arXiv:astro-ph/9903133v1 9 Mar 1999 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 30 July 2013 (MN L A T E X style file v1.4) The optical variability of the narrow line Seyfert 1 galaxy IRAS 13224-3809 A.J. Young 1 , C.S. Crawford 1 , A.C. Fabian 1 , W.N. Brandt 2 and P.T. O’Brien 3 1 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA 2 Department of Astronomy, The Pennsylvania State University, University Park, PA 16802, USA 3 Department of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH 30 July 2013 ABSTRACT We report on a short optical monitoring programme of the narrow-line Seyfert 1 Galaxy IRAS 13224-3809. Previous X-ray observations of this object have shown persistent giant variabil- ity. The degree of variability at other wavelengths may then be used to constrain the conditions and emission processes within the nucleus. Optical variability is expected if the electron pop- ulation responsible for the soft X-ray emission is changing rapidly and Compton-upscattering infrared photons in the nucleus, or if the mechanism responsible for X-ray emission causes all the emission processes to vary together. We find that there is no significant optical variability with a firm upper limit of 2 per cent and conclude that the primary soft X-ray emission region produces little of the observed optical emission. The X-ray and optical emission regions must be physically distinct and any reprocessing of X-rays into the optical waveband occurs some distance from the nucleus. The lack of optical variability indicates that the energy density of infrared radiation in the nucleus is at most equal to that of the ultraviolet radiation since lit- tle is upscattered into the optical waveband. The extremely large X-ray variability of IRAS 13224-3809 may be explained by relativistic boosting of more modest variations. Although such boosting enhances X-ray variability over optical variability, this only partially explains the lack of optical variability. Key words: galaxies: individual: IRAS 13224-3809 1 INTRODUCTION X-ray observations by Boller et al (1997) have shown persistent giant variability on short timescales, with an amplitude of vari- ability far in excess of that seen in a typical broad line Seyfert 1 galaxy. Such extreme variability is determined by the emission and variability mechanisms in the nucleus. The degree of variabil- ity at other wavelengths may be used to constrain the conditions and mechanisms. Optical variability will occur if, for example, the X-ray emitting electron population is rapidly changing and Comp- ton scattering infrared radiation in the nucleus, or if the mecha- nism responsible for the X-ray variations causes all the emission processes to vary together. It has also been suggested (Boller et al 1997) that such large amplitude X-ray variability may be due to relativistic boosting, the degree of which is spectrally dependent. We have therefore observed IRAS 13224-3809 on three consecu- tive nights in order to determine the level of optical variability in the source. Previous studies of the narrow line Seyfert 1 galaxy IRAS 13224-3809 have shown it to be variable both in the UV and X- ray wavebands. Long timescale Lyα line variability has been ob- served, with the line profile and flux changing between three IUE observations over four months (Mas-Hesse et al. 1994). UV contin- uum variability has been observed in 11 IUE observations spanning three years, with the flux varying by 24 per cent (Rodr´ ıguez-Pascual 1997). 2 OPTICAL OBSERVATIONS IRAS 13224-3809 (α2000 = 13 h 25 m 19 s , δ2000 = -38 h 24 m 53 s ) was observed in B, V, R and I on three consecutive nights from 1997 March 13 to 16 using the blue sensitive TEK4 CCD at the Cassegrain focus of the 1m Jacobus Kapteyn Telescope (JKT). The 5.6 × 5.6 arcmin CCD field contained the galaxy and a number of foreground stars (see Fig. 1) allowing us to perform relative photometry. The images were bias subtracted and flat fielded us- ing sky flats. Flux calibration was performed using the standard stars PG0942-029B, C and D of Landolt (1992). IRAS 13224-3809 has a foreground star 7.6 arcsec from the optical nucleus (10.7 arc- sec from the X-ray centroid position (Boller et al. 1997)), and the aperture used to collect the light from the nucleus was chosen to have a diameter of 6.6 arcsec. This was found to collect most of the light from the nucleus whilst minimising that from the surrounding galaxy and the foreground star. Repeating our analysis with differ- ent sized apertures did not significantly affect our results. The same c  0000 RAS