The simultaneous and coupled evolution of horizontal branch oscillation (HBO)
and normal branch oscillation (NBO) in Z-type sources suggests that the
production of HBO is connected to NBO and is caused by changes in the
physical/radiative properties of the inner accretion disk, although there is a
lack of substantial spectral evidence to support this. In this {\it Letter}, we
present the results of an analysis of a RXTE observation of a Z source GX~5-1,
where the 6 Hz NBO is simultaneously detected along with a HBO at 51 Hz. The
variations in the intensity and the associated power density spectrum indicate
that the HBO and NBO are strongly coupled, originating from the same location
in the inner accretion disk. The absence of HBO and NBO in the lower energy
bands, an increase in the rms amplitude with energy and a smooth transition
among them suggest that they are produced in the hot inner regions of the
accretion disk. Based on a spectral analysis, we found a signature of changing
or physically modified inner disk front during the coupled HBO and NBO
evolution. We explore the various models to explain the observed phenomenon and
propose that the NBO is affiliated to the oscillations in the thick/puffed-up
inner region of the accretion disk.Comment: 9 pages, 4 figures, Accepted for publication in ApJ Letter
