Radio Activity from the Rapidly Rotating T dwarf 2MASS 2228-4310

TL;DR

We report the detection of 2MASS J22282889-4310262 (2M2228), a rapidly rotating T6/T6.5 brown dwarf, in VLA C-band archival data across two epochs, with Stokes I peak flux densities of 67.3±4.9 and 107.2±5.2 μJy beam^-1 and Stokes V peaks of 14.4±3.0 and -20.7±1.2 μJy beam^-1, indicating highly polarized bursts (f_c>50%). Burst intervals are ~47 and ~58 minutes, with the first event roughly aligning with half the known infrared rotation period (~85.8±0.32 minutes), suggesting rotational modulation. Brightness temperatures reach ~$T_B\sim(4.9-7.8)\times10^{8}$ K and implied magnetic field strength $B\gtrsim1.4$ kG, consistent with Electron Cyclotron Maser Emission (ECME) and a steep spectrum with spectral indices $\alpha\approx-2.56\pm0.61$ and $-1.65\pm0.40$, yielding isotropic spectral luminosities $L_\nu\sim(9.1-14.5)\times10^{12}$ erg s^-1 Hz^-1 and radio luminosities $L_R\sim(3.65-5.81)\times10^{22}$ erg s^-1. The results position 2M2228 as a valuable laboratory for magnetospheric-current auroral models and motivate joint JWST-radio campaigns to probe the link between auroral activity and atmospheric dynamics in T dwarfs.

Abstract

We present the detection of 2MASS J22282889-4310262 (2M2228), a T6/T6.5 brown dwarf, using the Karl G. Jansky Very Large Array (VLA) archival data observed at C band (4-8 GHz) over two observing epochs ($2\times96$ minutes). 2M2228 is detected at time and frequency averaged Stokes I and V peak flux densities of $67.3\pm4.9\ μ \rm{Jy beam}^{-1}$ and $14.4\pm3.0\ μ\text{Jy beam}^{-1}$ in the first epoch and $107.2\pm5.2\ μ\rm{Jy\ beam}^{-1}$ and $-20.7\pm1.2\ μ\text{Jy beam}^{-1}$ in the second epoch. This discovery constitutes the eighth and, notably, the most rapidly rotating T dwarf detected to date at radio wavelengths. Our observations reveal highly polarised bursts at fractional polarisation ratios $f_\text{c}>50$%. Using Stokes I light curves, we measure occurrence intervals of $\sim47$ and $\sim58$ minutes in the two observing epochs respectively with the first burst aligning within a half period timescale of the the previously measured mid infrared photometric period of $85.8\pm0.32$ minutes. We attribute the emission to the electron cyclotron maser emission (ECME) and constrain the magnetic field strength to $B\gtrsim1.4$ kG. We emphasise that the periods inferred are provisional considering the short observing durations. The combination of previously demonstrated atmospheric stability and newly detected radio emission in 2M2228 makes it a promising laboratory for testing magnetospheric currents-driven auroral models and for guiding future coordinated James Webb Space Telescope (JWST) and radio observations to probe the link between auroral activity and atmospheric dynamics in T-type brown dwarfs.

Figures (2)

• Figure 1: Stokes I and V images of 2M2228 for the observations conducted over two epochs. Panels (a) and (b) correspond to observations of the first epoch, and panels (c) and (d) to epoch two observations. (a) The source is detected in the Stokes I at a peak flux density $\textcolor{black}{67.3\pm4.9\ \muJy beam^{-1}}$. The $1\sigma$ r.m.s. noise in the image is $3.1\ \mu\text{Jy\,beam}^{-1}$ resulting in a signal to noise ratio (SNR) $\sim22$. (b) The corresponding Stokes V image is detected at a peak flux density $\textcolor{black}{14.4\pm3.0\ \muJy\,beam^{-1}}$ at a $1\sigma$ r.m.s noise of $3.3\ \mu\text{Jy\,beam}^{-1}$ giving a SNR $\sim4$. (c) Stokes I image for the second observing epoch. The detection is at a peak flux density $\textcolor{black}{107.2\pm5.2\ \muJy\,beam^{-1}}$ at a $1\sigma$ r.m.s noise of $3.9\ \mu\text{Jy\,beam}^{-1}$ giving a SNR $\sim27$. (d) The corresponding Stokes V image for the observation. The detection is at a peak flux density $\textcolor{black}{-20.7\pm1.2\ \muJy\,beam^{-1}}$ at a $1\sigma$ r.m.s noise of $3.5\ \mu\text{Jy\,beam}^{-1}$ giving a SNR $\sim6$. The peak flux is negative, indicating the left-hand circular polarisation is dominant. All the contours are drawn at $5\sigma\times(-4,-2\sqrt{2},-2,-\sqrt{2},-1,1)$. In all the images, positive contours are represented by the solid line and negative contours by the broken line. The white ellipse with a black outline to the bottom left of the image represents the synthesised beam. All the positions are with respect to the detected positions at the first epoch.
• Figure 2: