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\input ibvs2.sty

\begin{document}

\IBVShead{5792}{28 August 2007}

\IBVStitle{UBVRI Photometry of DX A\lowercase{nd}: the 2006 Outburst}
\IBVSauth{Spogli, C.$^{1,2}$; Fiorucci, M.$^1$; Rocchi, G.$^2$; Capezzali, D.$^{1,2}$}
\IBVSinst{Physics Department, University of Perugia, Via A. Pascoli, 06123 
Perugia, Italy}
\IBVSinst{Porziano Astronomical Observatory, Via Santa Chiara 2, Assisi, Italy}

\SIMBADobj{DX And}
\IBVStyp{ UG }
\IBVSkey{photometry}
\IBVSabs{A new UBV(RI)c photometric sequence around DX And is presented,} 
\IBVSabs{together with the photometric data obtained during the 2006 Outburst}
\IBVSabs{of this cataclysmic variable.}


\begintext

The dwarf nova DX And is one of the few cataclysmic variables with the orbital period
length near the upper limit of the range (10.6 hours), together with
an exceptional long cycle length (270-330 days), a secondary star probably evolved off 
the main sequence, and a very low mass--transfer rate (\v{S}imon, 2000). For all these 
reasons, DX And can be considered representative of the upper limit of the distribution
of dwarf novae, and a detailed study of its activity can help to constrain 
theoretical models.
Nevertheless, only a few outbursts have been studied in detail, and rarely with 
multi--colors photometry (see \v{S}imon 2000 for an overview of the scarce database 
available in literature). 

In the contest of a long-term variability study of a sample of dwarf novae, we are 
monitoring DX And since 1994 and we have already obtained photometric data in the
$BVR_CI_C$ bands during two outbursts, in 1994 and 2005 (Spogli et al., 1998, 2006).
In this brief paper we present the results of our observations done in 2006, that 
includes also the $U$ broad band together with the usual $BVR_CI_c$ Johnson-Cousins 
filters. These are the firsts $U$ data during the rise and the maximum of the 
outburst, since we know only two other data reported in literature obtained during 
the descending phase (Echevarria, 1984).
The telescope we used was a 0.30-m f/6.5 Schmidt-Cassegrain reflector, equipped 
with an AP-32ME CCD camera (Kodak 3200-ME, 2184$\times$1470 pixels) and Schuler
$UBVR_CI_C$ filters, located on Mt. Subasio, Assisi (PG), Italy.
The exposure time was 120--600~s depending on the brightness of the object and the
filter used. The frames were first corrected for bias and flat-field, and then
processed by a PC-based aperture photometry package developed by one of the
authors using DAOPHOT routines (Stetson, 1987). 

All the data here reported were obtained in differential photometry using the 
photometric comparison sequence around DX And tabulated in Table~1. 
The $UBVR_CI_C$ magnitudes have been calibrated with CCD observations obtained in 
July-August 2006 during three different photometric nights with respect to a selected sample 
of standard stars (Landolt 1983, 1992). Color transformation equations were characterized by 
slopes always within the margins 0.9--1.1. The photometric stability of the 
comparison stars can be guaranteed for C1 and C2 because they have been checked by 
repeated observations since 1994 (Spogli et al., 1998), while for the other stars we can 
only say that they were stable during the four months reported in this paper.

DX And has been monitored from July 23 to November 15, for a total of 40 different nights 
(Table~2). During the minimum we used only the $R_C$ broad-band, because we already knew
that in quiescence the emission of DX And is dominated by the secondary
star (Spogli et al., 2006). Our data confirm that in this phase of activity the
$R_C$ magnitude oscillate between 14.4 and 14.6, probably ellipsoidal
variations superimposed to additional variability, a typical pattern for
long--period cataclysmic binaries (Hilditch, 1995).
The precedent outburst occurred at the end of September 2005 (Spogli et al., 2006),
so our aim was to observe the rise to the new outburst with the $UBVR_CI_C$ filters, and 
the outburst effectively went up at the middle of September 2006 (Fig.~1). 
We obtained data in all the photometric range during the rise up to the maximum,
observed in the night of September 23. Unfortunately, soon after the outburst we were not
able to use the $U$ filter for technical problems, so we followed the decline with the
$BVR_CI_C$ bands.


Fig.~2 shows the spectral flux distribution of DX And during the rise. The magnitudes 
have been converted in $f(\lambda)$ using the flux calibrations reported by Bessell (2000).
The increasing rate is more or less the same in all the filters, with the remarkable 
exception in the $U$, where the brightness continues to increase when in the other bands 
the maximum is already reached. This feature is quite common in outside--in outbursts, i.e.
when the thermal instability (that gives rise to the outburst) starts in the outer part
of the accretion disk and propagates inwards, producing an asymmetric light curve with
a rapid rise and slow decay. The figure shows the progressive increase of the disk emission, theoretically represented - in a first approximation - as a power--law 
$f(\lambda)\propto\lambda^{-7/3}$, during the final steps of the
outburst.

\IBVSfig{10cm}{5792-f1.eps}{$R_C$ light curve of DX And in July--October 2006. The maximum occurred in Sept.23}
\IBVSfigKey{5792-f1.eps}{DX And}{light curve}


\IBVSfig{9.5cm}{5792-f2.eps}{Spectral flux distribution of DX And during the rise to the
outburst. The data have been obtained during the nights of September 19 (circle), 
20 (diamond), 21 (triangle), 22 (cross) and 23 (box). The dotted line represents a generic
power-law function $f(\lambda)\propto\lambda^{-7/3}$.}
\IBVSfigKey{5792-f2.eps}{DX And}{}


\begin{table}
\small
\begin{center}
\caption{Magnitudes and their errors for the stars in the photometric sequence.}
\begin{tabular}{lcccccccc}
\hline
\hline
Id & GSC id & RA         & DEC         &      $U$       &      $B$       &     $V$        &    $R_C$       &    $I_C$      \\
   & 03242- & \multicolumn{2}{c}{(J2000)}\\
\hline
C1 & 00510  & 23 29 42.7 & +43 45 42   & 13.65$\pm$0.07 & 13.42$\pm$0.04 & 12.72$\pm$0.03 & 12.26$\pm$0.03 & 11.90$\pm$0.03\\
C2 & 00216  & 23 29 50.5 & +43 44 49   & 13.98$\pm$0.07 & 13.90$\pm$0.04 & 13.33$\pm$0.03 & 12.95$\pm$0.03 & 12.64$\pm$0.03\\
C3 & 00856  & 23 30 01.2 & +43 48 41   &  13.4$\pm$0.1  & 12.71$\pm$0.05 & 11.68$\pm$0.04 & 11.10$\pm$0.04 & 10.58$\pm$0.04\\
C4 & 00562  & 23 29 40.2 & +43 50 04   &  13.2$\pm$0.1  & 12.21$\pm$0.05 & 11.03$\pm$0.04 & 10.36$\pm$0.04 & ~9.84$\pm$0.04\\
C5 & 00990  & 23 29 24.5 & +43 43 27   &  12.6$\pm$0.1  & 12.58$\pm$0.05 & 12.12$\pm$0.04 & 11.80$\pm$0.04 & 11.55$\pm$0.04\\
\hline
\end{tabular}
\end{center}
\small
\begin{center}
\caption{$UBVR_CI_C$ magnitudes of DX And during the 2006 outburst}
\begin{tabular}{lrccccc}
\hline
\hline
UT date    &    J.D.  &      $U$       &      $B$       &     $V$        &    $R_C$       &    $I_C$      \\
           & 2453000+ &                &                &                &                &               \\
\hline
23/07/2006 &  939.534 &                &                &                & 14.60$\pm$0.02 &               \\
27/07/2006 &  944.401 &                &                &                & 14.58$\pm$0.02 &               \\
30/07/2006 &  947.378 &                &                &                & 14.48$\pm$0.03 &               \\
04/08/2006 &  952.359 &                &                &                & 14.63$\pm$0.02 &               \\
05/08/2006 &  953.391 &                &                &                & 14.52$\pm$0.02 &               \\
15/08/2006 &  963.369 &                &                &                & 14.67$\pm$0.03 &               \\
18/08/2006 &  966.354 &                &                &                & 14.45$\pm$0.02 &               \\
21/08/2006 &  969.335 &                &                &                & 14.47$\pm$0.02 &               \\
24/08/2006 &  972.353 &                &                &                & 14.55$\pm$0.01 &               \\
27/08/2006 &  975.329 &                &                &                & 14.51$\pm$0.01 &               \\
02/09/2006 &  981.325 &                &                &                & 14.43$\pm$0.02 &               \\
03/09/2006 &  982.327 &                &                &                & 14.62$\pm$0.02 &               \\
05/09/2006 &  984.352 &                &                &                & 14.45$\pm$0.05 &               \\
06/09/2006 &  985.375 &                &                &                & 14.67$\pm$0.03 &               \\
07/09/2006 &  986.343 &                &                &                & 14.58$\pm$0.03 &               \\
10/09/2006 &  989.316 &                &                &                & 14.55$\pm$0.02 &               \\
11/09/2006 &  990.335 &                &                &                & 14.50$\pm$0.02 &               \\
13/09/2006 &  992.342 &                &                &                & 14.39$\pm$0.02 &               \\
15/09/2006 &  994.345 &                &                &                & 14.38$\pm$0.03 &               \\
19/09/2006 &  998.371 & 12.70$\pm$0.10 & 13.31$\pm$0.03	& 13.12$\pm$0.03 & 12.91$\pm$0.02 & 12.80$\pm$0.06\\
20/09/2006 &  999.305 & 12.38$\pm$0.08 & 12.94$\pm$0.03	& 12.79$\pm$0.02 & 12.62$\pm$0.02 & 12.49$\pm$0.03\\
21/09/2006 & 1000.304 & 12.05$\pm$0.03 & 12.60$\pm$0.08	& 12.53$\pm$0.03 & 12.33$\pm$0.03 & 12.23$\pm$0.02\\
22/09/2006 & 1001.309 & 11.75$\pm$0.05 & 12.28$\pm$0.04	& 12.21$\pm$0.02 & 12.05$\pm$0.02 & 11.97$\pm$0.03\\
23/09/2006 & 1002.309 & 11.58$\pm$0.10 & 12.23$\pm$0.03	& 12.14$\pm$0.03 & 11.95$\pm$0.03 & 11.85$\pm$0.02\\
24/09/2006 & 1003.336 &                & 12.40$\pm$0.04	& 12.21$\pm$0.04 & 12.05$\pm$0.03 & 11.92$\pm$0.02\\
29/09/2006 & 1008.306 &                & 12.54$\pm$0.03	& 12.39$\pm$0.02 & 12.19$\pm$0.02 & 12.00$\pm$0.03\\
30/09/2006 & 1009.284 &                & 12.66$\pm$0.05	& 12.45$\pm$0.02 & 12.23$\pm$0.03 & 12.09$\pm$0.03\\
06/10/2006 & 1015.376 &                & 14.73$\pm$0.03	& 14.33$\pm$0.02 & 14.03$\pm$0.02 & 13.68$\pm$0.01\\
08/10/2006 & 1017.288 &                & 15.59$\pm$0.03 & 14.77$\pm$0.02 & 14.29$\pm$0.02 & 13.77$\pm$0.03\\
13/10/2006 & 1022.321 &                &                &                & 14.45$\pm$0.02 &               \\
14/10/2006 & 1023.278 &                &                &                & 14.54$\pm$0.02 &               \\
15/10/2006 & 1024.298 &                &                &                & 14.53$\pm$0.03 &               \\
27/10/2006 & 1036.305 &                &                &                & 14.52$\pm$0.03 &               \\
28/10/2006 & 1037.391 &                &                &                & 14.60$\pm$0.03 &               \\
29/10/2006 & 1038.227 &                &                &                & 14.52$\pm$0.02 &               \\
02/11/2006 & 1042.337 &                &                &                & 14.51$\pm$0.03 &               \\
03/11/2006 & 1043.267 &                &                &                & 14.49$\pm$0.02 &               \\
10/11/2006 & 1050.383 &                &                &                & 14.54$\pm$0.03 &               \\
14/11/2006 & 1054.302 &                &                &                & 14.51$\pm$0.01 &               \\
15/11/2006 & 1055.295 &                &                &                & 14.52$\pm$0.04 &               \\
\hline
\end{tabular}
\end{center}
\end{table}

\newpage

\references

Bessell, M., 2000, {\it Magnitude Scales and Photometric Systems}, in ``Encyclopedia  of Astronomy and Astrophysics'', P. Murdin (Ed.), Bristol Inst. of Physics Publishing
%\BIBCODE{2000eaa..bookE1939B}

Echevarria, J., 1984, {\it Rev. Mex. Astron. Astrofis.}, {\bf 9}, 99

Hilditch, R.W., 1995, {\it MNRAS}, {\bf 273}, 675

Landolt, A.U., 1983, {\it AJ}, {\bf 88}, 439

Landolt, A.U., 1992, {\it AJ}, {\bf 104}, 340

\v{S}imon, V., 2000, {\it A\&A}, {\bf 364}, 694

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

Spogli, C., Fiorucci, M., Capezzali, D., et al., 2006, {\it IBVS}, 5716

Stetson, P.B., 1987, {\it PASP}, {\bf 99}, 191

\endreferences

\end{document}
