Compton thick absorber in type 1 quasar 3C 345 revealed by Suzaku and Swift/BAT

Abstract

The archival data of 3C 345, a type 1 quasar at z = 0.5928, obtained with Suzaku and Swift/BAT are analysed. Though previous studies of this source applied only a simple broken power-law model, a heavily obscuring material is found to be required by considering Akaike information criteria. The application of the numerical torus model by Murphy & Yaqoob surprisingly reveals the existence of Compton thick type 2 nucleus with the line-of-sight hydrogen column density of the torus of N_H = 10^24.5 cm^-2 and the inclination angle of θ_inc = 90°. However, this model fails to account for the Eddington ratio obtained with the optical observations by Gu et al. and Shen et al., or requires the existence of a supermassive black hole binary, which was suggested by Lobanov & Roland, thus this model is likely to be inappropriate for 3C 345. A partial covering ionized absorber model that accounts for absorption in 'hard excess' type 1 active galactic nuclei (AGNs) is also applied, and finds a Compton thick absorber with the column density of N_H ≃ 10²⁵ cm^-2, the ionization parameter of log ξ ≳ 2 and the covering fraction of 75 per cent ≲ f_c ≲ 85 per cent. Since this model obtains a black hole mass of log (M_BH/M_⊙) = 9.8, which is consistent with the optical observation by Gu et al., this model is likely to be the best-fitting model of this source. The results suggest that 3C 345 is the most distant and most obscured hard excess AGN at this time.