A hard X-ray constraint on the presence of an AGN in the ultraluminous infrared galaxy Arp 220

We present X-ray results on the ultraluminous infrared galaxy Arp 220 obtained with BeppoSAX. 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 flat spectrum (Gamma similar to 1.7), similar to M82, and a luminosity of similar to1 x 10(41) erg s(-1). 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 similar or equal to 600 eV. This observation imposes the tightest constraint so far on an active nucleus if present in Arp 220. 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.