A hard X-ray constraint on the presence of an AGN in the ultra-luminous infrared galaxy Arp220

Abstract

We present X-ray results on the ultraluminous infrared galaxy Arp220 obtained with BeppoSAX. The X-ray emission up to 10 keV is detected. No significant signal is detected with the PDS detector in the higher energy band. The 2-10 keV emission has a hard spectrum (photon-index of 1.7), similar to M82, and a luminosity of 1e41 erg/s. A population of X-ray binaries may be a major source of this X-ray emission. The upper limit of an iron K line equivalent width at 6.4 keV is ~600 eV. This observation imposes so far the tightest constraint on an active nucleus if present in Arp220. We find that a column density of X-ray absorption must exceed 10**25 cm-2 for an obscured active nucleus to be significant in the energetics, and the covering factor of the absorption should be almost unity. The underluminous soft X-ray starburst emission may need a good explanation, if the bolometric luminosity is primarily powered by a starburst.