All Figures

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{6477fig1.eps}\end{figure}$ Figure 1: PKS2155-304 field (DSS-1 survey). Letters indicate stars used for calibration.
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In the text

$\begin{figure} \par\includegraphics[width=10.3cm,clip]{6477fig2.eps}\end{figure}$ Figure 2: Normalized light curves of PKS 2155-304. Flux is reported in arbitrary units (au). A typical error bar is plotted in each box.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fig3.eps}\end{figure}$ Figure 3: V/H flux ratio evolution during 2005. Error bars are comparable to the symbol size.
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In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{6477fig4.eps}\end{figure}$ Figure 4: Light curves in the H and V filters for five nights in November 2005, when the observations were more intensive. Dates of observations are reported in each box. The solid line in the V band - 4 Nov. box results from a linear regression analysis. The solid line in the H band - 8 Nov. box connects the four points of the flare-like structure. In each box a typical error bar is given. In V band - 4 Nov. box, the light curve of one comparison star is also plotted, with a fixed enhancement of 14 mJy.
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In the text

$\begin{figure} \par\includegraphics[width=10.4cm,clip]{6477fig5.eps}\end{figure}$ Figure 5: V/H flux ratio versus intensity for the five more intensively observed nights of epoch 3. A typical error bar is plotted in each box.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fig6.eps}\end{figure}$ Figure 6: September 2005 spectra for observations including the K and J filters. The spectral fit on average data with a single power law yields a spectral index $\alpha$ = 0.91 $\pm$ 0.07.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fig7.eps}\end{figure}$ Figure 7: 2006 June 29 spectrum. The spectral fit with a single power law yields a spectral index of $\alpha$ = 0.90 $\pm$ 0.16.
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In the text

$\begin{figure} \par\includegraphics[width=7.35cm,clip]{6477fig8.eps}\end{figure}$ Figure 8: 2005 May 13 spectrum - epoch 1. We also report the spectrum of the host galaxy (see text). The spectral fit with a single power law yields a spectral index of $\alpha$ = 0.77 $\pm$ 0.16.
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In the text

$\begin{figure} \par\includegraphics[width=7.35cm,clip]{6477fig9.eps}\end{figure}$ Figure 9: 2005 September 19 spectrum - epoch 2. For comparison we report the ESO 3.6 m telescope spectrophotometry that corresponds to a slightly lower state of the source. The spectral fit with a single power law yields a spectral index of $\alpha$ = 0.88 $\pm$ 0.05.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fi10.eps}\end{figure}$ Figure 10: 2005 November 4 spectrum - epoch 3. The spectral fit with a single power law yields a spectral index $\alpha$ = 1.33 $\pm$ 0.10. Error bars are comparable with symbol size.
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In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{6477fi11.eps}\end{figure}$ Figure 11: Different spectra of PKS 2155-304 from observations at other epochs reported in the literature. Symbols correspond to the following works: filled circles, this work (13/5/2005 data); filled squares, this work (19/9/2005 data); filled triangles, this work (4/11/2005 data); open diamonds, Bertone et al. (2000; 24/5/1996 data); open circles Pesce et al. (1997; 19/5/1994 data, the Hamuy & Maza (1989) calibration is used); open up triangles, Zhang & Xie (1996; 5/7/1994 data); open squares, Bersanelli et al. (1992; 17/1/1987 data); open crosses, Treves et al. (1989; 1/12/1983 data); open stars, Treves et al. (1989; 11/11/1984 data); asterisks, Miller & McAlister (1983; 19/11/1981 data). Spectral index values and V magnitudes for all data sets are reported in Table 5.
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In the text

$\begin{figure} \par\includegraphics[width=8.6cm,clip]{6477fi12.eps}\end{figure}$ Figure 12: $\alpha$ vs. V plot for data reported in Fig. 11. Symbols are the same as in Fig. 11.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fi13.eps}\end{figure}$ Figure 13: SED of PKS 2155-304 in two states, adapted from Chiappetti et al. (1999) (see text for details). Data from this work are also plotted. Filled triangles correspond to epoch 1 (13/5/2005 data), while filled hexagons belong to epoch 3 data (20/11/2005). Optical, UV and REM data are dereddened using E(B-V)=0.026 and parameters given by Cardelli et al. (1989).
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In the text

$\begin{figure} \par\includegraphics[width=7.65cm,clip]{6477fig1.eps}\end{figure}$	Figure 1: PKS2155-304 field (DSS-1 survey). Letters indicate stars used for calibration.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{6477fig2.eps}\end{figure}$	Figure 2: Normalized light curves of PKS 2155-304. Flux is reported in arbitrary units (au). A typical error bar is plotted in each box.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fig3.eps}\end{figure}$	Figure 3: V/H flux ratio evolution during 2005. Error bars are comparable to the symbol size.
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$\begin{figure} \par\includegraphics[width=10.4cm,clip]{6477fig5.eps}\end{figure}$	Figure 5: V/H flux ratio versus intensity for the five more intensively observed nights of epoch 3. A typical error bar is plotted in each box.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fig6.eps}\end{figure}$	Figure 6: September 2005 spectra for observations including the K and J filters. The spectral fit on average data with a single power law yields a spectral index $\alpha$ = 0.91 $\pm$ 0.07.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fig7.eps}\end{figure}$	Figure 7: 2006 June 29 spectrum. The spectral fit with a single power law yields a spectral index of $\alpha$ = 0.90 $\pm$ 0.16.
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$\begin{figure} \par\includegraphics[width=7.35cm,clip]{6477fig8.eps}\end{figure}$	Figure 8: 2005 May 13 spectrum - epoch 1. We also report the spectrum of the host galaxy (see text). The spectral fit with a single power law yields a spectral index of $\alpha$ = 0.77 $\pm$ 0.16.
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$\begin{figure} \par\includegraphics[width=7.35cm,clip]{6477fig9.eps}\end{figure}$	Figure 9: 2005 September 19 spectrum - epoch 2. For comparison we report the ESO 3.6 m telescope spectrophotometry that corresponds to a slightly lower state of the source. The spectral fit with a single power law yields a spectral index of $\alpha$ = 0.88 $\pm$ 0.05.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fi10.eps}\end{figure}$	Figure 10: 2005 November 4 spectrum - epoch 3. The spectral fit with a single power law yields a spectral index $\alpha$ = 1.33 $\pm$ 0.10. Error bars are comparable with symbol size.
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$\begin{figure} \par\includegraphics[width=8.6cm,clip]{6477fi12.eps}\end{figure}$	Figure 12: $\alpha$ vs. V plot for data reported in Fig. 11. Symbols are the same as in Fig. 11.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{6477fi13.eps}\end{figure}$	Figure 13: SED of PKS 2155-304 in two states, adapted from Chiappetti et al. (1999) (see text for details). Data from this work are also plotted. Filled triangles correspond to epoch 1 (13/5/2005 data), while filled hexagons belong to epoch 3 data (20/11/2005). Optical, UV and REM data are dereddened using E(B-V)=0.026 and parameters given by Cardelli et al. (1989).
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