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Published February 1, 2019 | Published + Submitted
Journal Article Open

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


Coherent radio bursts detected from M dwarfs have some analogy with solar radio bursts but reach orders of magnitude higher luminosities. These events trace particle acceleration, powered by magnetic reconnection, shock fronts (such as those formed by coronal mass ejections (CMEs)), and magnetospheric currents, in some cases offering the only window into these processes in stellar atmospheres. We conducted a 58 hr ultra-wideband survey for coherent radio bursts on five active M dwarfs. We used the Karl G. Jansky Very Large Array to observe simultaneously in three frequency bands covering a subset of 224–482 MHz and 1–6 GHz, achieving the widest fractional bandwidth to date for any observations of stellar radio bursts. We detected 22 bursts across 13 epochs, providing the first large sample of wideband dynamic spectra of stellar coherent radio bursts. The observed bursts have diverse morphology, with durations ranging from seconds to hours, but all share strong (40%–100%) circular polarization. No events resemble solar Type II bursts (often associated with CMEs), but we cannot rule out the occurrence of radio-quiet stellar CMEs. The hours-long bursts are all polarized in the sense of the x-mode of the star's large-scale magnetic field, suggesting that they are cyclotron maser emission from electrons accelerated in the large-scale field, analogous to auroral processes on ultracool dwarfs. The duty cycle of luminous coherent bursts peaks at 25% at 1–1.4 GHz, declining at lower and higher frequencies, indicating source regions in the low corona. At these frequencies, active M dwarfs should be the most common galactic transient source.

Additional Information

© 2019. The American Astronomical Society. Received 2018 May 7; revised 2018 October 1; accepted 2018 October 15; published 2019 February 1. The authors thank Stephen Bourke for writing the initial version of the tbavg code for producing dynamic spectra in CASA and for providing guidance on background source subtraction, as well as Michael Eastwood for testing direction-dependent calibration of our P-band data. J.R.V. thanks Tim Bastian for discussion of stellar radio bursts and feedback on this paper, Urvashi Rao for imaging advice, and Rick Perley and Frank Schinzel for help with P-band polarization calibration. The authors thank the referee for thoughtful and constructive feedback. This material is based upon work supported by the National Science Foundation under grant No. AST-1311098. J.R.V. additionally thanks the Troesh family and the PEO International Scholar Award program for financial support of her graduate research. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research has made use of NASA's Astrophysics Data System. This research has made use of Astropy, a community-developed core Python package for astronomy. The acknowledgments were compiled using the Astronomy Acknowledgment Generator. Facility: VLA. - Software: CASA, Astropy (Astropy Collaboration et al. 2013), Anaconda, SIMBAD, ADS, NumPy (Van Der Walt et al. 2011), SciPy (Jones et al. 2001), Matplotlib (Hunter 2007).

Attached Files

Published - Villadsen_2019_ApJ_871_214.pdf

Submitted - 1810.00855.pdf


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