UltraCompCAT: A comprehensive catalogue of ultra-compact and short orbital period X-ray binaries

Abstract

Ultracompact X-ray binaries (UCXBs) are a distinctive but elusive family of low-mass X-ray binaries (LMXBs) characterised by their tight orbits and degenerate donor stars. Here we present UltraCompCAT, the first online and comprehensive catalogue of UCXBs. The initial version of UltraCompCAT comprises 49 sources, including 20 confirmed UCXBs (those with a measured orbital period shorter than 80 min) and 25 systems that we label ‘candidates’ based on their multi-wavelength phenomenology. For completeness, we also include four LMXBs with orbital periods in the range of 80 to 120 min, since they might be related to (e.g. close progenitors) or even part of the UCXB population that evolved towards longer periods. We discuss the orbital period and Galactic distribution of the catalogue sample. We provide evidence for the presence of at least two separate groups of UCXBs, one formed by persistent systems with orbital periods shorter than 30 min and a second group of transient objects (70 %) with periods in the range of 40 to 60 min. We show that the former group is dominated by sources formed in globular clusters, while the latter accounts for the (known) UCXB population in the Galactic field. We discuss the possible evolutionary channels for the two groups.