The properties of the clumpy torus and BLR in the polar-scattered Seyfert 1 galaxy ESO 323-G77 through X-ray absorption variability

Abstract

We report results from multi-epoch X-ray observations of the polar-scattered Seyfert 1 galaxy ESO 323-G77. The source exhibits remarkable spectral variability from months to years timescales. The observed spectral variability is entirely due to variations of the column density of a neutral absorber towards the intrinsic nuclear continuum. The column density is generally Compton-thin ranging from a few times 10${}^{22}$ cm${}^{-2}$ to a few times 10${}^{23}$ cm${}^{-2}$. However, one observation reveals a Compton-thick state with column density of the order of 1.5 $\times$ 10${}^{24}$ cm${}^{-2}$. The observed variability offers a rare opportunity to study the properties of the X-ray absorber(s) in an active galaxy. We identify variable X-ray absorption from two different components, namely (i) a clumpy torus whose individual clumps have a density of $\le$ 1.7 $\times$ 10${}^8$ cm${}^{-3}$ and an average column density of $\sim$ 4 $\times$ 10${}^{22}$ cm${}^{-2}$, and (ii) the broad line region (BLR), comprising individual clouds with density of 0.1-8 $\times$ 10${}^9$ cm${}^{-3}$ and column density of 10${}^{23}$-10${}^{24}$ cm${}^{-2}$. The derived properties of the clumpy torus can also be used to estimate the torus half-opening angle, which is of the order of 47 ${}^{\circ }$. We also confirm the previously reported detection of two highly ionized warm absorbers with outflow velocities of 1000-4000 km s${}^{-1}$. The observed outflow velocities are consistent with the Keplerian/escape velocity at the BLR. Hence, the warm absorbers may be tentatively identified with the warm/hot inter-cloud medium which ensures that the BLR clouds are in pressure equilibrium with their surroundings. The BLR line-emitting clouds may well be the cold, dense clumps of this outflow, whose warm/hot phase is likely more homogeneous, as suggested by the lack of strong variability of the warm absorber(s) properties during our monitoring.

15 pages, 4 tables, and 9 figures. Accepted for publication in MNRAS

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