2016MNRAS.460.1640C

We report on the unveiling of the nature of the unidentified X-ray source 3XMM J005450.3-373849 as a Seyfert-2 galaxy located behind the spiral galaxy NGC 300 using Hubble Space Telescope data, new spectroscopic Gemini observations and available XMM-Newton and Chandra data. We show that the X-ray source is positionally coincident with an extended optical source, composed of a marginally resolved nucleus/bulge, surrounded by an elliptical disc-like feature and two symmetrical outer rings. The optical spectrum is typical of a Seyfert-2 galaxy redshifted to z=0.222±0.001, which confirms that the source is not physically related to NGC 300. At this redshift the source would be located at 909±4 Mpc (comoving distance in the standard model). The X-ray spectra of the source are well fitted by an absorbed power-law model. By tying N_H between the six available spectra, we found a variable index Γ running from ∼2 in 2000-2001 to 1.4-1.6 in the 2005-2014 period. Alternatively, by tying Γ, we found variable absorption columns of N_H ∼ 0.34 x 10^-22 cm^-2 in 2000-2001, and 0.54-0.75 x 10^-22 cm^-2 in the 2005-2014 period. Although we cannot distinguish between a spectral or absorption origin, from the derived unabsorbed X-ray fluxes, we are able to assure the presence of long-term X-ray variability. Furthermore, the unabsorbed X-ray luminosities of 0.8-2 x 10^43 ergs^-1 derived in the X-ray band are in agreement with a weakly obscured Seyfert-2 AGN at z ≃ 0.22.