  We present results from two Chandra imaging observations of Ross 154, a
nearby flaring M dwarf star. During a 61-ks ACIS-S exposure, a very large flare
occurred (the equivalent of a solar X3400 event, with L_X = 1.8x10^30 ergs/s)
in which the count rate increased by a factor of over 100. The early phase of
the flare shows evidence for the Neupert effect, followed by a further rise and
then a two-component exponential decay. A large flare was also observed at the
end of a later 48-ks HRC-I observation. Emission from the non-flaring phases of
both observations was analyzed for evidence of low level flaring. From these
temporal studies we find that microflaring probably accounts for most of the
`quiescent' emission, and that, unlike for the Sun and the handful of other
stars that have been studied, the distribution of flare intensities does not
appear to follow a power-law with a single index. Analysis of the ACIS spectra,
which was complicated by exclusion of the heavily piled-up source core,
suggests that the quiescent Ne/O abundance ratio is enhanced by a factor of
~2.5 compared to the commonly adopted solar abundance ratio, and that the Ne/O
ratio and overall coronal metallicity during the flare appear to be enhanced
relative to quiescent abundances. Based on the temperatures and emission
measures derived from the spectral fits, we estimate the length scales and
plasma densities in the flaring volume and also track the evolution of the
flare in color-intensity space. Lastly, we searched for a stellar-wind
charge-exchange X-ray halo around the star but without success; because of the
relationship between mass-loss rate and the halo surface brightness, not even
an upper limit on the stellar mass-loss rate can be determined.

Astrophysics