UltraCompCAT: a comprehensive Catalogue of Ultra-Compact and Short Orbital Period X-ray Binaries

TL;DR

UltraCompCAT is the first comprehensive online catalogue of ultracompact X-ray binaries, providing detailed classification, distribution, and evolutionary insights into these elusive systems with very short orbital periods.

Contribution

This paper introduces UltraCompCAT, the first extensive catalogue of UCXBs, including confirmed and candidate sources, with analysis of their distribution and potential evolutionary pathways.

Findings

Identification of two distinct UCXB groups based on orbital period and behavior.

Most short-period persistent UCXBs originate from globular clusters.

Field UCXBs predominantly exhibit transient behavior with longer periods.

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 as 'candidate' 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 (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's 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 per cent) 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 both groups.