LkH $ α $ 312 has been observed serendipitously with the ACIS-I detector on board the Chandra X-ray Observatory with 26 h continuous exposure .
This H \alpha 
emission line star belongs to M 78 ( NGC 2068 ) , one of the star-forming regions of the Orion B giant molecular cloud at a distance of 400 pc .
From the optical and the near-infrared ( NIR ) data , we show that LkH \alpha 312 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 rise phase ( \sim 8 h ) .
The X-ray emission was nearly constant during the first 18 h of the observation , and then increased by a factor of 13 during a fast rise phase ( \sim 2 h ) , and reached a factor of 16 above the quiescent X-ray level at the end of a gradual phase ( \sim 6 h ) showing a slower rise .
To our knowledge this flare , with \sim 0.4– \sim 0.5 cts s ^ { -1 } , has the highest count rate observed so far with Chandra from a PMS low-mass star .
By chance , the source position , 8.2 \arcmin 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 29 MK and 88 MK , respectively .
The quiescent and flare luminosities in the energy range 0.5–8 keV corrected from absorption ( N _ { H } \approx 1.7 10 ^ { 21 } cm ^ { -2 } ) are 6 10 ^ { 30 } erg s ^ { -1 } and \sim 10 ^ { 32 } erg s ^ { -1 } , respectively .
The ratio of the quiescent X-ray luminosity on the LkH \alpha 312 bolometric luminosity is very high with \log ( L _ { X } / L _ { bol } ) = -2.9 , implying that the corona of LkH \alpha 312 reached the ‘ saturation ’ level .
The X-ray luminosity of the flare peak reaches \sim 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 3.1 10 ^ { 10 } to 10 ^ { 11 } cm ( 0.2-0.5 R _ { \star } , i.e. , 0.5–1.3 R _ { \odot } ) .