Transient obscuration event captured in NGC 3227

Abstract

Context. A growing number of transient X-ray obscuration events in type I active galactic nuclei suggest that our line of sight to the central engine is not always free. Multiple X-ray obscuration events were reported in the nearby Seyfert 1.5 galaxy NGC 3227 from 2000 to 2016. In late 2019, another X-ray obscuration event was identified with Swift. Two coordinated target-of-opportunity observations with XMM-Newton, NuSTAR, and the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) were triggered in November and December 2019 to study this obscuration event. Aims. We aim to constrain the physical properties of the absorbing material (i.e., the obscurer) that caused the X-ray obscuration event in late 2019. We also aim to compare the handful of obscuration events in NGC 3227 and other Seyfert galaxies. Methods. For each observation, we analyzed the time-averaged X-ray spectra collected with XMM-Newton and NuSTAR. We performed photoionization modeling with the SPEX code, which allows us to constrain the intrinsic continuum simultaneously with various photoionized absorption and emission components. Results. Similar to previous transient X-ray obscuration events in NGC 3227, the one caught in late 2019 is short-lived (less than five months). If the obscurer has only one photoionized component, the two X-ray observations in late 2019 cannot be explained by the same obscurer that responds to the varying ionizing continuum. Due to the unknown geometry of the obscurer, its number density and distance to the black hole cannot be well constrained. The inferred distance covers at least two orders of magnitude, from the broad-line region to the dusty torus. Unlike some other X-ray obscuration events in Seyfert galaxies, such as NGC 5548 and NGC 3783, no prominent blueshifted, broad absorption troughs were found in the 2019 HST/COS spectra of NGC 3227 when compared with archival UV spectra. This might be explained if the X-ray obscurer does not intercept our line of sight to (a significant portion of) the UV-emitting region. Understanding the variety of observational differences in the X-ray obscuration events observed so far is not straightforward. Future observations with high-quality data are needed to unveil the nature of the X-ray obscuration events.