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We report on the unveiling of the nature of the unidentified X-ray source3XMM J005450.3-373849 as a Seyfert-2 galaxy located behind the spiral galaxyNGC 300 using Hubble Space Telescope data, new spectroscopic Geminiobservations and available XMM-Newton and Chandra data. We show that the X-raysource is positionally coincident with an extended optical source, composed bya marginally resolved nucleus/bulge, surrounded by an elliptical disc-likefeature and two symmetrical outer rings. The optical spectrum is typical of aSeyfert-2 galaxy redshifted to z=0.222 +/- 0.001, which confirms that thesource is not physically related to NGC 300. At this redshift the source wouldbe located at 909+/-4 Mpc (comoving distance in the standard model). The X-rayspectra of the source are well-fitted by an absorbed power-law model. By tying$N_\mathrm{H}$ between the six available spectra, we found a variable index$\Gamma$ running from ~2 in 2000-2001 years, to 1.4-1.6 in the 2005-2014period. Alternatively, by tying $\Gamma$, we found variable absorption columnsof N_H ~ 0.34 x $10^{-22}$ cm$^{-2}$ in 2000-2001 years, and 0.54-0.75 x$10^{-22}$ cm$^{-2}$ in the 2005-2014 period. Although we cannot distinguishbetween an spectral or absorption origin, from the derived unabsorbed X-rayfluxes, 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 obscuredSeyfert-2 AGN at $z \approx 0.22$.

Discovery of a deep Seyfert-2 galaxy at z = 0.222 behind NGC 300