Discovery of Kilohertz Quasi-periodic Oscillations from 4U 1820−303 with [ITAL]Rossi X-Ray Timing Explorer[/ITAL

Abstract

We have detected high-frequency (HF) quasi-periodic oscillations (QPOs) from the low-mass X-ray binary 4U 1820-303 during observations performed in 1996 October using the Rossi X-ray Timing Explorer. The QPOs are visible when the source occupies the low-state luminosity range LX = 2.4-3.1 × 1037 ergs s-1 (2-20 keV, at 6.4 kpc); the centroid frequency of the main QPO peak varies between 546 ± 2 Hz and 796 ± 6 Hz and is tightly correlated with the source count rate. The measured QPO widths are typically ~20 Hz, with mean rms amplitude 4.1% ± 0.3%. At the upper end of this luminosity range a second significant QPO peak appears with frequency 1065 ± 7 Hz, width 40 ± 20 Hz, and rms amplitude 3.2% ± 0.8%. When both QPOs are visible simultaneously, the difference between their frequencies is 275 ± 8 Hz. When the source brightens beyond LX = 3.1 × 1037 ergs s-1 (~10% of the Eddington limit for a helium-rich envelope), neither QPO is detected. Neither the magnetospheric beat frequency model nor the sonic point model of HF QPOs provides a perfect explanation of the phenomenology we observe. These results represent the first detection of kilohertz QPO activity in a globular cluster X-ray binary, and provide a new method of directly comparing the properties of cluster and noncluster neutron star binaries. If the highest QPO frequency we observe is identified with the marginally stable orbit in the accretion disk, the neutron star mass may be ~2 M☉, 35%-50% more massive than usually assumed. This may have consequences for the current evolutionary scenarios for this source and also for the debate about the evolution of millisecond pulsars in globular clusters.