Ultra-compact X-ray binaries with high luminosity: a key for a new scenario

TL;DR

This paper proposes a new scenario where high mass transfer rates in ultra-compact X-ray binaries are explained by the donor being a remnant core of a giant post-common envelope event, addressing previous theoretical gaps.

Contribution

It introduces a novel explanation for high mass transfer rates in long-period UCXBs involving remnants of giant cores from recent common envelope episodes.

Findings

High mass transfer rates can be sustained by remnant giant cores.

The proposed scenario explains observations of long-period UCXBs.

Current theories fail to account for these high transfer rates.

Abstract

Ultra-compact X-ray binaries (UCXBs) are accreting systems with periods less than 1 hour, which qualifies them to contain a degenerate donor-companion. One would expect such systems to have the easiest theoretical explanation, compared to other kinds of X-ray binaries. Nonetheless, current theory fails to explain high mass transfer (MT) rates in three recently well observed long-period UCXBs. We find that this range of MT rates can be maintained if the donor is a remnant of an out-of-thermal-equilibrium naked core of a giant which was revealed in a very recent episode of a common envelope (CE) event.