  We report on the results of a systematic study of X-ray flares from low-mass
young stellar objects, using Chandra observations of the main region of the Rho
Oph. From 195 X-ray sources, including class I-III sources and some young brown
dwarfs, we detected a total of 71 X-ray flares. Most of the flares have the
typical profile of solar and stellar flares, fast rise and slow decay. We
derived the time-averaged temperature (kT), luminosity (L_X), rise and decay
timescales (tau_r and tau_d) of the flares, finding that (1) class I-II sources
tend to have a high kT, (2) the distribution of L_X during flares is nearly the
same for all classes, and (3) positive and negative log-linear correlations are
found between tau_r and tau_d, and kT and tau_r. In order to explain these
relations, we used the framework of magnetic reconnection model to formulate
the observational parameters as a function of the half-length of the
reconnected magnetic loop (L) and magnetic field strength (B). The estimated L
is comparable to the typical stellar radius of these objects (10^{10-11} cm),
which indicates that the observed flares are triggered by solar-type loops,
rather than larger ones (10^{12} cm) connecting the star with its inner
accretion disk. The higher kT observed for class I sources may be explained by
a higher magnetic field strength (about 500 G) than for class II-III sources
(200-300 G).

Astrophysics