Transient obscuration event captured in NGC 3227

Abstract

The relationship between warm absorber (WA) outflows of active galactic nuclei and nuclear obscuration activities caused by optically thick clouds (obscurers) crossing the line of sight is still unclear. NGC 3227 is a suitable target for studying the properties of both WAs and obscurers because it matches the following selection criteria: WAs in both ultraviolet (UV) and X-rays, suitably variable, bright in UV and X-rays, and adequate archival spectra for making comparisons with the obscured spectra. In the aim of investigating WAs and obscurers of NGC 3227 in detail, we used a broadband spectral-energy-distribution model that is built in findings of the first paper in our series together with the photoionization code of SPEX software to fit the archival observational data taken by XMM-Newton and NuSTAR in 2006 and 2016. Using unobscured observations, we find four WA components with different ionization states (log ξ [erg cm s−1] ∼ −1.0, 2.0, 2.5, 3.0). The highest-ionization WA component has a much higher hydrogen column density (∼1022 cm−2) than the other three components (∼1021 cm−2). The outflow velocities of these WAs range from 100 to 1300 km s−1, and show a positive correlation with the ionization parameter. These WA components are estimated to be distributed from the outer region of the broad line region (BLR) to the narrow line region. It is worth noting that we find an X-ray obscuration event in the beginning of the 2006 observation, which was missed by previous studies. We find that it can be explained by a single obscurer component. We also study the previously published obscuration event captured in one observation in 2016, which needs two obscurer components to fit the spectrum. A high-ionization obscurer component (log ξ ∼ 2.80; covering factor Cf ∼ 30%) only appears in the 2016 observation, which has a high column density (∼1023 cm−2). A low-ionization obscurer component (log ξ ∼ 1.0 − 1.9; Cf ∼ 20%−50%) exists in both 2006 and 2016 observations, which has a lower column density (∼1022 cm−2). These obscurer components are estimated to reside within the BLR by their crossing time of transverse motions. The obscurers of NGC 3227 are closer to the center and have larger number densities than the WAs, which indicate that the WAs and obscurers might have different origins.