X-ray detection of the substellar twin 2MASS J11011926-7732383 AB

INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: stelzer@astropa.unipa.it

Received: 27 March 2007
Accepted: 5 June 2007

Abstract

Context.2MASS J11011926-7732383 AB (hereafter 2M 1101 AB), located in the Cha I star forming region, is a rare wide-separation brown dwarf binary. Being spatially resolvable in many wavebands, it is a unique target for studying the properties of substellar twins.

Aims. Here, we exploit the coeval pair 2M 1101 AB to examine the influence of physical parameters (mass, bolometric luminosity and effective temperature) on X-ray emission from substellar objects.

Methods. We determine the X-ray properties of 2M 1101 A and B using XMM-Newton and Chandra observations.

Results. 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 2 M1101's X-ray emission. Combined with previous sensitive X-ray observations from low-mass members of Cha I, we find a decline of X-ray luminosity with decreasing (sub)stellar mass that is typical for star forming regions.

Conclusions. 2M 1101 B is the coolest (spectral type M8.25) and least massive brown dwarf of Cha I detected in X-rays so far. It is also among the youngest (~1 Myr) substellar Cha I 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.