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 flat spectrum ( \Gamma \sim 1.7 ) , similar to M82 , and a luminosity of \sim 1 \times 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 \simeq 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 .