A hard X-ray constraint on the presence of an AGN in the ultraluminous infrared galaxy Arp 220 |
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Authors: | K. Iwasawa G. Matt M. Guainazzi A.C. Fabian |
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Affiliation: | 1Institute of Astronomy, Madingley Road, Cambridge CB3 0HA;2Dipartimento di Fisica, Universita degli Studi Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy;3XMM-Newton SOC, VILSPA, ESA, Apartado 50727, 28080 Madrid, Spain |
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Abstract: | 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 (Γ∼1.7) , similar to M82, and a luminosity of ∼ 1×1041 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 ≃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 1025 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. |
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Keywords: | galaxies: active galaxies: individual: Arp 220 galaxies: starburst X-rays: galaxies |
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