Measurement of Hard Lags and Coherences in the X-Ray Flux of Accreting Neutron Stars and Comparison with Accreting Black Holes

Using the Rossi X-ray Timing Explorer we have measured lags of the 9 to 33 keV photons relative to the 2 to 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 CygX–1 and GX339–4 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 radians in all 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 non-uniform medium can perhaps explain the lags. However, the magnitudes of the lags imply that the hot electron gas extends to more than 10 Schwarzschild radii. This may constitute an energy problem. We argue that while a large hot cloud is possible for Astronomical Institute, “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands Facultad de Ciencias Astronómicas y Geof́ısicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, 1900 La Plata, Argentina. University of Alabama in Huntsville, Department of Physics, Huntsville, AL 35899 Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138