Chandra ACIS eclipse profiles of W2, folded to the best period of P = 8649 s. Top panel: profiles for the three different epochs where the modulation was detected: 2000 (red line and filled circles), 2002 (black lines), and 2014 (blue line and filled squares). A correction of about 20% was applied to the 2000 datasets in order to take into account the difference in CCD efficiency between ACIS-I and ACIS-S. We used the reference epoch MJD 52551.999(1) (1σ c.l.) for the 2002 datasets, while the 2000 and 2014 light curves have been arbitrarily shifted along the x-axis in order to align the minima, corresponding to the 8-min eclipse. All light curves have 20 phase-bins of ∼432 s duration each. Bottom panel: folded light curves of the four longest observations, carried out in 2002 a few days apart from each other. From top to bottom: ObsID. 2735, 2736, 2737 and 2738. The profiles are complex and variable on time scales of less than a day.