Distributions of values (null distribution of the LRT statistic) obtained from a Monte Carlo test that uses 10 000 simulated spectra to compare a baseline model (null hypothesis) parameterized with an LP function, and a narrow-feature model parameterized with an LP+EP function. Top panel: results derived with the observed VHE spectrum; bottom panel: results obtained with the EBL-corrected VHE spectrum. The green dashed line indicates the (LRT_data) obtained when comparing the LP and LP+EP fit results on the spectral data from Fig. 5. The blue, red, and purple solid lines depict the nominal χ² distribution for 1, 2, and 3 degrees of freedom. See text in Sect. 3.4 for further details.