A Radio Flaring, Chromospherically-Inactive K Dwarf

TL;DR

This study reports on a peculiar radio-emitting K dwarf star with unusual polarization and variability, suggesting possible auroral emission, and highlights the need for further multi-wavelength observations to understand its nature.

Contribution

The paper presents multi-wavelength radio observations of a K dwarf with unique polarized emission and variability, proposing potential auroral origins and identifying a close-in companion.

Findings

Detected highly polarized coherent radio emission at 1.5 GHz

Identified a close-in M5.5V companion star

Suggested auroral emission as the likely origin of radio activity

Abstract

We report on an unusual radio source J180526-292953, initially identified as a steep spectrum, polarized point source toward the Galactic bulge and found to coincide with the nearby K dwarf HD317101A. We conducted a multi-wavelength radio study utilizing new GMRT observations and archival data from ASKAP, MeerKAT, and the VLA. At 1.5 GHz, HD317101A exhibits highly polarized coherent emission with variable activity lasting several hours with an apparent period of 3.7 days, which is consistent with electron cyclotron maser (ECM) emission. The behavior at 3 GHz is distinctive, with a short burst lasting tens of seconds to minutes, a flat spectrum, and no detected polarization, possibly suggesting gyro-synchrotron emission. High-resolution optical spectroscopy from CHIRON/SMARTS confirms HD317101A as a mature, chromospherically inactive K7V star, while Gaia astrometry, combined with speckle imaging from Zorro/Gemini-S, indicates the presence of a close-in M5.5V companion. We evaluated three possible origins for the combined radio behavior: chromospheric activity, auroral emission (possibly from a star-planet interaction), or an ultra-long-period transient. The bulk of the evidence favors an auroral origin, but the dominant stellar source of the ECM emission remains uncertain. Future VLBI observations, long-term TESS monitoring, high resolution spectroscopy and further radio characterization will be key to distinguishing between various scenarios.