Contributed Talk - Splinter Compact
Studying quasi-periodic oscillations during state transition
In low-mass black hole X-ray binaries, a stellar-mass black hole accretes matter of its low-mass companion star through an accretion disc. At the beginning and end of an outburst these systems are observed in the hard-state that is dominated by comptonised emission. More than halve of the systems show a transition to a soft-state that is dominated by thermal emission of an accretion disc. The transition between these two states and the physics behind it, is still not well understood and a topic of ongoing research. GX 339–4, which can be regarded as text-book example of a low-mass black hole X-ray binary, showed an outburst last year that was monitored with the Neutron star Interior Composition Explorer (NICER) onboard the International Space Station. Thanks to the dense coverage of the outburst and the enhanced sensitivity of NICER compared to previous missions we were able to study in detail the evolution of quasi-periodic oscillations (QPOs) and noise components when the source transited from the hard into the soft state. The evolution between these states is a somewhat erratic process showing several transitions. Our study allowed us to follow in more detail the formation and disappearance of the different types of QPOs and to gain further insights in their evolution. In addition to power-density spectra we also studied the evolution of spectral parameters, but only found a strong correlation between the hardness ratio and the type of QPO observed. We will present the results of our study and discuss the implications our findings have on the appearance and coherence of type-B QPOs and their relation to changes in the accretion geometry of the system.