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 detectedtwo simultaneous quasi-periodic oscillation (QPO) peaks in the low mass X-raybinary and atoll source 4U 1735-44. The lower and higher frequency QPOs havefrequencies varying between 632 and 729 Hz, and 982 and 1026 Hz, respectively.The fractional rms amplitudes are 3.7 to 8.1% and 5.0 to 5.8%. The frequencyseparation between the two QPOs changes from 341+-7 Hz to 296+-12 Hz. Theinferred mass accretion rate during our observations is relatively low comparedto that during the previous observations, where only a single QPO was present.There is weak evidence that the frequency of the QPOs correlates with the massaccretion rate, as observed in other binaries. Five X-ray bursts were recordedwith no detectable oscillations with upper limits for the rms fraction of 4% to13%.