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

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 L rad = ν L ν = (9 ± 3) × 10 23 erg s −1 and log 10 L rad / L bol = −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 S ν ∝ν −α with α ≳ 0.5. The emission is quiescent, with no evidence of variability or bursts over three hours 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, make 2MASS J1315−2649AB an important test for proposed radio emission mechanisms in ultracool dwarfs.