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The XMM-Newton Extended Survey of the TMC (XEST) is a large program designedto systematically investigate the X-ray properties of young stellar/substellarobjects in the TMC. In particular, the area surveyed by 15 XMM-Newton pointings(of which three are archival observations), supplemented with one archivalChandra observation, allows us to study 17 BDs with M spectral types. Half ofthis sample (9 out of 17 BDs) is detected; 7 BDs are detected here for thefirst time in X-rays. We observed a flare from one BD. We confirm severalprevious findings on BD X-ray activity: a log-log relation between X-ray andbolometric luminosity for stars (with L*<10 Lsun) and BDs detected in X-rays; ashallow log-log relation between X-ray fractional luminosity and mass; alog-log relation between X-ray fractional luminosity and effective temperature;a log-log relation between X-ray surface flux and effective temperature. Wefind no significant log-log correlation between the X-ray fractional luminosityand EW(Halpha). Accreting and nonaccreting BDs have a similar X-ray fractionalluminosity. The median X-ray fractional luminosity of nonaccreting BDs is about4 times lower than the mean saturation value for rapidly rotating low-massfield stars. Our TMC BDs have higher X-ray fractional luminosity than BDs inthe Chandra Orion Ultradeep Project. The X-ray fractional luminosity declinesfrom low-mass stars to M-type BDs, and as a sample, the BDs are less efficientX-ray emitters than low-mass stars. We thus conclude that while the BDatmospheres observed here are mostly warm enough to sustain coronal activity, atrend is seen that may indicate its gradual decline due to the drop inphotospheric ionization degree (abridged).

X-ray emission from the young brown dwarfs of the Taurus molecular cloud