Fe K emission in the ultraluminous infrared galaxy Arp 220

Prominent Fe Kα line emission is detected in the XMM-Newton spectrum of the ultraluminous infrared galaxy Arp220. The centroid of the line is found at an energy of 6.7 keV and the equivalent width of the line is EW ∼ 1.9 keV (at 3.5σ significance). A few other spectral features are found at various degrees of significance in the lower energy range on a hard 2.5–10 keV continuum (Γ ∼ 1). The large EW of the Fe K line 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 ∼ 7 keV modified by absorption of NH ≃ 3 × 10 cm, 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 (∼ 2 SNe yr) required by both interpretations. Highly photoionized, low-density gas illuminated by a hidden Compton-thick AGN 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.