All Figures

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{H4368F1.eps}}\end{figure}$ Figure 1: The observed UVX average structure function (thick solid) and the best-fitting (in a least-square sense) average structure functions predicted by microlensing scenarios with parameter constraints $\Omega _{\rm compact}\leq 0.15$ (thin solid), $M_{\rm compact}\geq 10^{-1} \ M_\odot$ (thick dashed), and $R_{\rm QSO}\geq 3\times 10^{13}$ m (thin dashed). All simulated average structure functions obeying these parameter constraints but having larger residuals are located below the ones plotted.
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In the text

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{H4368F2.eps}}\end{figure}$ Figure 2: The observed UVX average structure function (thick solid) and the ten best-fitting average structure functions derived from microlensing scenarios (dashed).
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In the text

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{H4368F3.eps}}\end{figure}$ Figure 3: The cumulative probabilities $P(\leq$ $\delta m)$ of observing quasars with amplitudes lower than $\delta m$ for the UVX sample (thick solid). The other lines represent the corresponding distributions derived from microlensing simulations assuming $\Omega _{\rm compact}=0.3$ , $R_{\rm QSO}=10^{12}$ m, $\sigma _{\rm v,compact}=400$ km s^-1 and $M_{\rm compact}=1 \ M_\odot$ (thin solid), $10^{-1} \ M_\odot$ (thick dashed), $10^{-2} \ M_\odot$ (thin dashed), $10^{-3} \ M_\odot$ (thick dash-dotted), $10^{-4} \ M_\odot$ (thin dash-dotted), $10^{-5} \ M_\odot$ (thick dotted).
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In the text

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{H4368F4.eps}}\end{figure}$ Figure 4: Same as Fig. 3 except $R_{\rm QSO}=3\times 10^{13}$ m.
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In the text

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{H4368F5.eps}}\end{figure}$ Figure 5: The fraction of objects with amplitudes higher than 0.35 mag predicted by microlensing scenarios for different $M_{\rm compact}$ , assuming $\Omega _{\rm compact}=0.3$ , $\sigma _{\rm v,compact}=600$ km s^-1 and $R_{\rm QSO}=10^{12}$ m (thick solid), $3\times 10^{12}$ m (thin solid), 10¹³ m (thick dash-dotted), $3\times 10^{13}$ m (thin dash-dotted). The thick dashed line marks the observed fraction (93%) of the UVX quasar sample.
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In the text

$\begin{figure} \par\resizebox{8.6cm}{!}{\includegraphics{H4368F6.eps}}\end{figure}$ Figure 6: The empirical relation between mean amplitude and redshift for the UVX quasar sample (thick black) and the corresponding relations derived from the 432 different microlensing parameter combinations generated (thin grey). The error bars indicate the uncertainty in the position of the observed mean.
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In the text

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{H4368F7.eps}}\end{figure}$ Figure 7: The empirical relation between mean amplitude and redshift for the UVX quasar sample (thick solid) and the corresponding predictions from microlensing simulations assuming different background cosmologies: the standard $\Omega _{\rm M}=0.3$ , $\Omega _\Lambda =0.7$ case (thin solid), $\Omega _{\rm M}=0.4$ , $\Omega _\Lambda =0.6$ (thick dashed), $\Omega _{\rm M}=0.2$ , $\Omega _\Lambda =0.8$ (thin dashed) and $\Omega _{\rm M}=1.0$ , $\Omega _\Lambda =0.0$ (thick dash-dotted). All simulations assume $\Omega _{\rm compact}=\Omega _{\rm M}$ , $M_{\rm compact}=10^{-4} \ M_\odot$ , $R_{\rm QSO}=10^{12}$ m and $\sigma _{\rm v,compact}=400$ km s^-1. The error bars indicate the uncertainty in the position of the observed mean.
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In the text

$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{H4368F2.eps}}\end{figure}$	Figure 2: The observed UVX average structure function (thick solid) and the ten best-fitting average structure functions derived from microlensing scenarios (dashed).
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$\begin{figure} \par\resizebox{8.8cm}{!}{\includegraphics{H4368F4.eps}}\end{figure}$	Figure 4: Same as Fig. 3 except $R_{\rm QSO}=3\times 10^{13}$ m.
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$\begin{figure} \par\resizebox{8.6cm}{!}{\includegraphics{H4368F6.eps}}\end{figure}$	Figure 6: The empirical relation between mean amplitude and redshift for the UVX quasar sample (thick black) and the corresponding relations derived from the 432 different microlensing parameter combinations generated (thin grey). The error bars indicate the uncertainty in the position of the observed mean.
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