The optical variability of the narrow-line Seyfert 1 galaxy IRAS 13224-3809

We report on a short optical monitoring programme of the narrow-line Seyfer t 1 galaxy IRAS 13224-3809. Previous X-ray observations of this object have shown persistent giant variability. The degree of variability at other wav elengths may then be used to constrain the conditions and emission processe s within the nucleus. Optical variability is expected if the electron popul ation responsible for the soft X-ray emission is changing rapidly and Compt on-upscattering infrared photons in the nucleus, or if the mechanism respon sible for X-ray emission causes all the emission processes to vary together . We find that there is no significant optical variability, with a firm upp er limit of 2 per cent, and conclude that the primary soft X-ray emission r egion produces little of the observed optical emission. The X-ray and optic al emission regions must be physically distinct and any reprocessing of X-r ays into the optical waveband occurs some distance from the nucleus. The la ck of optical variability indicates that the energy density of infrared rad iation in the nucleus is at most equal to that of the ultraviolet radiation , because Little is upscattered into the optical waveband. The extremely la rge X-ray variability of IRAS 13224-3809 may be explained by relativistic b oosting of more modest variations. Although such boosting enhances X-ray va riability over optical variability, this only partially explains the lack o f optical variability.