Detection of Radio Emission From the Hyperactive L Dwarf 2mass J13153094−2649513ab

Abstract

We report the detection of radio emission from the unusually active L5e + T7 binary 2MASS J13153094−2649513AB made with the Australian Telescope Compact Array. Observations at 5.5 GHz reveal an unresolved source with a continuum flux of 370±50 µJy, corresponding to a radio luminosity of Lrad = νLν = (9±3)×1023 erg s−1 and log10 Lrad/Lbol = −5.44±0.22. No detection is made at 9.0 GHz to a 5σ limit of 290 µJy, consistent with a power law spectrum Sν ∝ ν−α with α & 0.5. The emission is quiescent, with no evidence of variability or bursts over 3 hr of observation, and no measurable polarization (V/I < 34%). 2MASS J1315−2649AB is one of the most radio-luminous ultracool dwarfs detected in quiescent emission to date, comparable in strength to other cool sources detected in outburst. Its detection indicates no decline in radio flux through the mid-L dwarfs. It is unique among L dwarfs in having strong and persistent Hα and radio emission, indicating the coexistence of a cool, neutral photosphere (low electron density) and a highly active chromosphere (high electron density and active heating). These traits, coupled with the system’s mature age and substellar secondary, makes 2MASS J1315−2649AB an important test for proposed radio emission mechanisms in ultracool dwarfs.