1 Hz QPO {#1Hz:appearence}
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In the 2002 outburst, the 1Hz QPO appears immediately after the pulse phase drift is complete, but with the flux still decreasing in the fast decay stage. In 2005 the 1 Hz QPO appears in a similar position (see black vertical lines in Fig. \[decay\]). In both the 2002 and 2005 outbursts, data gaps prevent the observation of the exact moment when the 1 Hz QPO appears. Taking account of the gaps, the flux level of the first 1 Hz QPO appearance is consistent between the two outbursts.

To track the appearance of the 1 Hz QPO on time scales as short as a few hundred seconds, we investigated the observation available in which the 1 Hz QPO first appears in the 2005 outburst. The observation (Obs-Id 91056-01-04-03) shows two chunks of data separated by a gap of 5000 s (chunk A with a length of 3200 s and chunk B with length $3500$s) that we analyzed separately.

In chunk A the power spectrum has already changed with respect to the earlier observations: the power at higher frequencies has disappeared, and there is only power in the range 0.05–10 Hz. Probably we are witnessing the 