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Astron. Astrophys. 361, 121-138 (2000)

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Low and high frequency quasi-periodic oscillations in 4U1915-05

L. Boirin 1, D. Barret 1, J.F. Olive 1, P.F. Bloser 2 and J.E. Grindlay 2

1 Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, 9 Avenue du Colonel Roche, 31028 Toulouse Cedex 04, France (boirin@cesr.fr)
2 Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received 1 October 1999 / Accepted 6 July 2000


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 [FORMULA] and [FORMULA] ergs s-1 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 [FORMULA] Hz. 4U1915-05 displays red noise below [FORMULA] 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 [FORMULA] 5 and 80 Hz, as well as high frequency QPOs (HFQPOs) between [FORMULA] 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 [FORMULA] Hz.

Key words: stars: individual: 4U1915-05 – stars: neutron – X-rays: stars

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000