2004A&A...419..653G

objS{EM* LkHA 312}{LkHα312} has been observed serendipitously with the ACIS-I detector on board the Chandra X-ray Observatory with a 26h continuous exposure. This Hα emission line star belongs to M78 (NGC2068), one of the star-forming regions of the Orion B giant molecular cloud at a distance of 400pc. From the optical and the near-infrared (NIR) data, we show that LkHα312 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 unusually long rise phase (∼8 h). The X-ray emission was nearly constant during the first 18h of the observation, 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 of a 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 from a PMS low-mass star. By chance, the source position, 8.2' off-axis, protected this observation from pile-up. We make a spectral analysis of the X-ray emission versus time, showing that the plasma temperature of the quiescent phase and the flare peak reaches 29MK and 88MK, respectively. The quiescent and flare luminosities in the energy range 0.5-8keV corrected from absorption (N_H≃1.7x10²¹cm^–2) are 6x10³⁰erg/s and ∼10³²erg/s, respectively. The ratio of the quiescent X-ray luminosity on the LkHα312 bolometric luminosity is very high with log(L_X/L_bol)= -2.9, implying that the corona of LkHα312 reached the ``saturation'' level. The X-ray luminosity of the flare peak reaches ∼2% of the stellar bolometric luminosity. The different phases of this flare are finally discussed in the framework of solar flares, which leads to the magnetic loop height from 3.1x10¹⁰ to 10¹¹cm (0.2-0.5R_*, i.e., 0.5-1.3R_☉).