What can be learnt from a highly informative X-ray occultation event in NGC 6814? A marvellous absorber

Abstract

ABSTRACT A unique X-ray occultation event in NGC 6814 during an XMM–Newton observation in 2016 has been reported, providing useful information of the absorber and the corona. We revisit the event with the aid of the hardness ratio (HR)–count rate (CR) plot and comparison with two other absorption-free XMM exposures in 2009 and 2021. NGC 6814 exhibits a clear ‘softer-when-brighter’ variation pattern during the exposures, but the 2016 exposure significantly deviates from the other two in the HR–CR plot. While spectral fitting does yield transient Compton-thin absorption corresponding to the eclipse event in 2016, rather than easing the tension between exposures in the HR–CR plot, correcting the transient Compton-thin absorption results in new and severe deviation within the 2016 exposure. We show that the eclipsing absorber shall be clumpy (instead of a single Compton-thin cloud), with an inner denser region composed of both Compton-thin and Compton-thick clouds responsible for the previously identified occultation event, and an outer sparser region with Compton-thin clouds which eclipses the whole 2016 exposure. With this model, all the tension in the HR–CR plots could be naturally erased, with the observed spectral variability during the 2016 exposure dominated by the variation of absorption. Furthermore, the two warm absorbers (with different ionization and column densities but similar outflowing velocities) detected in the 2016 exposure shall also associate with the transient absorber, likely due to ablated or tidal stretched/disrupted fragments. This work highlights the unique usefulness of the HR–CR plot while analysing rare occultation events.