2003ApJ...587L..51T

We report the X-ray detection of the brown dwarf (BD) companion TWA 5B in an ≃12 Myr old pre-main-sequence binary system. We clearly resolve the faint companion (35 photons) separated from the X-ray luminous primary by 2" in a Chandra ACIS image. TWA 5B shows a soft X-ray spectrum with a low plasma temperature of only 0.3 keV and a constant flux during the 3 hr observation, of which the characteristics are commonly seen in the solar corona. The X-ray luminosity is 4x10²⁷ ergs/s (0.1-10 keV band) or logL_X/L_bol=-3.4. Comparing these properties to both younger and older BDs, we discuss the evolution of the X-ray emission in BDs. During their first few megayears, they exhibit high levels of X-ray activity, as seen in higher mass pre-main-sequence stars. The level in TWA 5B is still high at t≃12 Myr in logL_X/L_bol, while kT has already substantially cooled.