Results of the cross-spectra analysis in regions with the shock (top left), bubble (top middle) and spiral (top right) (see Fig. 2). Top row: amplitude of the volume emissivity fluctuations, in the soft, A_{k, aa}, (purple) and hard, A_{k, bb}, (blue) bands and the cross-amplitude, A_{k, ab}, (black dashed curve); coherence C and ratio R obtained from the observed spectra [equations (9) and (10)]. Blue and purple dotted regions show 1σ statistical and stochastic uncertainties. For clarity, we do not plot the uncertainties on the cross-amplitudes. Our conservative estimates of statistical uncertainties on the measured C and R are shown with the dotted red regions. Bottom row: maps of coherence C (left) and ratio R (right) for a mixture of isobaric, adiabatic and isothermal perturbations in the 3.2 keV gas. Colour bars show the values of C and R [equations (11) and (12)]. The dashed grid shows the values of α_i, associated with each type of fluctuation (see Section 2). X-axis: contribution of adiabatic fluctuations α_adiab.; Y-axis: contribution of isobaric fluctuations α_isob.. The contribution of isothermal fluctuations is |$\alpha _{\rm isoth.}=\sqrt{1-\alpha _{\rm adiab.}^2-\alpha _{\rm isob.}^2}$|⁠. The maps are schematically divided into three regions where one of the types of perturbations is dominant in terms of total variance. Ellipses show the regions of α_isob., α_adiab. and α_isoth. that correspond to the values of R and C taken from the figures in the top row. Black ellipse: the region with shock, blue ellipse: bubble, white ellipse: spiral. The size of each ellipse reflects the uncertainties associated with Poisson noise, the choice of the underlying model and the choice of the weighting scheme in calculating the power spectra. The locus of C and R is in the adiabatic area if measured in the region with the shock in Perseus, in the isobaric area if obtained from the region with spiral structure and in the isothermal area when the region with the bubble is considered.