We have analysed the soft X-ray emission from the nuclear source of the nearby spiral galaxy M 81 , using the available data collected with ROSAT , ASCA , BeppoSAX and Chandra .
The source flux is highly variable , showing ( sometimes dramatic : a factor of 4 in 20 days ) variability at different timescales , from 2 days to 4 years , and in particular a steady increase of the flux by a factor of \gtrsim 2 over 4 years , broken by rapid flares .
After accounting for the extended component resolved by Chandra , the nuclear soft X-ray spectrum ( from ROSAT/PSPC , BeppoSAX/LECS and Chandra data ) can not be fitted well with a single absorbed power-law model .
Acceptable fits are obtained adding an extra component , either a multi-color black body ( MCBB ) or an absorption feature .
In the MCBB case the inner accretion disk would be far smaller than the Schwartzchild radius for the 3 - 60 \times 10 ^ { 6 } M _ { \odot } 
nucleus requiring a strictly edge-on inclination of the disk , even if the nucleus is a rotating Kerr black hole .
The temperature is 0.27 keV , larger than expected from the accretion disk of a Schwartzchild black hole , but consistent with that expected from a Kerr black hole .
In the power-law + absorption feature model we have either high velocity ( 0.3 c ) infalling C v clouds or neutral C i absorption at rest .
In both cases the C : O overabundance is a factor of 10 .