New Results on Time lags of quasi-periodic oscillations in the low-mass X-ray binary 4U 1636-53

We present an analysis of the energy and frequency dependence of the Fourier time lags of the hectoHertz QPOs and of the QPOs at the frequency at which the PDS shows a break in the NS-LMXB 4U 1636-53, using data obtained with the RXTE satellite. We found that: (i) For the break frequency QPO: for low frequencies the lag is positive, but it is decreasing with increasing frequency, reaching zero lag at ˜ 20 Hz. Between 20 and 35 Hz there is some fluctuation around zero, then the lags become positive again increasing slightly above zero up to 65 Hz. (ii) For the hHz QPO: we see that when the frequency is ˜ 100 Hz the lag is negative, but it increases to zero at ˜ 110 Hz, being consistent with this value up to 130 Hz, increasing to 0.5 msec at ˜140 Hz. From 140 Hz the lag decreases being strongly negative for hHz ≥ 220 Hz. (iii) We see no significant dependence of the lags on energy for both QPOs studied here. We compare these lags with our previous results for the lags of the kiloHertz QPOs in this system and discuss possible scenarios for producing the lags.