Discovery of a Second Kilohertz Quasi-periodic Oscillation Peak in 4U 1608−52

Using a new technique to improve the sensitivity to weak Quasi-PeriodicOscillations (QPO) we discovered a new QPO peak at about 1100 Hz in the March1996 outburst observations of 4U 1608-52, simultaneous with the ~ 600-900 Hzpeak previously reported from these data. The frequency separation between theupper and the lower QPO peak varied significantly from $232.7 \pm 11.5$ Hz onMarch 3, to $293.1 \pm 6.6$ Hz on March 6. This is the first case of a variablekHz peak separation in an atoll source. We discuss to what extent this resultcould be accommodated in beat-frequency models such as proposed for the kHzQPOs. We measured the rms fractional amplitude of both QPOs as a function ofenergy, and we found that the relation is steeper for the lower than for theupper frequency peak. This is the first source where such difference betweenthe energy spectrum of the two kHz QPOs could be measured.