AstroSat/UVIT observations of IC 4329A: constraining the accretion disc inner radius

Abstract

We present a study of far and near-ultraviolet emission from the accretion disk in a powerful Seyfert 1 galaxy IC4329A using observations performed with the Ultraviolet Imaging Telescope (UVIT) onboard AstroSat. These data provide the highest spatial resolution and deepest images of IC4329A in the far and near UV bands acquired to date. The excellent spatial resolution of the UVIT data has allowed us to accurately separate the extended emission from the host galaxy and the AGN emission in the far and near UV bands. We derive the intrinsic AGN flux after correcting for the Galactic and internal reddening, as well as for the contribution of emission lines from the broad and narrow-line regions. The intrinsic UV continuum emission shows a marked deficit compared to that expected from the "standard" models of the accretion disk around an estimated black hole mass of 1-2x10^8Msun when the disk extends to the innermost stable circular orbit. We find that the intrinsic UV continuum is fully consistent with the standard disk models, but only if the disk emits from distances larger than 80-150 gravitational radii.