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We present results from two Chandra imaging observations of Ross 154, anearby flaring M dwarf star. During a 61-ks ACIS-S exposure, a very large flareoccurred (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 ofthe flare shows evidence for the Neupert effect, followed by a further rise andthen a two-component exponential decay. A large flare was also observed at theend of a later 48-ks HRC-I observation. Emission from the non-flaring phases ofboth observations was analyzed for evidence of low level flaring. From thesetemporal studies we find that microflaring probably accounts for most of the`quiescent' emission, and that, unlike for the Sun and the handful of otherstars that have been studied, the distribution of flare intensities does notappear 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/Oratio and overall coronal metallicity during the flare appear to be enhancedrelative to quiescent abundances. Based on the temperatures and emissionmeasures derived from the spectral fits, we estimate the length scales andplasma densities in the flaring volume and also track the evolution of theflare in color-intensity space. Lastly, we searched for a stellar-windcharge-exchange X-ray halo around the star but without success; because of therelationship between mass-loss rate and the halo surface brightness, not evenan upper limit on the stellar mass-loss rate can be determined.

X-Ray Flaring on the dMe Star, Ross 154