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 to 8.1% and 5.0 to 5.8%. The frequency
separation between the two QPOs changes from 341+-7 Hz to 296+-12 Hz. The
inferred mass accretion rate during our observations is relatively low compared
to 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 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% to
13%.Comment: accepted by ApJ Letters, for more information see the QPO page at
http://www.astro.uva.nl/ecford/qpos.htm