Coupled HBO and NBO variations in the Z source GX 5-1: inner accretion disk as the location of QPOs

TL;DR

This study analyzes simultaneous HBO and NBO oscillations in the Z source GX 5-1, linking their origin to the inner accretion disk's physical changes and providing spectral evidence of their coupled nature.

Contribution

It presents observational evidence of coupled HBO and NBO in GX 5-1 and links their origin to the inner accretion disk, proposing a model involving oscillations in the disk's inner region.

Findings

HBO and NBO are strongly coupled and originate from the same inner disk region.

Spectral analysis indicates changes in the inner disk during coupled oscillations.

Oscillations are associated with the hot, puffed-up inner accretion disk region.

Abstract

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.