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 ( \sim 7 \times 10 ^ { 23 } cm ^ { -2 } ) power law plus an Fe K \alpha emission line .
The iron line ( detected at more than 99 % c.l . )
is broad ( \sigma = 0.26 ^ { +0.37 } _ { -0.17 } 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 can not be singularly resolved with high statistical significance : the neutral component is detected at \sim 2.5 \sigma and the Fe XXV line at \sim 2 \sigma 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 ( 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 L _ { 2 - 10 \mathrm { keV } } \sim 7 \times 10 ^ { 42 } erg s ^ { -1 } , corresponding to \sim 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 0.2 - 0.7 \times 10 ^ { 42 } 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 z = 0.458 , which lies in the field of view of Mrk 273 .