DISCOVERY OF KILOHERTZ QUASI-PERIODIC OSCILLATIONS IN THE ALTOLL X-RAY BINARY 4U-1705-44

In observations with the Rossi X-Ray Timing Explorer, we have discover ed quasi-periodic oscillations (QPOs) near 1 kHz from 4U 1705-44, a lo w-mass X-ray binary with a neutron star classified as an atoll source. In six separate observations, we detect one QPO with a frequency rang ing 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 inte rval. The separation in frequency of the two QPOs is 298 +/- 21 Hz. Th e 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 detect ed with upper limits to the rms fraction of about 2%. At the lowest ac cretion rates, the upper limits are about 4%. The QPO frequency increa ses with inferred mass accretion rate. This is expected in models wher e the QPO frequency is generated by motion at an inner edge of the acc retion disk. An increased mass accretion rate causes the disk edge to move in, increasing the orbital frequency. Five type I X-ray bursts ar e observed with no detectable oscillations.