A Temporal Scale to Track the Spectral Transitions in Low-Mass X-ray Binaries

TL;DR

This study introduces a temporal scale based on wavelet analysis to track spectral transitions in low-mass X-ray binaries, revealing potential markers to distinguish black hole and neutron star systems.

Contribution

It develops a novel minimal time scale metric from high-resolution lightcurves to compare spectral state evolution in LMXBs, aiding in source classification.

Findings

High fractional RMS and hardness correlate with both BH and NS sources.

Atolls show large MTS, Z-type sources show small MTS at low RMS.

BH sources occupy a unique low-hardness, small-MTS region in the hardness-MTS plane.

Abstract

The results of a temporal analysis of observations for a sample of nine low-mass X-ray binaries (LMXBs) are presented. Of these sources, five host a neutron star (NS) primary (4U1608-52, Aql X-1, 4U1705-44, GX17+2 and Cyg X-2), and four host a black hole (BH) (GX339-4, XTE J1859+226, H1743-322 and MAXI J1659-152). The NS group includes three Atolls and two Z-type sources. We utilized archival Proportional Counter Array (PCA)/RXTE data to construct high-resolution lightcurves. A wavelet transform of the lightcurves is deployed to extract a minimal time scale (MTS) associated with the spectral state of the sources. The MTS, together with the fractional root-mean-square (RMS) and hardness ratios, is used to construct RMS-MTS and hardness-MTS diagrams that enable a direct comparison of the evolution of spectral transitions in the target sources. Observations with high fractional RMS and high hardness cluster in a broad region occupied jointly by BH and NS sources. For low fractional RMS observations the Atolls exhibit large MTS whereas Z-type sources exhibit small MTS. This new feature raises the possibility of discriminating between these two types of sources. Moreover, in the hardness-MTS plane, BH sources are the sole occupiers of the low-hardness and small-MTS domain thus potentially signaling a unique property for distinguishing BH and NS hosts in LMXBs.