Galactic Ultracompact X-ray Binaries: Empirical Luminosities

TL;DR

This study empirically measures the luminosity function of Galactic ultracompact X-ray binaries, revealing their limited contribution to high-luminosity X-ray sources and challenging some theoretical predictions.

Contribution

It provides the first detailed empirical luminosity function of UCXBs in the Milky Way, contrasting with existing theoretical models and explaining their role in X-ray binary populations.

Findings

UCXBs show significant luminosity variability.

Measured luminosity function extends to lower luminosities than some models.

UCXBs constitute a small fraction of high-luminosity X-ray binaries.

Abstract

Ultracompact X-ray binaries (UCXBs) are thought to have relatively simple binary evolution post-contact, leading to clear predictions of their luminosity function. We test these predictions by studying the long-term behavior of known UCXBs in our Galaxy, principally using data from the MAXI All-Sky Survey and the Galactic Bulge Scans with RXTE's PCA instrument. Strong luminosity variations are common (and well-documented) among persistent UCXBs, which requires an explanation other than the disk instability mechanism. We measure the luminosity function of known UCXBs in the Milky Way, which extends to lower luminosities than some proposed theoretical luminosity functions of UCXBs. The difference between field and globular cluster X-ray luminosity functions in other galaxies cannot be explained by an increased fraction of UCXBs in globular clusters. Instead, our measured luminosity function suggests that UCXBs only make up a small fraction of the X-ray binaries above a few times 10^36 ergs/s in both old field populations and globular clusters.