1999ApJ...524L..59C

I report the discovery of hard X-ray phase lags associated with the famous 67 Hz quasi-periodic oscillation (QPO) in microquasar GRS 1915+105. The QPO is seen on multiple occasions. For this investigation, I have chosen one particular observation when the oscillation is the strongest. The measured hard lags show strong energy dependence. With respect to the 2-2.5 keV band, the phase lag increases from insignificant detection at 5.2-7.0 keV to as much as 2.3 radians (which corresponds a time lag of ∼5.6 ms) above ∼13 keV. In the same observation, one narrowly peaked, strong QPO at 67 mHz, along with its first three harmonics, one weak QPO at ∼400 mHz, and one broad QPO at ∼840 mHz are also detected. Similar cross spectral analyses have been performed on these QPOs (and the harmonics), in order to measure any phase lags associated with the features. Phase lags are detected in all, with similar energy dependence. For the 67 mHz QPO, the results are quite intriguing: only the first- and third-harmonic components show hard lags, while the fundamental and second-harmonic components actually display smaller soft lags. Coupled with the energy dependence of the QPO amplitude, the results seem to indicate a complicated change in the signal profile with photon energy. I will discuss the implication of the phase lags on possible origins of the QPOs.