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Figure 1: The XMM-Newton Optical/UV Monitor (OM) image in the U-band of the young brown dwarf (BD) 2MASS J0414. This sky image is the sum of ten OM central window exposures; the total on-source exposure was about 7 h. The image size is with North up and East left, with pixels of ; the stretch of the color scale is logarithmic. This image is centered on the weak-line T Tauri star V773 Tau located only  away from the BD. The Classical T Tauri star FM Tau is also visible. The -radius circle centered on 2MASS J0414 indicates the aperture used in the photometry measurement. The FWHM of the OM in the U-band is .

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In the text

	Figure 2: The OM light curve of 2MASS J0414 in the U-band. The image sequence shows enlargements (field-of-view of with North up and East left, and with pixel-size) of the OM central small window exposures (2480 s) in the U-band; the stretch of the color scale is linear. The OM observed for 6 h showed an increase in the brightness (  mag).
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In the text

Figure 3: X-ray image of V773 Tau in the 0.5-2 keV energy band obtained with XMM-Newton/EPIC (pn+MOS1+MOS2). The field-of-view and orientation are the same as that in Fig. 1. The stretch of the color scale is logarithmic. 2MASS J0414 is located on the PSF wings of V773 Tau, where no X-ray counterpart was found with the source detection algorithm. The -radius circle indicates the region used for the selection of the background+source X-ray events. The -width annulus centered on V773 Tau indicates the region used for the selection of the background X-ray events; the circular region centered on FM Tau, another X-ray source, was excluded from this annulus.

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In the text

Figure 4: XMM-Newton/EPIC pn X-ray light curve of 2MASS J0414. The solid lines show the source+background light curve at the location of 2MASS J0414 (see the definition of selected regions in Fig. 3). The dashed/dotted lines show the estimate of the background level inside the extraction region for comparison. The time bin size is one hour. The scale of the time axis is identical to the one used in Fig. 2. 2MASS J0414 is not detected in X-rays.

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In the text