Correlations in Horizontal Branch Oscillations and Break Components in XTE J1701-462 and GX 17+2

TL;DR

This study analyzes the correlations of horizontal branch oscillations and break components in two neutron star X-ray binaries, revealing their physical origins and differences in spectral states, and suggesting a boundary between persistent and transient sources.

Contribution

It provides a detailed comparison of HBO and break components in XTE J1701-462 and GX 17+2, proposing a common physical mechanism and a boundary separating different source types.

Findings

Frequencies of XTE J1701-462 follow known correlations, GX 17+2 is shifted.

HBO and break frequencies are independent of accretion rate.

HBO and break components likely originate from Comptonization in the corona.

Abstract

We studied the horizontal branch oscillations (HBO) and the band-limited components observed in the power spectra of the transient neutron star low-mass X-ray binary XTE J1701-462 and the persistent "Sco-like" Z source GX 17+2. These two components were studied based on the state-resolved spectra. We found that the frequencies of XTE J1701-462 lie on the known correlations (WK and PBK), showing consistency with other types of X-ray binaries (black holes, atoll sources and millisecond X-ray pulsars). However, GX 17+2 is shifted from the WK correlation like other typical Z sources. We suggest that the WK/PBK main track forms a boundary which separates persistent sources from transient sources. The characteristic frequencies of break and HBO are independent of accretion rate in both sources, although it depends on spectral models. We also report the energy dependence of the HBO and break frequencies in XTE J1701-462 and how the temporal properties change with spectral state in XTE J1701-462 and GX 17+2. We studied the correlation between rms at the break and the HBO frequency. We suggest that HBO and break components for both sources probably arise from a similar physical mechanism: Comptonization emission from the corona. These two components could be caused by same kind of oscillation in a corona who with uneven density, and they could be generated from different areas of corona. We further suggest that different proportions of the Comptonization component in the total flux cause the different distribution between GX 17+2 and XTE J1701-462 in the $rms_{\rm{break}}$-$rms_{\rm{HBO}}$ diagram.