Period--luminosity relation for persistent LMXBs in the near-infrared

TL;DR

This paper establishes a near-infrared period-luminosity relation for persistent low-mass X-ray binaries, enabling orbital period estimates from X-ray and infrared data, aiding future galactic population studies.

Contribution

It introduces a new correlation between X-ray luminosity, orbital period, and infrared magnitude for LMXBs, useful for estimating orbital periods from observational data.

Findings

Correlation between L_X, P_orb, and M_K with RMS scatter of ~0.3 mag.

Spectral energy distribution modeled by accretion disc and secondary star.

Potential application in population studies with upcoming surveys.

Abstract

We study relations between the X-ray luminosity, orbital period and absolute near-infrared magnitude of persistent low-mass X-ray binaries (LMXBs). We show that often optical and near-infrared spectral energy distribution of LMXBs can be adequately described by a simple model of an accretion disc and a secondary star reprocessing X-ray emission of a central compact object. This gives us an evidence that using an X-ray luminosity and an absolute infrared magnitude of a persistent LMXB one can make reliable estimate of its orbital period. Using a sample of well-known LMXBs, we have constructed a correlation of L_X, P_orb and M_K values which can be approximated by a straight line with the RMS scatter at the level of ~0.3 mag. Such a correlation, being to some extent an analogous to the correlation, found by van Paradijs & McClintock 1994, might be helpful for future population studies especially in the light of forthcoming surveys of the Galaxy in X-ray and infrared spectral domains.