X-ray intraday variability of the TeV blazar Mrk 421 with Chandra.

Abstract

We present an extensive study of 72 archival Chandra light curves of the high-frequencypeaked 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 ∼84 per cent is present in these data. Variability time-scales, 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.