We present an extensive study of 72 archival Chandra light curves of the high-frequency-peaked type blazar Mrk 421 , the first strong extragalactic object to be detected at TeV energies .
Between 2000 and 2015 Mrk 421 often displayed intraday variability in the 0.3-10.0 keV energy range , as quantified through fractional variability amplitudes that range up to 21.3 per cent .
A variability duty cycle of \sim 84 per cent is present in these data .
Variability timescales , with values ranging from 5.5 to 30.5 ks , appear to be present in seven of these observations .
Discrete correlation function analyses show positive correlations between the soft ( 0.3–2.0 keV ) and hard ( 2.0–10.0 keV ) X-ray energy bands with zero time lags , indicating that very similar electron populations are responsible for the emission of all the X-rays observed by Chandra .
The hardness ratios of this X-ray emission indicate a general ‘ ‘ harder-when-brighter '' trend in the spectral behaviour of Mrk 421 .
Spectral index–flux plots provide model independent indications of the spectral evolution of the source and information on the X-ray emission mechanisms .
Brief discussions of theoretical models that are consistent with these observations are given .