A CHANDRA OBSERVATION OF THE TW HYDRAE ASSOCIATION BROWN DWARF 2MASSW J1139511-315921

We report on a sequence of Chandra X-Ray Observatory observations of the TW Hydrae brown dwarf (BD) 2MASSW J1139511-315921 (2M1139). In the combined 31 ks ACIS-S exposure, 2M1139 is detected at the 3 sigma confidence level. We find an X-ray luminosity of L-X = 1.4(-1.0)(+2.7) x 10(26) erg s(-1) or log L-X/L-bol = -4.8 +/- 0.3. This object is similar to another TW Hydrae BD member, CD-33 7795B (TWA 5B): both have Ha emission, both show no signatures of accretion, and both have comparable ages and spectral types. TWA 5B was previously detected in X-rays with a luminosity of L-X = 4 x 10(27) erg s(-1) or log L-X/L-bol = -3.4, an order of magnitude more luminous in X-rays than 2M1139. We find that the discrepancy between the X-ray luminosity of 2M1139 and TWA 5B is consistent with the spread in X-ray luminosity in the Orion Nebula Cluster for BDs of similar spectral types. Though rotation may play a role in the X-ray activity of ultracool dwarfs like 2M1139 and TWA 5B, the discrepancy cannot be explained by rotation alone. We also examine two X-ray bright objects in the field of view of our Chandra observations and find one to be of spectral type K0IV and identify it as a possible RS Canum Venaticorum, and another X-ray bright object whose light curve clearly shows the decay phase of an X-ray flare.