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$\begin{figure} \par\includegraphics[width=16.2cm,clip]{5559fig1.eps} \end{figure}$ Figure 1: The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST). Contours show the ¹²CO emission (Dame et al. 1987) of the Taurus Molecular Cloud (TMC), overlaid on a visual extinction map (linear colour scale, with black colour corresponding to $A_{\rm V}\sim 6$ mag; based on DSS I, 6 $^\prime$ -resolution map, Dobashi et al. 2005). The 27 XMM-Newton fields of view of the XEST are plotted with continuous/dashed circles (of which seven are archival observations and one is a separate program on T Tau; dashed circles). Note the outlying XMM-Newton fields of view around SU Aur (NE corner) and L1551 (S). The two squares near labels 3 and 8 show the archival Chandra fields of view used in this work (see Table A.1). The continuous line shows the survey of brown dwarfs (BDs) performed with the Canada-France-Hawaii telescope (Guieu et al. 2006). This region hosts 42 young BDs of the TMC (Briceño et al. 2002,1998; Luhman 2004,2000,2006; Martín et al. 2001; Guieu et al. 2006). Black dots indicate the 25 BDs of this region which were not surveyed in X-rays with the XEST. White dots show the 8 BDs not detected in X-rays. Crosses show the 9 BDs detected in X-rays (Table 2) with labeled numbers referring to BD names. Only 2 BDs of TMC were previously detected in X-rays by ROSAT: MHO 4 (Neuhäuser et al. 1999; Carkner et al. 1996) and CFHT-BD-Tau 4 (Mokler & Stelzer 2002; Briceño et al. 1999).
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In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{5559fig2.eps} \end{figure}$ Figure 2: HR diagram of the 17 BDs of the TMC surveyed in the XEST. The references for the computation of the effective temperatures and the luminosities are given in Table 1. The pre-main-sequence tracks from Baraffe et al. (1998) are shown for comparison. Continuous lines show mass tracks from 0.1 down to 0.02 $M_\odot$ in steps of 0.01 $M_\odot$ ; the dashed line indicates the stellar/substellar boundary at 0.075 $M_\odot$ , which is equivalent to spectral type later than M6V for a typical TMC member age of 3 Myr. A cross indicates typical uncertainties. Two circles have been slightly moved in spectral type to avoid overlaps. The 9 BDs detected in X-rays are marked with "X''.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{5559fig3.eps} \end{figure}$ Figure 3: Light curve of the X-ray flare from CFHT-BD-Tau 1. The bin size is 1000 s. The grey thick line shows the fit of the non-zero count rates using a quiescent level plus an exponential rise and decay.
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In the text

$\begin{figure} \par\begin{tabular}{@{}c@{}c@{}c@{}} \includegraphics[width=6.7cm... ...CMJN.eps} & \includegraphics[width=4cm]{5559fi4f.eps}\end{tabular} \end{figure}$ Figure 4: X-ray spectra of TMC BDs. From top to bottom and left to right: 2MASS J0422 (pn spectrum), MHO 4 ( XMM-Newton and Chandra spectra), CFHT-Tau 5 ( XMM-Newton spectra), CFHT-BD-Tau 1 ( XMM-Newton spectra), CFHT-BD-Tau 4 ( Chandra spectra). Black, red, and green code for XMM-Newton/EPIC pn, MOS1, and MOS2 spectra, respectively. The continuous lines show our best fits obtained with an absorbed single-temperature or two-temperature (MHO 4 spectra) plasma model (see Table 3).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{5559fi5a.eps}\par\vspace*{2.5mm} \includegraphics[width=8.8cm,clip]{5559fi5b.eps}\par \end{figure}$ Figure 5: Quantile diagram for pn ( top) and MOS1+MOS2 ( bottom). The grid of hydrogen column density and plasma temperature were computed following the prescriptions of Hong et al. (2004) using in SHERPA an absorption model ( WABS; Morrison & McCammon 1983) multiplied with a single optically thin thermal plasma model (MEKAL; Mewe et al. 1995) with 0.3 times the solar elemental abundances. The background map in grey levels indicates the degeneracy level of grid parameters (see online Appendix C). The MOS1+MOS2 quantile diagram is used only for X-ray sources without available pn data. In the PN quantile diagram, asterisks and diamond mark the plasma parameters obtained from spectral fitting; the dashed line indicates the X-ray colour locus corresponding to the parameter uncertainties of source #4a (diamond; Table 3). The spectrum of #3 requires a two-temperature plasma model (see Table 3).
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In the text

$\begin{figure} \par\includegraphics[height=9cm,clip]{5559fig6.eps} \end{figure}$ Figure 6: X-ray luminosity vs. bolometric luminosity for the TMC BDs (diamonds; and arrows for upper limits) and the TMC members detected in the XEST (white and black dots represent low-mass stars and protostars, respectively; Güdel et al. 2007). The dotted lines indicate from bottom to top an X-ray fractional luminosity, $\eta=\log (L_{\rm X}/L_*)$ , of -5, -4, -3, -2. The grey stripe shows a linear regression fit (continuous line) and standard deviation (dashed lines) for detected objects with bolometric luminosities lower than 10 $L_\odot$ .
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In the text

$\begin{figure} \par\includegraphics[height=9cm,clip]{5559fig7.eps} \end{figure}$ Figure 7: X-ray fractional luminosity vs. mass for the young BDs of the TMC and single TMC members of the XEST. The symbols are as in Fig. 6. The vertical dotted lines indicates the stellar/substellar boundary. The solid and dashed lines show a linear regression fit and standard deviation, respectively.
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In the text

$\begin{figure} \par\includegraphics[height=9cm,clip]{5559fig8.eps} \end{figure}$ Figure 8: X-ray fractional luminosity vs. effective temperature for the young BDs of the TMC and single TMC members of the XEST. The symbols are as in Fig. 6. The vertical dotted lines indicates the stellar/substellar boundary. The solid and dashed lines show a linear regression fit and standard deviation, respectively.
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In the text

$\begin{figure} \par\includegraphics[height=9cm,clip]{5559fig9.eps} \end{figure}$ Figure 9: X-ray surface flux vs. effective temperature for the young BDs of the TMC and single TMC members of the XEST with spectral type M0 or later. The symbols are as in Fig. 6. The solid and dashed lines show a linear regression fit and standard deviation, respectively.
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In the text

$\begin{figure} \par\includegraphics[width=9cm]{5559fi10.eps} \end{figure}$ Figure 10: X-ray fractional luminosity of TMC BDs vs. ${\rm H}\alpha$ . TMC BDs detected in X-rays are marked with "X''.
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In the text

$\begin{figure} \par\includegraphics[width=9cm]{5559fi11.eps} \end{figure}$ Figure 11: Equivalent width of H $\alpha$ emission lines vs. spectral type for TMC BDs. TMC BDs detected in X-rays are marked with "X''. The dashed line shows the saturation limit of chromospheric activity at $\log[L({\rm H\alpha})/L_*]=-3.3$ (determined in the open clusters; Barrado y Navascués & Martín 2003).
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In the text

$\begin{figure} \par\includegraphics[width=9cm]{5559fi12.eps} \end{figure}$ Figure 12: Cumulative distributions of the X-ray fractional luminosities for nonaccreting (continuous line) and accreting (dashed line) BDs. In the top panel, diamonds and arrows indicate detections and upper limits, respectively of the two samples. Only Kaplan-Meier estimator error bars of the nonaccreting sample are shown to clarify the plot. Two-population statistical methods show that both samples are drawn from the same underlying distribution.
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In the text

$\begin{figure} \par\includegraphics[width=9cm]{5559fi13.eps} \end{figure}$ Figure 13: Cumulative distributions of the X-ray fractional luminosities of XEST (continuous line) and COUP (dashed line) BDs. In the top panel, diamonds and arrows indicate detections and upper limits, respectively of the two samples. Only Kaplan-Meier estimator error bars of the nonaccreting sample are plotted to clarify the plot. Two-population statistical methods show that XEST BDs are more active in X-rays than COUP BDs.
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In the text

$\begin{figure} \par\includegraphics[angle=180,width=12cm,clip]{5559fi14.eps} \end{figure}$ Figure 14: X-ray fractional luminosity vs. spectral type for objects of type M 5 and later. Detections of late M field stars from Fleming et al. (1993) are shown as asterisks. The circles show low-mass stars of the TMC detected in X-rays (Güdel et al. 2007). Diamonds and thick arrows show BDs in the TMC. The other X-ray detected BDs (see Preibisch et al. 2005b, and references therein) are shown by gray filled squares. For very cool objects with strong flares, the values at flare peak are shown by triangles, connected by dotted lines to the quiescent emission. Some symbols have been slightly moved in spectral type to avoid overlaps.
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In the text

$\begin{figure} \par\includegraphics[width=5.5cm]{5559fB1a.eps}\includegraphics[w... ...th=5.5cm]{5559fB1e.eps}\includegraphics[width=5.5cm]{5559fB1f.eps} \end{figure}$ Figure B.1: X-ray light curves of BDs in the TMC. From top to bottom and left to right: 2MASS J0422 (only pn data), KPNO-Tau 5, CFHT-BD-Tau 3, CFHT-Tau 6 (EPIC), CFHT-BD-Tau 4 (ACIS-I), and 2MASS J0455 (only MOS data). On the Chandra light curve, black dots indicate the arrival time and energy of individual X-ray photons. These light curves are consistent with quiescent emission.

In the text

$\begin{figure} \par\includegraphics[width=5.5cm,clip]{5559fB2a.eps}\hspace*{3mm}... ...ps}\hspace*{3mm} \includegraphics[width=5.5cm,clip]{5559fB2d.eps} \end{figure}$ Figure B.2: X-ray light curves of BDs in the TMC observed at two different epochs. Top and bottom row show MHO 4 and CFHT-Tau 5 light curves, respectively. During the second XMM-Newton observation of CFHT-Tau 5, the source is located on an EPIC pn gap, which explains the count rate difference observed between the two epochs.

In the text

$\begin{figure} \par\begin{tabular}{@{}cc@{}} \includegraphics[width=9cm]{5559fC1a.eps} & \includegraphics[width=9cm]{5559fC1b.eps}\end{tabular} \end{figure}$ Figure C.1: Degeneracy map of the absorption column density parameter ( $N_{\rm H}$ ) in the quantile diagram. Left: quantile diagram showing in the (x, y) space colours the loci of constant $N_{\rm H}$ values (dotted lines) for a WABS $\times$ MEKAL plasma model with 0.3 times the solar elemental abundances using XMM-Newton EPIC pn RMF and ARF. Grey levels indicate for each (x, y) value the number of corresponding absorption column density values. Regions where a (x, y) value correspond to a unique value of $N_{\rm H}$ are coloured in light grey. Right: 3-dimensional shape of the surface generated by $N_{\rm H}$ from the colour space of the quantile diagram (x, y). Folds produce the degeneracy observed in the quantile diagram.

In the text

$\begin{figure} \begin{tabular}{@{}cc@{}} \includegraphics[width=9cm]{5559fC2a.eps} & \includegraphics[width=9cm]{5559fC2b.eps}\end{tabular}%\end{figure}$ Figure C.2: Degeneracy map of the plasma temperature parameter (kT) in the quantile diagram. Left: quantile diagram showing in the (x, y) space colours the loci of constant kT values (dotted lines) for a WABS $\times$ MEKAL plasma model with 0.3 solar elemental abundance using XMM-Newton EPIC pn RMF and ARF. Grey levels indicate for each (x, y) value the number of corresponding plasma temperature values. Regions where a (x, y) value correspond to a unique value of kT are coloured in light grey. Right: 3D shape of the surface generated by kT from the parameter space of the quantile diagram (x, y). Folds produce the degeneracy observed in the quantile diagram.

In the text

$\begin{figure} \par\begin{tabular}{@{}cc@{}} \includegraphics[width=9cm]{5559fC3a.eps} & \includegraphics[width=9cm]{5559fC3b.eps}\end{tabular} \end{figure}$ Figure C.3: Illustration of degenerated solutions. Left: horizontal cut of the $N_{\rm H}$ surface (Fig. C.1 right) at y=1.3. Right: horizontal cut of the kT surface (Fig. C.2 right) at the same elevation. The line colour indicates the degeneracy level of the parameter. Numbers label the boundary/changing points when one moved on these surfaces along the xdirection, and give the correspondance between the two parameters.

In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{5559fig3.eps} \end{figure}$	Figure 3: Light curve of the X-ray flare from CFHT-BD-Tau 1. The bin size is 1000 s. The grey thick line shows the fit of the non-zero count rates using a quiescent level plus an exponential rise and decay.
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$\begin{figure} \par\includegraphics[height=9cm,clip]{5559fig7.eps} \end{figure}$	Figure 7: X-ray fractional luminosity vs. mass for the young BDs of the TMC and single TMC members of the XEST. The symbols are as in Fig. 6. The vertical dotted lines indicates the stellar/substellar boundary. The solid and dashed lines show a linear regression fit and standard deviation, respectively.
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$\begin{figure} \par\includegraphics[height=9cm,clip]{5559fig8.eps} \end{figure}$	Figure 8: X-ray fractional luminosity vs. effective temperature for the young BDs of the TMC and single TMC members of the XEST. The symbols are as in Fig. 6. The vertical dotted lines indicates the stellar/substellar boundary. The solid and dashed lines show a linear regression fit and standard deviation, respectively.
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$\begin{figure} \par\includegraphics[height=9cm,clip]{5559fig9.eps} \end{figure}$	Figure 9: X-ray surface flux vs. effective temperature for the young BDs of the TMC and single TMC members of the XEST with spectral type M0 or later. The symbols are as in Fig. 6. The solid and dashed lines show a linear regression fit and standard deviation, respectively.
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$\begin{figure} \par\includegraphics[width=9cm]{5559fi10.eps} \end{figure}$	Figure 10: X-ray fractional luminosity of TMC BDs vs. ${\rm H}\alpha$ . TMC BDs detected in X-rays are marked with "X''.
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$\begin{figure} \par\includegraphics[width=9cm]{5559fi11.eps} \end{figure}$	Figure 11: Equivalent width of H $\alpha$ emission lines vs. spectral type for TMC BDs. TMC BDs detected in X-rays are marked with "X''. The dashed line shows the saturation limit of chromospheric activity at $\log[L({\rm H\alpha})/L_*]=-3.3$ (determined in the open clusters; Barrado y Navascués & Martín 2003).
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