Time Lags in Low Mass X-ray Binaries
  Using RXTE/PCA data, we have studied the time lag (TL) properties of a sample of four accreting neutron stars (NSs), namely 1E1724-3045, GS1826-238, 4U1705-44 and 4U1728-34. The aim of the study is to identify the spectral and timing state(s) in which TLs are detected. Along this work, we have discovered TLs between the 7-40 keV hard and 2-7 keV soft photons from 4U1728-34 with amplitudes similar to those seen in 4U1705-44 (i.e. 2 ms at 5 Hz). We show that the TLs are only seen in the low states of those sources, but that within the so-called ``island'' spectral state, some sources display TLs whereas some do not. On the other hand, we have found that TLs are detected when the associated Power Density Spectrum (PDS) shows excess power at high frequencies (above 1 Hz). 
J. F. Olive and D. Barret