1999ApJ...512L..31F

Using the Rossi X-ray Timing Explorer, we have measured lags of the 9-33 keV photons relative to the 2-9 keV photons in the timing noise between 0.01 and 100 Hz in the accreting neutron stars 4U 0614+09 and 4U 1705-44. We performed similar measurements on the accreting black hole candidates Cyg X-1 and GX 339-4 as a comparison. During the observations, these sources were all in low (hard) states. We find phase lags of between 0.03 and 0.2 rad in all of these sources, with a variation in frequency much less than expected for a lag constant in time. We also measure a coherence consistent with unity in all sources. As already noted for the black hole candidates, these data are inconsistent with simple Comptonization models invoking a constant time delay. Comptonization in a nonuniform medium can perhaps explain the lags. However, the magnitudes of the lags imply that the hot electron gas extends to more than 10³ Schwarzschild radii. This may constitute an energy problem. We argue that while a large hot cloud is possible for black holes, which may hide some of their accretion energy in advection, such a distribution may not be possible for neutron stars, in which all the accretion energy is eventually released at the neutron star surface. This casts doubt on the Comptonization model, although the energy problem may be resolved, for example, by a wind from the inner disk.