Discovery of Kilohertz QPOs in the Atoll X-Ray Binary 4U 1705-44

Abstract

In observations with the Rossi X-ray Timing Explorer we have discovered quasi-periodic oscillations (QPOs) near 1 kHz from 4U 1705-44, a low--mass X-ray binary with a neutron star classified as an atoll source. In six separate observations, we detect one QPO with a frequency ranging between 770 and 870 Hz and a 4% rms fraction in the full detector energy band. There is evidence for a second QPO at 1073 Hz in one interval. The separation in frequency of the two QPOs is 298+-11 Hz. The QPOs are present only in observations where the mass accretion rate is inferred to be at an intermediate level, based on the atoll source phenomenology. At the highest accretion rates, the QPOs are not detected with upper limits to the rms fraction of about 2%. At the lowest accretion rates the upper limits are about 4%. The QPO frequency increases with inferred mass accretion rate. This is expected in models where the QPO frequency is generated by motion at an inner edge of the accretion disk. An increased mass accretion rate causes the disk edge to move in, increasing the orbital frequency. Five Type-I X-ray bursts are observed with no detectable oscillations.