We detected a major X-ray outburst from M82 with a duration of 79 days , an average flux of 5 \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } in the 2-10 keV band , and strong variability .
The X-ray spectrum remained hard throughout the outburst .
We obtained a Chandra observation during the outburst that shows that the emission arises from the ultraluminous X-ray source X41.4+60 .
This source has an unabsorbed flux of ( 5.4 \pm 0.2 ) \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } in the 0.3-8 keV band , equivalent to an isotropic luminosity of 8.5 \times 10 ^ { 40 } erg s ^ { -1 } .
The spectrum is adequately fitted with an absorbed power-law with a photon index of 1.55 \pm 0.05 .
This photon index is very similar to the value of 1.61 \pm 0.06 measured previously while the flux was ( 2.64 \pm 0.14 ) \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } .
Thus , the source appears to remain in the hard state even at the highest flux levels observed .
The X-ray spectral and timing data available for X41.4+60 are consistent with the source being in a luminous hard state and a black hole mass in the range of one to a few thousand solar masses .