Photoionisation modelling of the X-ray emission line regions within the Seyfert 2 AGN NGC 1068

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
e-mail: sam.waters.17@ucl.ac.uk
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁵ Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel

Received: 24 July 2020
Accepted: 24 March 2021

Abstract

Aims. We investigate the photoionised X-ray emission line regions (ELRs) within the Seyfert 2 galaxy NGC 1068 to determine if there are any characteristic changes between observations taken 14 years apart.

Methods. We compared XMM-Newton observations collected in 2000 and 2014, simultaneously fitting the reflection grating spectrometer and EPIC-pn spectra of each epoch, for the first time, with the photoionisation model, PION, in SPEX.

Results. We find that four PION components are required to fit the majority of the emission lines in the spectra of NGC 1068, with log ξ = 1−4, log N_H >  26 m⁻², and v_out = −100 to −600 km s⁻¹ for both epochs. Comparing the ionisation state of the components shows almost no difference between the two epochs, while there is an increase in the total equivalent column density. To estimate the locations of these plasma regions from the central black hole, we compare distance methods, excluding the variability arguments as there is no spectral change between observations. Although the methods are unable to constrain the distances for each plasma component, the locations are consistent with the narrow line region, with the possibility of the higher ionised component being part of the broad line region; we cannot conclude this for certain, but the photoionisation modelling does suggest this is possible. In addition, we find evidence for emission from collisionally ionised plasma, while previous analysis had suggested that collisional plasma emission was unlikely. However, although PION is unable to account for the Fe XVII emission lines at 15 and 17 Å, we do not rule out that photoexcitation is a valid processes to produce these lines as well.

Conclusions. NGC 1068 has not changed, both in terms of the observed spectra or from our modelling, within the 14 year time period between observations. This suggests that the ELRs are fairly static relative to the 14 year time frame between observations, or there is no dramatic change in the spectral energy distribution, resulting from a lack of black hole variability.