  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 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-hour, ultra-wideband survey for coherent radio
bursts on 5 active M dwarfs. We used the Karl G. Jansky Very Large Array (VLA)
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 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.
