摘要：The ultra-luminous infrared galaxy (ULIRG) Mrk 273 contains two infrared nuclei, N and SW, separated by 1 arcsecond. AChandraobservation has identified the SW nucleus as an absorbed X-ray source withNH~ 4 × 1023cm−2but also hinted at the possible presence of a Compton-thick AGN in the N nucleus, where a black hole of ~109 M⊙is inferred from the ionized gas kinematics. The intrinsic X-ray spectral slope recently measured byNuSTARis unusually hard (Γ~ 1.3) for a Seyfert nucleus, for which we seek an alternative explanation. We hypothesize a strongly absorbed X-ray source in N, of which X-ray emission rises steeply above 10 keV, in addition to the known X-ray source in SW, and test it against theNuSTARdata, assuming the standard spectral slope (Γ= 1.9). This double X-ray source model gives a good explanation of the hard continuum spectrum, deep Fe K absorption edge, and strong Fe K line observed in this ULIRG, without invoking the unusual spectral slope required for a single source interpretation. The putative X-ray source in N is found to be absorbed byNH= 1.4+0.7−0.4× 1024cm−2. The estimated 2−10 keV luminosity of the N source is 1.3 × 1043erg s−1, about a factor of 2 larger than that of SW during theNuSTARobservation. Uncorrelated variability above and below 10 keV between theSuzakuandNuSTARobservations appears to support the double source interpretation. Variability in spectral hardness and Fe K line flux between the previous X-ray observations is also consistent with this picture.
