Using x-ray colour-colour diagrams to study AGN variability

Abstract

Identifying X-ray variability in active galactic nuclei (AGN) has been a longstanding issue due to spectral model degeneracies. It often occurs on timescales too short to employ regular spectral analysis, but colour-colour analysis provides a unique opportunity to get past this problem. Studies like in Grinberg et al. (2020) have shown the application of colour-colour diagrams to high-mass X-ray binary variability for determining partial absorber properties. Colour-colour diagrams were generated for absorbed power law, partial covering, and relativistic blurred reflection models to assess parameter variability. Comparisons were done with both simulated data sets, and real data from an observation of NGC 6814. The use of simulated data demonstrated that these diagrams have the ability to predict parameter changes, but can have difficulties distinguishing between parameter variations. Real data showed that grids need to have more consistency to accurately compare to real data and predict correct parameter changes. Some consistency was seen when comparing to current analyses on NGC 6814, but it warrants further studies. Colour-colour diagrams have shown promise, and future detailed works involving the inclusion of other components, should be quite useful.