A Radio Flaring, Chromospherically Inactive K Dwarf

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 HD 317101A. We conducted a multiwavelength radio study utilizing new GMRT observations and archival data from ASKAP, MeerKAT, and the Very Large Array. At 1.5 GHz, HD 317101A 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 HD 317101A 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 ultralong-period transient. The bulk of the evidence favors an auroral origin, but the dominant stellar source of the ECM emission remains uncertain. Future very-long baseline interferometry observations, long-term TESS monitoring, high-resolution spectroscopy, and further radio characterization will be key to distinguishing between various scenarios.