WISE J1517+05 emits powerful radio bursts every 1.149 hours

The T8 brown dwarf WISE J1517+05 produces radio bursts that repeat every 1.149 hours, making it one of the fastest-spinning ultracool dwarfs detected in radio waves. Its bursts are unusually bright but show relatively weak, changing circular polarisation. Their properties point to aurora-like emissions shaped by the dwarf’s magnetic field and surrounding plasma.

Star system: WISE J1517+0529 | Distance from the Sun: 68.1 light years

The T8 brown dwarf WISE J151721.13+052929.3, also known as WISE J1517+05, produces radio bursts that repeat every 1.149 hours. That regular rhythm places it among the fastest-spinning ultracool dwarfs detected through radio emissions. Its bursts are also exceptionally bright, making WISE J1517+05 the brightest ultracool dwarf with periodic radio bursts reported to date.

ASKAP and MeerKAT radio telescopes detected the broadband bursts, whose intensity varies over time. Their brightness and changing pattern as the dwarf rotates point to an auroral source. In this process, electrons moving through a magnetic field generate radio waves, much as magnetic activity creates auroras on planets. The bursts have a brightness temperature above 100 billion kelvin. This measure describes how intense the radio emission is, rather than the brown dwarf’s actual temperature.

The bursts show circular polarisation, a property in which radio waves’ electric fields trace circles, but the measured fraction is typically about 20 percent and varies. This is lower than expected for this type of auroral emission. The signal may lose some of its original polarisation while travelling through the dwarf’s complex magnetic environment. One burst also showed a reversal in polarisation, which remains unexplained.

A model of the radio emission, using a dipole-shaped magnetic field and one active field line, estimates that the dwarf’s rotation axis is tilted about 48 degrees to our line of sight. The magnetic axis is offset from the rotation axis by about 40 degrees. These results extend the range of known radio-emitting ultracool dwarfs to brighter sources with weaker polarisation. Searches that require strong circular polarisation may therefore miss some objects. The findings also support a link between rapid rotation, strong magnetic fields and auroral radio emissions in these dwarfs.

Key findings
WISE J1517+05 produces radio bursts that repeat every 1.149 ± 0.002 hours.
Its peak radio spectral luminosity is 5.6 × 10^15 erg s⁻¹ Hz⁻¹, the highest reported for a periodic ultracool dwarf.
The bursts have brightness temperatures above 10^11 kelvin and are consistent with auroral electron cyclotron maser emission, in which electrons generate radio waves in a magnetic field.
The bursts typically have about 20 percent circular polarisation, and one burst shows an unexplained reversal.
Modelling estimates an inclination of about 48 degrees and an angle of about 40 degrees between the rotation and magnetic axes.

Scientific study

Periodic radio emission from a rapidly rotating T8 ultracool dwarf (external link)