Discovery of a high confidence soft lag from an X-ray flare of Markarian 421

Abstract

We present the X-ray variability properties of the X-ray and TeV bright blazar Mrk 421 with a ∼60 ks long XMM-Newton observation performed on November 9–10, 2005. The source experienced a pronounced flare, of which the inter-band time lags were determined with a very high confidence level. The soft (0.6–0.8 keV) X-ray variations lagged the hard (4–10 keV) ones by 1.09_‑0.12^+0.11 ks, and the soft lag increases with increasing difference in the photon energy. The energy-dependent soft lags can be well fitted with the difference of the energy-dependent cooling timescales of the relativistic electron distribution responsible for the observed X-ray emission, which constrains the magnetic field strength and Doppler factor of the emitting region to be Bδ^1/3∼1.78 Gauss.