Angular power spectra for the extracted line signal assuming energy bins 3.0−3.2 keV and 3.3−3.5 keV along with F_lim = 10^-13 erg/s/cm². For clarity the errors are only displayed for Model I. To calculate the level of photon noise we have assumed that flux 10^-13 erg/s/cm² corresponds to 500 counts. Upper panel: angular spectra including two- and one-halo terms. The gray shaded area shows 1σ uncertainty region as calculated from Eq. (18). Here we have assumed perfect photon sampling, i.e., the photon noise is taken to be zero. Middle panel: angular power spectra with one-halo term subtracted. The gray band shows error corridor as calculated from Eq. (19). The level of photon noise, which is not yet included in calculation of errors, is shown by a short-dashed line. Lower panel: the same as middle panel but with the effect of photon noise included (Eq. (21)). The light gray band and dark gray histogram present errors without and with binning, respectively.