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

COMBI, J.; GARCÍA, F.; RODRIGUEZ, M.J.; GAMEN, R. C.; CELLONE, S.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2016 vol. 460 p. 1640 - 1644

0035-8711

We report on the unveiling of the nature of the unidentified X-raysource 3XMM J005450.3-373849 as a Seyfert-2 galaxy located behind thespiral galaxy NGC 300 using Hubble Space Telescope data, newspectroscopic Gemini observations and available XMM-Newton and Chandradata. We show that the X-ray source is positionally coincident with anextended optical source, composed of a marginally resolvednucleus/bulge, surrounded by an elliptical disc-like feature and twosymmetrical outer rings. The optical spectrum is typical of a Seyfert-2galaxy redshifted to z = 0.222 ± 0.001, which confirms that thesource is not physically related to NGC 300. At this redshift the sourcewould be located at 909 ± 4 Mpc (comoving distance in thestandard model). The X-ray spectra of the source are well fitted by anabsorbed power-law model. By tying NH between the sixavailable 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 NH? 0.34 × 10-22 cm-2 in 2000-2001, and0.54-0.75 × 10-22 cm-2 in the 2005-2014period. Although we cannot distinguish between a spectral or absorptionorigin, from the derived unabsorbed X-ray fluxes, we are able to assurethe presence of long-term X-ray variability. Furthermore, the unabsorbedX-ray luminosities of 0.8-2 × 1043 erg s-1derived in the X-ray band are in agreement with a weakly obscuredSeyfert-2 AGN at z ≈ 0.22.