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LkHA312 has been observed serendipitously with the ACIS-I detector on boardChandra with 26h continuous exposure. This H_alpha emission line star belongsto the star-forming region M78 (NGC2068). From the optical and NIR data, weshow that it is a pre-main sequence (PMS) low-mass star with a weak NIR excess.This genuine T Tauri star displayed an X-ray flare with an unusual long risephase (~8h). The X-ray emission was nearly constant during the first 18h of theobservation, and then increased by a factor of 13 during a fast rise phase(~2h), and reached a factor of 16 above the quiescent X-ray level at the end ofa gradual phase (~6h) showing a slower rise. To our knowledge this flare, with\~0.4-~0.5 cts/s, has the highest count rate observed so far with Chandra froma PMS low-mass star. By chance, the source position, 8.2' off-axis, protectedthis observation from pile-up. We make a spectral analysis of the X-rayemission versus time, showing that the plasma temperature of the quiescentphase and the flare peak reaches 29MK and 88MK, respectively. The quiescent andflare luminosities in the energy range 0.5--8keV corrected from absorption(N_H~1.7E21 cm^{-2}) are 6E30erg/s and ~1E32erg/s, respectively. The ratio ofthe quiescent X-ray luminosity on the LkHA312 bolometric luminosity is veryhigh with log(L_X/L_bol)= -2.9, implying that the corona of LkHA312 reached the`saturation' level. The X-ray luminosity of the flare peak reaches ~2% of thestellar bolometric luminosity. The different phases of this flare are finallydiscussed in the framework of solar flares, which leads to the magnetic loopheight from 3.1E10 to 1E11 cm (0.2-0.5 R*, i.e., 0.5-1.3 R_sun).Comment: 13 pages and 10 Figure. Accepted for publication by Astronomy &  Astrophysic

Chandra observation of an unusually long and intense X-ray flare from a young solar-like star in M 78