2000A&A...361..121B

The X-ray burster and dipper 4U 1915-05 (also known as XB 1916-053) 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 2-20keV luminosities are about 1.4x10³⁷ and 3.2x10³⁶erg/s in the two regimes respectively (assuming a source distance of 9.3kpc). We confirm that 4U 1915-05 is an atoll source based on its spectral behaviour and its aperiodic variability in the frequency range ∼4x10^–3-128Hz. 4U 1915-05 displays red noise below ∼1Hz, 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 80Hz, as well as high frequency QPOs (HFQPOs) between ∼200 and 1300Hz. 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±12Hz.