Testing a double AGN hypothesis for Mrk 273

Abstract

The ultra-luminous infrared galaxy (ULIRG) Mrk 273 contains two infrared nuclei, N and SW, separated by 1 arcsecond. A Chandraobservation has identified the SW nucleus as an absorbed X-ray source with N_H ~ 4 × 10^(23) cm^(−2) but also hinted at the possible presence of a Compton-thick AGN in the N nucleus, where a black hole of ~10^9 M⊙ is inferred from the ionized gas kinematics. The intrinsic X-ray spectral slope recently measured by NuSTAR is 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 the NuSTAR data, 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 by N_H = 1.4^(+0.7)_(−0.4) × 10^(24) cm^(−2). The estimated 2−10 keV luminosity of the N source is 1.3 × 10^(43) erg s^(−1), about a factor of 2 larger than that of SW during the NuSTAR observation. Uncorrelated variability above and below 10 keV between the Suzaku and NuSTAR observations 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.

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