  2MASS J11011926-7732383 AB (hereafter 2M1101AB), located in the Cha I star
forming region, is a rare wide-separation brown dwarf binary. XMM-Newton and
Chandra observations of 2M1101AB have allowed us to examine the influence of
physical parameters (mass, bolometric luminosity and effective temperature) on
X-ray emission from a coeval pair of substellar objects. The spatial resolution
of XMM-Newton is not sufficient to separate contributions from the two
components in the binary. The X-ray source detected with XMM-Newton has a
column density compatible with the infrared extinction of component A. On the
other hand, the binary is resolved with Chandra, and the bulk of the X-ray
emission is clearly associated with the photospherically cooler component B.
These apparently contradictory results point at strong variability of 2M1101's
X-ray emission. Combined with previous sensitive X-ray observations from
low-mass members of ChaI, we find a decline of X-ray luminosity with decreasing
(sub)stellar mass that is typical for star forming regions. 2M1101B is the
coolest (spectral type M8.25) and least massive brown dwarf of ChaI detected in
X-rays so far. It is also among the youngest (~1 Myr) substellar ChaI members,
and therefore relatively luminous. Most bona fide brown dwarfs of Cha I have
remained below the sensitivity limits of available X-ray observations, because
of their low luminosity associated with higher age.
