Low and High Frequency Quasi-Periodic Oscillations in 4U1915-05

Abstract

The X-ray burster and dipper 4U1915-05 (also known as XB1916-053) was observed 19 times for a total exposure of roughly 140 ks between 1996 February and October with the proportional counter array (PCA) aboard Rossi X-ray Timing Explorer (RXTE). We present here the detailed timing study of its X-ray emission. The source was observed in both a high intensity/soft spectral and a low intensity/hard spectral state. The 2-20 keV luminosities are about 1.4x10^37 and 3.2x10^36 ergs/s in the two regimes respectively (assuming a source distance of 9.3 kpc). We confirm that 4U1915-05 is an atoll source based on its spectral behaviour and its aperiodic variability in the frequency range 4x10^{-3}-128 Hz. 4U1915-05 displays red noise below 1 Hz, but is especially remarkable for its quasi-periodic variability detected for the first time at several timescales. We detect low frequency quasi-periodic oscillations (LFQPOs) between 5 and 80 Hz, as well as high frequency QPOs (HFQPOs) between 200 and 1300 Hz. Both LFQPOs and HFQPOs frequencies positively correlate with the mass accretion rate inferred from the position of the source on the color-color diagram. In a narrow range of luminosity, we also detect twin simultaneous HFQPOs separated by 348 +/- 12 Hz.