XMM-Newton spectral properties of the ultraluminous IRAS galaxy Mrk 273

Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany e-mail: balestra@mpe.mpg.de

Received: 2 June 2005
Accepted: 12 July 2005

Abstract

We present a 23 ks XMM-Newton observation of the Ultraluminous Infrared Galaxy (ULIRG) Mrk 273. The hard X-ray spectrum can be modeled by a highly absorbed (~ cm^-2) power law plus an Fe Kα emission line. The iron line (detected at more than 99% c.l.) is broad ( keV), 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 cannot be individually resolved with high statistical significance: the neutral component is detected at and the Fe XXV line at ~ 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 reflection off photoionized gas, as has been observed in a number of nearby Compton-thick Seyfert 2 galaxies (e.g. NGC 1068, Circinus, Mrk 3, NGC 4945). 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 luminosity of Mrk 273 is  erg s^-1, corresponding 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 approximately  erg s^-1, comparable to those found in NGC 6240 and other starburst–dominated ULIRGs. We also analyze the XMM-Newton spectrum of Mrk 273x, an unabsorbed Seyfert 2 galaxy at redshift , which lies in the field of view of Mrk 273.