A temporal scale to track the spectral transitions in low-mass X-ray binaries

Abstract

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 light-curves. A wavelet transform of the light curves 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 signalling a unique property for distinguishing BH and NS hosts in LMXBs.