On the variability of the iron line in Mrk 841

Abstract

Petrucci et al. (2002; hereafter P02) have reported extraordinary behaviour of the iron K alpha line in the type 1 AGN Mrk 841. At the XMM-Newton/EPIC resolution, a narrow line was observed in a short observation, which then apparently disappeared around 0.5 days later when another observation was performed. The limits are such that the line cannot both be narrow, and variable at this level, as the light-travel time places the material at a radius where broadening by the black hole's gravitational field is inevitable. Adding an additional observation taken before the P02 data, we present a different interpretation of the apparent variability. The data support the hypothesis that the line in fact varies in width, not only in flux. Rather than showing no line, and despite the strong Compton reflection component, the final observation exhibits broad emission at ~ 6.2-6.4 keV, with a width of ~0.8 keV, typical of a relativistic accretion disc. We suggest that the apparently narrow line in the early observations arises from local illumination by a flare inducing an hotspot in the inner disc, which then becomes progressively broadened as the disc rotates.