The BL Lac object Mkn 421 was observed three times by the X-ray observatory BeppoSAX in consecutive days during 1997 April and May .
The source was in a quiescent state , with an average 2–10 keV flux of 9.0 \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } .
Flux variation by a factor of \simeq 2 on timescales as short as a few 10 ^ { 4 } s were more pronounced in the hard ( i.e .
above \simeq 3 keV ) than in the soft X-rays .
The broadband ( 0.1–40 keV ) spectrum is concave and can be most easily explained with a power-law model which steepens gradually with energy .
In this framework , neither photoabsorption edges nor resonant absorption lines are required , strengthening the case against the ubiquity of such features in BL Lac objects , which had been previously suggested by Einstein observations .
The broadband spectrum hardens with hard X-ray flux , mostly due to a flattening above \simeq 4 keV .
This suggests that the relativistic highest energy electron distribution properties drive the X-ray spectral dynamics : either a stratification of the distribution in the jet with energy or inhomogeneities in the electron injection mechanism could be consistent with the observed variability pattern .