Discovery of a second kilohertz QPO in the X-ray binary 4U 1735-44

In recent observations with the Rossi X-Ray Timing Explorer, we have detected two simultaneous quasi-periodic oscillation (QPO) peaks in the low-mass X-ray binary and atoll source 4U 1735-44. The lower and higher frequency QPOs have frequencies varying between 632 and 729 Hz and 982 and 1026 Hz, respectively. The fractional rms amplitudes are 3.7%-8.1% and 5.0%-5.8%. The frequency separation between the two QPOs changes from 341 +/- 7 to 296 +/- 12 Hz. The inferred mass accretion rate during our observations is relatively low compared to that during the previous observations, in which only a single QPO was present. There is weak evidence that the frequency of the QPOs correlates with the mass accretion rate, as observed in other binaries. Five X-ray bursts were recorded with no detectable oscillations with upper limits for the rms fraction of 4%-13%.