  We present the Chandra ACIS and ASCA GIS results for a series of four
long-term observations on DoAr 21, ROXs 21 and ROXs 31; the X-ray brightest
T-Tauri stars (TTSs) in the Rho Ophiuchi cloud. In the four observations with a
net exposure of ~600 ksec, we found six, three and two flares from DoAr 21,
ROXs 21 and ROXs 31, respectively; hence the flare rate is fairly high. The
spectra of DoAr 21 are well fitted with a single-temperature plasma model,
while those of ROXs 21 and ROXs 31 need an additional soft plasma component.
Since DoAr 21 is younger (~10^5 yr) than ROXs 21 and ROXs 31 (~10^6 yr), these
results may indicate that the soft component gradually increases as T-Tauri
stars age. The abundances are generally sub-solar and vary from element to
element. Both high-FIP (first ionization potential) and low-FIP elements show
enhancement over the mean abundances. An unusual giant flare is detected from
ROXs 31. The peak luminosity and temperature are ~10^33 ergs s^-1 and ~10 keV,
respectively. The temperature reaches its peak value before the flux maximum,
and is nearly constant (4--5 keV) during the decay phase, indicating successive
energy release during the flare. The abundances and absorption show dramatic
variability from the quiescent to flare phase.

Astrophysics