Recurring obscuration in NGC 3783

TL;DR

This study analyzes archival X-ray data of NGC 3783 to characterize the frequency, duration, and properties of obscuration events caused by high-velocity gas streams, revealing they occur more often than previously recognized.

Contribution

It provides new constraints on the duty cycle and frequency of obscuration events in NGC 3783 using multi-epoch archival X-ray observations.

Findings

Obscuration events occur in about half of the observed time.

Obscuration is more frequent than previously thought, including in archival data from 1993 and 1996.

Obscuration can significantly block ionizing X-ray and EUV photons.

Abstract

Obscuration of the continuum emission from active galactic nuclei by streams of gas with relatively high velocity (> 1000 km/s) and column density (>3E25 per m2) has been seen in a few Seyfert galaxies. This obscuration has a transient nature. In December 2016 we have witnessed such an event in NGC 3783. The frequency and duration of these obscuration events is poorly known. Here we study archival data of NGC 3783 in order to constrain this duty cycle. We use archival Chandra/NuSTAR spectra taken in August 2016. We also study the hardness ratio of all Swift XRT spectra taken between 2008-2017. In August 2016, NGC 3783 also showed evidence for obscuration. While the column density of the obscuring material is ten times lower than in December 2016, the opacity is still sufficient to block a significant fraction of the ionising X-ray and EUV photons. From the Swift hardness ratio behaviour we find several other epochs with obscuration. Obscuration with columns >1E26 per m2 may take place in about half of the time. Also in archival X-ray data taken by ASCA in 1993 and 1996 we find evidence for obscuration. Obscuration of the ionising photons in NGC 3783 occurs more frequently than previously thought. This may not always have been recognised due to low spectral resolution observations, too limited spectral bandwidth or confusion with underlying continuum variations.