The active nucleus of the ULIRG IRAS F00183-7111 viewed by NuSTAR

Abstract

We present an X-ray study of the ultra-luminous infrared galaxy IRAS F00183–7111 (z = 0.327), using data obtained from NuSTAR, Chandra X-ray Observatory, Suzaku and XMM-Newton. The Chandra imaging shows that a point-like X-ray source is located at the nucleus of the galaxy at energies above 2 keV. However, the point source resolves into diffuse emission at lower energies, extending to the east, where the extranuclear [OIII]λ5007 emission, presumably induced by a galactic-scale outflow, is present. The nuclear source is detected by NuSTAR up to the rest-frame 30 keV. The strong, high-ionization Fe K line, first seen by XMM-Newton, and subsequently by Suzaku and Chandra, is not detected in the NuSTAR data. The line flux appears to have been declining continuously between 2003 and 2016, while the continuum emission remained stable to within 30%. Further observations are needed to confirm this. The X-ray continuum below 10 keV is characterised by a hard spectrum caused by cold absorption of N_H ~ 1 × 10^(23) cm^(-2), compatible to that of the silicate absorption at 9.7 μm, and a broad absorption feature around 8 keV which we attribute to a high-ionization Fe K absorption edge. The latter is best described by a blueshifted, high-ionization (log ξ ~ 3) absorber with a column density of N_H ~ 1 × 10^(24) cm^(-2), similar to the X-ray high-velocity outflows observed in a number of active nuclei. No extra hard component, which would arise from a strongly absorbed (i.e. Compton-thick) source, is seen in the NuSTAR data. While a pure reflection scenario (with a totally hidden central source) is viable, direct emission from the central source of L_(2−10 keV) ≃ 2 × 10^(44) erg s^(-1), behind layers of cold and hot absorbing gas may be an alternative explanation. In this case, the relative X-ray quietness (L_x/L_(bol,AGN) ≤ 6 × 10^(-3)), the high-ionization Fe line, strong outflows inferred from various observations, and other similarities to the well-studied ULIRG/QSO Mrk 231 point that the central source in this ULIRG might be accreting close to the Eddington limit.

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