154 MHz Detection of Faint, Polarized Flares from UV Ceti

Abstract

We have detected four flares from UV Ceti at 154 MHz using the Murchison Widefield Array. The flares have flux densities between 10-65 mJy -a factor of 100 fainter than most flares in the literature at these frequencies -and are only detected in polarization. The circular polarized fractions are limited to > 27% at 3σ confidence and two of the flares exhibit polarity reversal. We suggest that these flares occur periodically on a time scale consistent with the rotational period of UV Ceti. During the brightest observed flare, we also detect significant linear polarization with polarization fraction > 18%. Averaging the data in 6-minute, 10 MHz frequency bins we find that the flux density of these flares does not vary over the 30 MHz bandwidth of the Murchison Widefield Array, however we cannot rule out finer time-frequency structure. Using the measured flux densities for the flares, we estimate brightness temperatures between (10 13 − 10 14 ) K, indicative of a coherent emission mechanism. The brightness temperature and polarization characteristics point to the electron cyclotron maser mechanism. We also calculate the flare rates given our four observed flares and compare them to flare rates for the set of M dwarf stars with known 100-200 MHz flares. Our measurement is the first for flares with intensities < 100 mJy at 100-200 MHz.

An all-sky survey of circular polarization at 200 MHz

“…The all-sky survey presented here was not ideal for detecting transient emission from sources such as flare stars and exoplanets. Transient sources and sources that can change sign in polarisation, such as seen with the flare star UV Ceti (Lynch et al 2017b), require an alternate observing and processing strategy. To avoid diluting the signal, the integration of the snapshot images should not exceed the time-scale of the expected emission before a sign flip occurs or the emission stops.…”

“…To confirm the previous M dwarf stellar flare rates and flux densities at 100 − 200 MHz, Lynch et al (2017b) targeted UV Ceti, a magnetically active M dwarf star. As the radio flares from UV Ceti were expected to be highly circularly polarised, this search was focused in the circularly polarised images rather than in total intensity.…”

“…For small fields of view, a simple two-dimensional plane is sufficient to model the leakage across the beam, e.g. Lynch et al (2017b), however for larger fields there is significant warping in the leakage behaviour and the fit errors increase significantly at the beam edges. To better model the leakage we fit a two-dimensional quadratic surface (over both spatial directions) to the grid in order to interpolate over the entire beam.…”

“…Transient searches are also simplified as they are less affected by source confusion (e.g. Lynch et al 2017b).…”

Ultra-wideband Detection of 22 Coherent Radio Bursts on M Dwarfs

“…Other low-frequency observations produced marginal or uncertain detections (e.g., Jackson et al 1990). Recent Low-Frequency Array (LO-FAR) beam-formed observations of YZ CMi at 10-190 MHz yielded no detections (Crosley et al 2016), while Lynch et al (2017) detected multiple 154-MHz bursts on UV Cet using interferometric imaging with the Murchison Widefield Array (MWA). VLA observations of EQ Peg at 230-470 MHz detected two coherent bursts (Crosley & Osten 2018b), but no Type II-like events in spite of an optical flare rate implying that this star may produce CMEs at least once per hour (Crosley & Osten 2018a).…”