FeK emission in the ultraluminous infrared galaxy Arp 220

Prominent Fe K line emission is detected in the XMM-Newton spectrum of the ultraluminous infrared galaxy Arp 220. The centroid of the line is found at an energy of 6.7 keV and the equivalent width of the line is EW similar to 1.9 keV (at 3.5sigma significance). A few other spectral features are found at various degrees of significance in the lower energy range on a hard 2.5-10 keVcontinuum (Gamma similar to 1). The large EWof the Fe Kline poses a problem with interpreting the hard X-ray emission as integrated X-ray binary emission. A thermal emission spectrum with a temperature of kT similar to 7 keV modified by absorption of N-H similar or equal to 3 x 10(22) cm(-2), can describe the 2.5-10 keV continuum shape and the Fe K emission. A hot bubble that is shocked internally in a starburst region would have a similar temperature and gives a good explanation for the observed X-ray properties with a high star formation rate. An ensemble of radio supernovae in a dense environment, as suggested from VLBI imaging, could be another possibility, if such powerful supernovae are produced continuously at a high rate. However, the apparent lack of emission from X-ray binaries is incompatible with the high supernova rate (similar to2 SNe yr(-1)) required by both interpretations. Highly photoionized, low-density gas illuminated by a hidden Compton-thick active galactic nucleus is a possible alternative for the hard X-ray emission, which can be tested by examining whether radiative recombination continua from highly ionized Ca and Fe are present in better quality data from a forthcoming observation.