All Figures

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{3677f1.eps} \end{figure}$ Figure 1: BDs and BD candidates with known age on the model tracks from Baraffe et al. (1998) and Chabrier et al. (2000) in steps of $0.01~M_\odot$ . The $0.08~M_\odot$ track marks the separation from stars to BDs. Only objects that were detected with XMM-Newton or Chandra are shown. These include: squares - BDs in IC 348; triangles - BDs in Cha I; open circles - further BDs compiled from the literature, see labels on top of the figure; filled symbols - BDs observed in the Chandra program described in this paper. Objects on which an X-ray flare was detected are marked by a star symbol. The grey-shades indicate distinct mass and temperature ranges we study in this paper.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=4.5cm,clip]{3677f2a.ps} \hspace*... ....ps}\hspace*{4mm} \includegraphics[width=4.5cm,clip]{3677f2c.ps} }\end{figure}$ Figure 2: X-ray images of stellar systems with BD secondary observed with Chandra ACIS-S: left - HR 7329 (12 Myr), middle - Gl 659 (300 Myr), right - HD 130948 (550 Myr). The primaries are labeled "A''.
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In the text

$\begin{figure} \par\resizebox{8.5cm}{!}{\includegraphics{3677f3a.eps}}\par\resizebox{8.5cm}{!}{\includegraphics{3677f3b.eps}} \par\end{figure}$ Figure 3: Top: Chandra spectrum of HR 7329 B, best fit 1-T APEC model, and residuals. The dashed lines represent the spectrum of Gl 569 Bab (see Stelzer 2004), shown for comparison. Bottom: Chandra lightcurve of HR 7329 B. The background, also shown and scaled to the source extraction area, is negligible.
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In the text

$\begin{figure} \par\includegraphics[width=5.8cm,height=3.2cm,clip]{3677f4a.eps}\includegraphics[width=2.8cm,height=3.2cm,clip]{3677f4b.eps} \end{figure}$ Figure 4: X-ray lightcurve of HD 130948 A and background; source photons were extracted from an annulus with inner radius of $1.5^{\prime \prime }$ , thus avoiding the central portion of the PSF which is subject to pile-up.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{3677f5.eps}\end{figure}$ Figure 5: Median photon energy vs. X-ray temperature obtained from simulations of a 1-T APEC spectrum with photo-absorption. The individual curves correspond to spectra with different column density $N_{\rm H}$ . We assumed a standard extinction law of $N_{\rm H}$ [cm $^{-2}] = 2.0\times 10^{21}$ A_V[mag] (Ryter 1996). Squares denote BDs in the ONC from Preibisch et al. (2005a).
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In the text

$\begin{figure} \par\includegraphics[width=13.5cm,clip]{3677f6a.eps}\par\vspace*{... ...s}\par\vspace*{2mm} \includegraphics[width=13.7cm,clip]{3677f6c.eps}\end{figure}$ Figure 6: X-ray activity of BDs with $M \sim 0.05{-}0.07~M_\odot$ versus $L_{\rm bol}$ a), versus $T_{\rm eff}$ b) and versus age c). Plotting symbols: small circles - Time-averaged $L_{\rm x}$ from Chandra and XMM-Newton observations; star symbols - $L_{\rm x}$ in the flare peak; large circles - Quiescent $L_{\rm x}$ . Dotted lines in the uppermost panel indicate $L_{\rm x}/L_{\rm bol}$ levels of 10^-3, 10^-4, and 10^-5.
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In the text

$\begin{figure} \par\includegraphics[width=13.8cm,clip]{3677f7a.eps}\par\vspace*{2mm} \includegraphics[width=13.8cm,clip]{3677f7b.eps} \end{figure}$ Figure 7: X-ray activity for BDs with $T_{\rm eff} = 2400...2800$ K: top - $L_{\rm x}$ versus $L_{\rm bol}$ , bottom - $L_{\rm x}/L_{\rm bol}$ ratio versus age. BDs above the Baraffe et al. (1998) and Chabrier et al. (2000) models in the HR diagram are arbitrarily placed at an age of 0.5 Myr, and given open plotting symbols.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{3677f8.eps} \end{figure}$ Figure 8: Ratio of $\log{L_{\rm x}}$ versus $\log{L_{\rm bol}}$ for young BDs and BD candidates (spectral type M 5 and later): triangles, squares, open circles - BDs in Cha I, IC 348, TW Hya. The plot includes objects with spectral type M 5 and later with published measurement of H $\alpha _{\rm 10}$ . Accretors are marked with "A'' and a larger plotting symbol. Two objects in Cha I that have flared during the X-ray observation are marked with an asterisk.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{3677f9.eps} \end{figure}$ Figure 9: Spectra of the primaries, from top to bottom Gl 569 A during flare phase, Gl 569 A during quiescence, and average of HD 130948 A. The spectra of Gl 569 A are offset in the vertical direction for clarity. The enhanced bin at $\sim$ 1.85 keV in the flare spectrum of Gl 569 A may point at strong Si XIII emission.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=4.5cm,clip]{3677f2a.ps} \hspace... ....ps}\hspace{4mm} \includegraphics[width=4.5cm,clip]{3677f2c.ps} }\end{figure}$	Figure 2: X-ray images of stellar systems with BD secondary observed with Chandra ACIS-S: left - HR 7329 (12 Myr), middle - Gl 659 (300 Myr), right - HD 130948 (550 Myr). The primaries are labeled "A''.
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$\begin{figure} \par\resizebox{8.5cm}{!}{\includegraphics{3677f3a.eps}}\par\resizebox{8.5cm}{!}{\includegraphics{3677f3b.eps}} \par\end{figure}$	Figure 3: Top: Chandra spectrum of HR 7329 B, best fit 1-T APEC model, and residuals. The dashed lines represent the spectrum of Gl 569 Bab (see Stelzer 2004), shown for comparison. Bottom: Chandra lightcurve of HR 7329 B. The background, also shown and scaled to the source extraction area, is negligible.
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$\begin{figure} \par\includegraphics[width=5.8cm,height=3.2cm,clip]{3677f4a.eps}\includegraphics[width=2.8cm,height=3.2cm,clip]{3677f4b.eps} \end{figure}$	Figure 4: X-ray lightcurve of HD 130948 A and background; source photons were extracted from an annulus with inner radius of $1.5^{\prime \prime }$ , thus avoiding the central portion of the PSF which is subject to pile-up.
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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{3677f5.eps}\end{figure}$	Figure 5: Median photon energy vs. X-ray temperature obtained from simulations of a 1-T APEC spectrum with photo-absorption. The individual curves correspond to spectra with different column density $N_{\rm H}$ . We assumed a standard extinction law of $N_{\rm H}$ [cm $^{-2}] = 2.0\times 10^{21}$ A_V[mag] (Ryter 1996). Squares denote BDs in the ONC from Preibisch et al. (2005a).
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$\begin{figure} \par\includegraphics[width=13.8cm,clip]{3677f7a.eps}\par\vspace*{2mm} \includegraphics[width=13.8cm,clip]{3677f7b.eps} \end{figure}$	Figure 7: X-ray activity for BDs with $T_{\rm eff} = 2400...2800$ K: top - $L_{\rm x}$ versus $L_{\rm bol}$ , bottom - $L_{\rm x}/L_{\rm bol}$ ratio versus age. BDs above the Baraffe et al. (1998) and Chabrier et al. (2000) models in the HR diagram are arbitrarily placed at an age of 0.5 Myr, and given open plotting symbols.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{3677f9.eps} \end{figure}$	Figure 9: Spectra of the primaries, from top to bottom Gl 569 A during flare phase, Gl 569 A during quiescence, and average of HD 130948 A. The spectra of Gl 569 A are offset in the vertical direction for clarity. The enhanced bin at $\sim$ 1.85 keV in the flare spectrum of Gl 569 A may point at strong Si XIII emission.
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