Modelling the environment around five ultracool dwarfs via the radio domain

TL;DR

This study uses upgraded VLA radio observations to analyze the magnetic environments of five ultracool dwarfs, revealing unique polarized burst phenomena and modeling their magnetic fields.

Contribution

First detection of a broadband, polarized radio burst below 4 GHz with both positive and negative frequency drifts in an ultracool dwarf, and modeling of its magnetic field structure.

Findings

Detected a unique polarized burst with both positive and negative frequency drifts.

Modeled the magnetic field as a tilted dipole to reproduce burst characteristics.

Identified different polarization states among observed ultracool dwarfs.

Abstract

We present the results of a series of short radio observations of six ultracool dwarfs made using the upgraded VLA in S (2--4GHz) and C (4--7GHz) bands. LSR J1835+3259 exhibits a 100 percent right-hand circularly polarised burst which shows intense narrowband features with a fast negative frequency drift of about $-30$ MHz $\textrm{s}^{-1}$. They are superimposed on a fainter broadband emission feature with a total duration of about 20 minutes, bandwidth of about 1 GHz, centred at about 3.5 GHz, and a slow positive frequency drift of about 1 MHz $\textrm{s}^{-1}$. This makes it the first such event detected below 4 GHz and the first one exhibiting both positive and negative frequency drifts. Polarised radio emission is also seen in 2MASS J00361617+1821104 and NLTT 33370, while LP 349-25 and TVLM 513-46546 have unpolarised emission and BRI B0021-0214 was not detected. We can reproduce the main characteristics of the burst from LSR J1835+3259 using a model describing the magnetic field of the dwarf as a tilted dipole. We also analyse the origins of the quiescent radio emission and estimate the required parameters of the magnetic field and energetic electrons. Although our results are non-unique, we find a set of models which agree well with the observations.