A Possible Explanation for the “Parallel Tracks” Phenomenon in Low‐Mass X‐Ray Binaries

An explanation is proposed for the fact that in LMXBs the correlation betweenmost observable X-ray spectral and timing parameters (such as kHz QPOfrequency) on the one hand, and Lx on the other, while generally good in agiven source on a time scale of hours, is absent both on longer time scales andbetween sources. This leads to parallel tracks in plots of such parameters vs.Lx. Where previous explanations require at least two time-variable independentparameters, e.g. Mdot through the disk and through a radial inflow, one is infact sufficient if the systemic response to time variations in this variablehas both a prompt and a time-averaged component. I explore a scenario in whichmost observable spectral and timing parameters to first order depend on diskaccretion rate normalized by its own long-term average rather than on anyindividual Mdot; Lx just depends on total Mdot. Thus, parameters can beuncorrelated to Mdot, yet vary in response to Mdot variations. Numericalsimulations of the model describing the dependence of kHz QPO frequency on Lx,which observationally is characterized by a striking pattern of parallel tracksboth in individual sources and between sources, reproduce the observationsremarkably well. A physical interpretation involving a radial inflow with arate that derives through a time averaging process from the disk accretionrate, and an inner disk radius that depends on the balance between theaccretion through the disk and the total luminosity seems particularlypromising. The consequences of this idea for our understanding of states andtracks in LMXBs are discussed, and the applicability of the idea to black-holecandidates, where the observational situation is more complex, is brieflyaddressed.Comment: 17 pages 3 figures - version accepted for publication in the ApJ; tentatively scheduled for the v561 n2 p1 ApJ November 10, 2001 issue. Some corrections and clarifications w/r to details of the argumen