Unprecedented X-ray Flaring Activity of Mrk 421 in 2013 April

Abstract

Mrk 421 showed an unprecedented X-ray flaring behaviour in 2013 April 10–17 when the con- secutive outbursts by a factor of 8–16 were observed in the 0.3–10 keV and 3–79 keV bands, and it became one of the brightest objects in the X-ray sky. During these events, the source was also very active on intraday timescales with fractional variability amplitude of 2–47 per cent, and showed the flux doubling and halving time scales of 1.16–7.20 hr and 1.04–3.54 hr, respectively. We have revealed up to 20 cases when the flux varied with 3σ significance within 1 ks. In this period, Mrk421 also underwent the most extreme X-ray spectral variability ever reported for BL Lacertae objects. The location of the synchrotron SED peak moved from about 0.1 keV to almost 20 keV. The photon index at 1 keV and curvature parameter showed the ranges a=1.68–2.83 and b=0.09–0.57, respectively, and varied along with the flux on diverse time scales down to the in- tervals shorter than 1 ks. While the X-ray outburst were accompanied by very strong TeV-flares, the source was relatively less active in the radio-UV and GeV energy ranges.