X-ray variability analysis of a large series of XMM–Newton +NuSTAR observations of NGC 3227

Abstract

We present a series of X-ray variability results from a long XMM–Newton + NuSTAR campaign on the bright, variable AGN NGC 3227. We present an analysis of the light curves, showing that the source displays typically softer-when-brighter behaviour, although also undergoes significant spectral hardening during one observation which we interpret as due to an occultation event by a cloud of absorbing gas. We spectrally decompose the data and show that the bulk of the variability is continuum-driven and, through rms variability analysis, strongly enhanced in the soft band. We show that the source largely conforms to linear rms-flux behaviour and we compute X-ray power spectra, detecting moderate evidence for a bend in the power spectrum, consistent with existing scaling relations. Additionally, we compute X-ray Fourier time lags using both the XMM–Newton and – through maximum-likelihood methods – NuSTAR data, revealing a strong low-frequency hard lag and evidence for a soft lag at higher frequencies, which we discuss in terms of reverberation models.