A rapid occultation event in NGC 3227

TL;DR

This study reports a rapid occultation event in NGC 3227, revealing a transient gas cloud crossing the line of sight, associated with the broad-line region, using coordinated X-ray observations.

Contribution

First detection of a rapid occultation event in NGC 3227 using combined XMM-Newton and NuSTAR data, identifying a transient gas cloud in the inner BLR.

Findings

A gas cloud with NH ~ 5 x 10^22 cm^-2 occulted 60% of the continuum.

The occultation lasted about a day, indicating a transient event.

The event is associated with clouds in the inner broad-line region.

Abstract

NGC 3227 exhibits rapid flux and spectral variability in the X-ray band. To understand this behaviour we conducted a co-ordinated observing campaign using 320 ks of XMM-Newton exposures together with 160 ks of overlapping NuSTAR observations, spanning a month. Here, we present a rapid variability event that occurs toward the end of the campaign. The spectral hardening event is accompanied by a change in the depth of an unresolved transition array, whose time-dependent behaviour is resolved using the RGS data. This UTA fingerprint allows us to identify this as a transit event, where a clump of gas having NH ~ 5 x 10^22 atoms/ cm^2, log xi ~2 occults ~60% of the continuum photons over the course of approximately a day. This occulting gas is likely associated with clouds in the inner BLR. An additional zone of gas with lower column and higher ionization, matches the outflow velocity of the variable zone, and may represent transmission through the cloud limb.