Simulated coherence spectra for an obscuring gas responding to variations of the ionising continuum. Panel a: dependency of the intensity of the drop of coherence on the average ionisation parameter: the colours indicate different degrees of ionisation of the gas, with log ξ varying within the ranges log ξ = 0.33 − 0.61 (cyan), log ξ = 1.53 − 1.82 (blue), log ξ = 2.14 − 2.42 (purple), log ξ = 2.33 − 2.61 (magenta). These respectively correspond to distances of d = 40, 10, 5, and 4 light days of the gas from the X-ray source. Panel b: dependency of the intensity of the drop of coherence caused by variations of the ionisation parameter (within the range log ξ = 1.53 − 1.82), on the average column density of the obscurer: the different values of column density are obtained by changing the depth of the gas, Δd = 3.8 × 10¹² cm (red), 3.8 × 10¹³ cm (orange), 4.8 × 10¹³ cm (yellow), and 7.7 × 10¹³ cm (green). For comparison with the observations, the measured coherence during O1-O2 is overplotted (grey squares) on the simulated coherence spectra.