The NUSTAR view of a QPO evolution of GRS 1915+105

We report a timing analysis of the black hole binary GRS 1915+105 with the Nuclear Spectroscopic Telescope Array Observatory. A strong type-C quasi-periodic oscillation (QPO) below 2 Hz appears in the power density spectrum during the whole observation, whose frequency is correlated with the 3–25 keV count rate. The QPO shows a sudden increase in frequency along with an increase in flux and a softening of the spectrum. We discuss the possible origin of the QPO and the reasons that lead to the QPO frequency variation. It is suggested that the reflection component has little influence on QPO frequency and the increase in QPO frequency could be associated with the inward motion of the outer part of the disk.