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\IBVShead{5983}{31 March 2011}

\IBVStitle{On the Optical Variations of AH Herculis}

\IBVSauth{Spogli, C.$^{1,2}$;  Fiorucci, M.$^{1,3}$; Rocchi, G.$^2$;  Ciprini, S.$^{4,1,5}$; Brunozzi, P.$^2$;\\ Fagotti, P.$^2$; Mancinelli, V.$^2$; Vergari, D.$^2$; Capezzali, D.$^2$}

\IBVSinst{Physics Department, University of Perugia, via A. Pascoli, 06123 Perugia, Italy}
\IBVSinst{Porziano Astronomical Observatory, via Santa Chiara 2, 06081 Assisi (PG), Italy}
\IBVSinst{INAF - Padova and Asiago Astronomical Observatory, via Osservatorio, 36012 Asiago (VI), Italy}
\IBVSinst{ASI Science Data Center (ASDC), c/o ESRIN, via G. Galilei, 00044 Frascati (Roma), Italy}
\IBVSinst{Tuorla Observatory, University of Turku, V\"{a}is\"{a}l\"{a}ntie 20, 21500, Piikki\"{o}, Finland}


\SIMBADobj{AH Her} 
\IBVStyp{CWA} 
\IBVSkey{photometry} 
\IBVSkey{variability}
\IBVSabs{In this paper we report 2007-2009 multicolour observations of the}
\IBVSabs{dwarf nova AH Her aiming to test the steady-state model}
\IBVSabs{for accretion disk emission.}


\begintext

In the context of a long-term variability study of a sample of dwarf novae, we have been
monitoring AH Her since 1994 (Spogli et al., 2001, 2002) and we have obtained photometric data during
many outbursts and standstills with the aim to constrain theoretical models. AH Her is
one of the intrinsically brightest dwarf novae in quiescence, with the optical emission
dominated by the accretion disk and the secondary (probably a K2~V~star, Bruch, 1987).
The system has an inclination angle $i \simeq 41\deg$. Moreover, the FUV flux of AH Her is
completely dominated by the accretion disk, with only a marginal fraction of the total
light generated by the White Dwarf: $ \simeq 3\%$ in quiescence (Urban \& Sion, 2006) and $ \leq 0.5\%$
during the outburst (Hamilton et al., 2007). For all these reasons, AH~Her is a perfect
candidate to study the accretion disk emission, thanks to the marginal contribution of the
primary star and of the boundary layer region. The principal aim of this work is to test
the steady state model making use of multicolour observations of AH~Her on different
parts of the outburst light curve. Since the secondary star is the same for the different
observations, a set of disk spectra should exist which reproduces the different shapes of
the simultaneous optical observations and the brightness differences.

All the observations have been obtained with 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 $UBVR_{C}I_{C}$ Johnson-Cousins filters. The exposure time was 120--600 s depending
on the brightness of the object and the filter used. The CCD frames were first corrected
for de-biasing and flat-fielding, then processed for aperture photometry. All the $BVR_{C}I_{C}$
data were obtained in differential photometry using the photometric comparison sequence
reported by Spogli et al. (2001). The U magnitudes have been measured only during good
photometric nights with respect to a selected sample of standard stars (Landolt, 1992).
Color transformation equations were characterized by slopes always within the margins
0.9--1.1.


AH Her has been monitored more intensively in the years 2007-2009, for a total of 121 different nights
and 393 photometric points (see Table 1 in the electronic version). During the
2007 campaign many fast outbursts are evident in the light curve (Fig. 1), while during
the 2008 campaign AH Her showed longer outbursts and standstill phases (Fig. 2).

\IBVSedata{5983-t1.txt}
\IBVSdataKey{5983-t1.txt}{AH Her}{photometry}


\newpage

\IBVSfig{7cm}{5983-f1.eps}{$R_c$ data of AH Her from April 24th to October 16th 2007 from Table 1 (filled circles). Small crosses represent visual
estimates available from AFOEV (\texttt{http://cdsweb.u-strasbg.fr/afoev/}).}

\IBVSfig{7cm}{5983-f2.eps}{$R_c$ data of AH Her from May 6th to October 11th 2008 from Table 1 (filled circles). Small crosses represent visual
estimates available from AFOEV (\texttt{http://cdsweb.u-strasbg.fr/afoev/}).}

\vspace*{-1cm}

To study the behaviour of the optical continuum of AH Her we corrected our observations by the interstellar extinction $E_{(B-V)} = 0.03$, then we converted the $UBVR_{C}I_{C}$ magnitudes in fluxes $f(\lambda)$ using the conversion factors reported by Bessell (1979). With
this raw fluxes the spectral flux distribution is sensibly different from that expected by a
standard accretion disk with the canonical power-law spectrum $f(\lambda)\propto \lambda^{7/3}$ (Lin \& Papaloizou, 1996). For many
times this behaviour has been considered an evidence that a steady and optically thick
model disk cannot account for the continuum distribution of dwarf novae during the outburst (see, for example, Spogli et al., 1998). However, for AH Her the model is consistent
with the observations if we take into consideration the secondary emission.

Fig. 3 shows the spectral flux distribution of AH Her at different stages of the outbursts
cycle. The spectral slope is flatter in quiescence and steeper during the maximum, and
in general is different from the spectral slope expected by a steady-state accretion disk.
The same behaviour has been observed by many authors, for example by Hamilton et al.
(2007) in the IUE spectra. Now we have considered the optically thick and geometrically
thin disk model described by la Dous (1989) with angle of inclination $i = 45\deg$, and Fig.~3
shows that our observations are consistent with the superposition of this disk emission
with a K5 V secondary star (Kurucz 1992, $T=4250 K$, $\log g=4.5$, $\log Z=0.0$). The disk
emission during the maximum is $\simeq 13$ times brighter than at quiescence.


\IBVSfig{9cm}{5983-f3.eps}{Examples of $UBVR_{C}I_{C}$ fluxes of AH Her from quiescence to the outburst (points). The solid
lines represent the superposition of a K2 V secondary (dashed line) with an accretion disk with different
levels of brightness (dotted lines). The dwarf nova and hot spot emissions have been omitted because
they give a marginal contribution in the UV only.}

\vspace*{-2cm}

\references

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\endreferences


\IBVSefigure{5983-f4.eps}
\IBVSecomment{Table reporting our database of regular multicolour photometric observations obtained from 2007 to 2009 with the addiction of a few data obtained in 2005. See 5983-t1.txt.}


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