XMM-Newton spectral properties of the Ultraluminous IRAS Galaxy Mrk 273

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Abstract
We present a 23 ks XMM-Newton observation of the ULIRG Mrk 273. The hard X-ray spectrum can be modeled by a highly absorbed power law plus an Fe Kalpha emission line. The iron line is broad, suggesting possible superposition of a neutral iron line at 6.4 keV, and a blend of ionized iron lines from Fe XXV and Fe XXVI. Given the relatively short exposure, the three line components can not be singularly resolved with high statistical significance: the neutral component is detected at ~2.5sigma and the Fe XXV line at ~2sigma c.l., while for the Fe XXVI line we can only estimate an upper limit. The broad band spectrum requires, in addition to a highly absorbed power law, at least three collisionally ionized plasma components, which may be associated with star-forming regions. The temperatures of the three plasmas are about 0.3, 0.8 and 6 keV, where the highest of the three is sufficient to produce ionized iron emission lines. An alternative interpretation for the origin of the soft emission might also be given in terms of reflection off some photoionized gas, as has been observed in a number of nearby Compton-thick Seyfert 2 galaxies. A hot gas, photoionized by the primary, continuum can also produce ionized iron lines. Unfortunately, given the limited statistics and the lack of high resolution spectroscopy, it is not possible to distinguish between the two models investigated. We further compare the XMM-Newton findings with the Chandra data obtaining consistent spectral results. The absorption corrected hard X-ray (2-10 keV) luminosity of Mrk 273 corresponds to ~0.2% of the far-IR luminosity, similar to typical values found in pure starbursts. The thermal contribution to the soft X-ray luminosity is ~0.2-0.7x10^42 erg s^-1, comparable to those found in NGC 6240 and other starburst dominated ULIRGs.

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