, {\it IBVS}, No.\ 4977

Thorstensen, J.R., Ringwald, F.A., 1997, {\it PASP}, {\bf 109}, 483
% \BIBCODE{1997PASP..109..483T}

Warner, B., 1995, ``Cataclysmic Variable Stars'', Cambridge University 
Press, Cambridge % \BIBCODE{1995cvs..book.....W}

\endreferences


\end{document}

