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 detect ed 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 QP Os have frequencies varying between 632 and 729 Hz and 982 and 1026 Hz, res pectively. The fractional rms amplitudes are 3.7%-8.1% and 5.0%-5.8%. The f requency separation between the two QPOs changes from 341 +/- 7 to 296 +/- 12 Hz. The inferred mass accretion rate during our observations is relative ly low compared to that during the previous observations, in which only a s ingle QPO was present. There is weak evidence that the frequency of the QPO s correlates with the mass accretion rate, as observed in other binaries. F ive X-ray bursts were recorded with no detectable oscillations with upper l imits for the rms fraction of 4%-13%.