Discovery of a kHz QPO in 2S 0918-549

Peter G. Jonker; Michiel van der Klis; Jeroen Homan; Mariano Mendez; Jan van Paradijs; Tomaso Belloni; Chryssa Kouveliotou; Walter Lewin; Eric C. Ford

arXiv · astro-ph/0101150

Discovery of a kHz QPO in 2S 0918-549

Peter G. Jonker·Michiel van der Klis·Jeroen Homan·Mariano Mendez·Jan van Paradijs·Tomaso Belloni·Chryssa Kouveliotou·Walter Lewin·Eric C. Ford

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Abstract

We report the discovery of a kilohertz quasi-periodic oscillation (kHz QPO) in the low-mass X-ray binary (LMXB) 2S 0918-549. The kHz QPO has a frequency of 1156+-9 Hz, a FWHM of 70+-20 Hz, and a fractional rms amplitude of 18%+-2%. We also detected for the first time a type I X-ray burst in this source. The compact object must therefore be a neutron star. Finally, we were able to classify the source as an atoll source exhibiting the full complement of spectral/timing states of this class (island and banana). From the peak burst flux an upper limit on the persistent luminosity can be derived of 0.5% of the Eddington luminosity, making 2S 0918-549 one of the least luminous LMXBs showing a kHz QPO. We compare the fractional rms amplitudes of the upper kHz QPO across the ensemble of LMXBs. We find a strong anticorrelation with luminosity. In LMXBs with luminosities ~100 times lower than those of Z-sources, the fractional rms amplitude is a factor ~10 larger.

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Peter G. Jonker, Michiel van der Klis, Jeroen Homan, Mariano Mendez, Jan van Paradijs, Tomaso Belloni, Chryssa Kouveliotou, Walter Lewin, Eric C. Ford. 2001-01-10. Discovery of a kHz QPO in 2S 0918-549. https://arxiv.org/abs/astro-ph/0101150

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