Correlations between QPO frequencies and spectral parameters of GRS 1915+105 using AstroSat observations

Abstract

ABSTRACT In this work, we study the correlation between quasi-periodic oscillation (QPO) frequency and the spectral parameters during various X-ray states in the black hole binary GRS 1915+105 which matches well with the predicted relativistic dynamic frequency (i.e. the inverse of the sound crossing time) at the truncated radii. We have used broad-band data of Large Area X-ray Proportional Counter and Soft X-ray Telescope instruments onboard AstroSat. Spectral fitting shows that the accretion rate varies from ∼0.1 to ∼5.0 × 1018 gm s−1 and the truncated radius changing from the last stable orbit of an almost maximally spinning black hole, ∼1.2 to ∼19 gravitational radii. For this wide range, the frequencies of the C-type QPO (2–6 Hz) follow the trend predicted by the relativistic dynamical frequency model and interestingly, the high-frequency QPO at ∼70 Hz also follows the same trend, suggesting they originate from the innermost stable circular orbit with the same mechanism as the more commonly observed C-type QPO. While the qualitative trend is as predicted, there are quantitative deviations between the data and the theory, and the possible reasons for these deviations are discussed.