On the variability of the iron Kα line in Mrk 841

Petrucci et al. have recently reported the extraordinary behaviour of the iron Kα line in the Type I active galactic nucleus Mrk 841. At the XMM-Newton/EPIC resolution, a narrow line was observed in a short observation, which then apparently disappeared around 0.5 d 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 traveltime places the material at a radius where broadening by the black hole gravitational field is inevitable. Adding an additional observation taken before the Petrucci et al. 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 σ~ 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 a hotspot in the inner disc, which then becomes progressively broadened as the disc rotates.