A statistical study on the low-frequency quasi-periodic oscillation amplitude spectrum and amplitude in GRS 1915+105

Abstract

A statistical study was made on both the energy dependence of the\nlow-frequency quasi-periodic oscillation (QPO) amplitude (LFQPO amplitude\nspectrum) and the LFQPO amplitude from all the RXTE observations of GRS\n1915+105. Based on the two-branch correlation of the LFQPO frequency and the\nhardness ratio, the observations that were suitable for evaluating the LFQPO\namplitude spectrum were divided into two groups. According to a comparison\nbetween the radio and X-ray emissions, we deduced that the jets during the two\ngroups of observations are very different. A negative correlation between the\nLFQPO frequency and the radio flux was found for one group. The LFQPO amplitude\nspectrum was fitted by a power-law with an exponential cutoff in order to\ndescribe it quantitatively. It reveals that as the LFQPO frequency increases,\nthe power-law hardens. And the cutoff energy firstly decreases, and then\nsmoothly levels off. The fit also shows that the LFQPO amplitude spectra of the\ntwo groups are essentially the same, suggesting that the LFQPO seems not to\noriginate from the jet. The LFQPO amplitude spectra are hard, indicating a\npossible origin of the LFQPO in the corona. As the LFQPO frequency increases,\nthe LFQPO amplitude firstly increases and then decreases. The effects of the\nlow pass filter and the jet on the LFQPO amplitude are discussed.\n