Poster abstract details

The neutron star low mass X-ray binary 4U 1608-52 is known to show kHz QPOs as well as low frequency broad band noise. The energy dependence of the fractional r.m.s of these variations reflect the underlying radiative mechanism responsible for the phenomena. In this work we compute the energy dependence for 26 instances of kHz QPO observed by RXTE. We typically find as reported before, that the r.m.s increases with energy with slope of ~0.5. This indicates that the variation is in the hot thermal comptonization component and in particular the QPO is likely to be driven by variation in the thermal heating rate of the hot plasma. For the same data, we compute the energy dependent r.m.s variability of the low frequency noise component by considering light curves binned at 10 sec. In contrast to the behaviour seen for the kHz QPO, the energy dependence is nearly flat i.e. with slope ~ 0. This indicates that the driver here may be the soft photon source. Thus the radiative mechanism driving the low frequency noise and the high frequency QPO are different in nature. This is against a simple resonance model where the QPO is simply a resonance at a particular high frequency which amplifies the underlying noise. Alternatively, if the QPO is a resonance, then the noise at kHz frequencies is of different origin that that observed at low frequencies.