Periodic radio emission from a rapidly rotating T8 ultracool dwarf
Abstract
We report the discovery of periodic radio bursts from the T8 brown dwarf WISE J151721.13+052929.3 (WISE J1517+05), detected with ASKAP and MeerKAT. The emission is characterised by a stable periodicity of 1.149 ± 0.002 hr, placing WISE J1517+05 among the fastest rotating radio-detected ultracool dwarfs (UCDs). The radio bursts are broadband and highly variable, reaching a peak spectral luminosity of Lν = 5.6 × 1015 erg s−1 Hz−1, making WISE J1517+05 the brightest periodic UCD to date. The high brightness temperature of the radio bursts (TB>1011 K), combined with the rotational modulation, strongly favour an auroral electron cyclotron maser emission (ECME) origin. In contrast to most known radio-bursting UCDs, the bursts exhibit relatively low and variable circular polarisation fractions (typically ∼ 20%). These properties suggest significant depolarisation of intrinsically highly polarised ECME, likely due to propagation effects in a structured magnetospheric plasma. Notably, we also observe a polarisation reversal within a single burst profile, which is currently unexplained. We model the emission using a dipolar magnetic field with emission arising from a single active field line, and find a moderate inclination angle of i = 47.9+11.4−9.2 degrees and a magnetic obliquity of β= 40.1+9.2−10.9 degrees. WISE J1517+05 expands the parameter space of radio-active UCDs toward higher luminosities and lower polarisation fractions, suggesting that current surveys may be sensitive to a larger population than previously recognised, while searches relying on methods that assume high circular polarisation fractions risk missing a subset of UCDs. More broadly, this discovery reinforces the idea that rapid rotation plays a key role in generating strong dipolar magnetic fields in UCDs that can give rise to auroral radio emission.
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BibTeX
@misc{ruiter2026periodic,
title = {{Periodic radio emission from a rapidly rotating T8 ultracool dwarf}},
author = {Iris de Ruiter and Joshua Pritchard and Tara Murphy and Andrew K. Hughes and David L. Kaplan and Kovi Rose and Andrew Zic},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.36336},
eprint = {2610.36336},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.36336},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Periodic radio emission from a rapidly rotating T8 ultracool dwarf
AU - Iris de Ruiter
AU - Joshua Pritchard
AU - Tara Murphy
AU - Andrew K. Hughes
AU - David L. Kaplan
AU - Kovi Rose
AU - Andrew Zic
PY - 2026
DA - 2026/10/26
PB - arXiv
DO - 10.48550/arxiv.2610.36336
UR - https://arxiv.org/abs/2610.36336
AB - We report the discovery of periodic radio bursts from the T8 brown dwarf WISE J151721.13+052929.3 (WISE J1517+05), detected with ASKAP and MeerKAT. The emission is characterised by a stable periodicity of 1.149 ± 0.002 hr, placing WISE J1517+05 among the fastest rotating radio-detected ultracool dwarfs (UCDs). The radio bursts are broadband and highly variable, reaching a peak spectral luminosity of L_ν = 5.6 × 10^15 erg s^−1 Hz^−1, making WISE J1517+05 the brightest periodic UCD to date. The high brightness temperature of the radio bursts (T_B>10^11 K), combined with the rotational modulation, strongly favour an auroral electron cyclotron maser emission (ECME) origin. In contrast to most known radio-bursting UCDs, the bursts exhibit relatively low and variable circular polarisation fractions (typically ∼ 20%). These properties suggest significant depolarisation of intrinsically highly polarised ECME, likely due to propagation effects in a structured magnetospheric plasma. Notably, we also observe a polarisation reversal within a single burst profile, which is currently unexplained. We model the emission using a dipolar magnetic field with emission arising from a single active field line, and find a moderate inclination angle of i = 47.9^+11.4_−9.2 degrees and a magnetic obliquity of β= 40.1^+9.2_−10.9 degrees. WISE J1517+05 expands the parameter space of radio-active UCDs toward higher luminosities and lower polarisation fractions, suggesting that current surveys may be sensitive to a larger population than previously recognised, while searches relying on methods that assume high circular polarisation fractions risk missing a subset of UCDs. More broadly, this discovery reinforces the idea that rapid rotation plays a key role in generating strong dipolar magnetic fields in UCDs that can give rise to auroral radio emission.
ER -