Detection of Radio Emission from the Hyperactive L Dwarf 2MASS J13153094-2649513AB

TL;DR

This paper reports the detection of persistent radio emission from the ultracool dwarf 2MASS J1315-2649AB, providing insights into magnetic activity and emission mechanisms in cool, substellar objects.

Contribution

It presents the first detection of quiescent radio emission from this specific ultracool dwarf, challenging previous assumptions about radio flux decline in mid-L dwarfs.

Findings

Radio luminosity is comparable to other active ultracool dwarfs.

No variability or bursts observed over 3 hours.

Strong and persistent Halpha and radio emission coexist in this object.

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 microJy, corresponding to a radio luminosity of L_rad = nuL_nu = (9+/-3)x10^23 erg/s and log10(L_rad/L_bol) = -5.44+/-0.22. No detection is made at 9.0 GHz to a 5 sigma limit of 290 microJy, consistent with a power law spectrum S_nu ~ nu^-a with a > 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 Halpha 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.