The 2023 identification of the Be X-ray binary CPD −29 2176 (SGR 0755−2933) as the first high-mass X-ray binary descended from an ultra-stripped supernova left open the exact nature of the core collapse, the efficiency of the earlier non-conservative mass transfer, the system's future evolution toward a double neutron star, and the demographics of the ultra-stripped/failed-supernova channel. A 2024 Matters-Arising exchange in Nature sharpened rather than settled the core: the same orbital data admit both ultra-stripped and normal Type Ib/c solutions, and the discriminating neutron-star observables (spin period, period derivative, magnetic field) have never been measured. The problem is to determine the formation channel and future fate of this system — and, through an expanded sample, the incidence of the low-ejecta, low-kick channel invoked to produce double neutron stars and gravitational-wave sources.

Background

An X-ray binary is a pair in which a compact object — a neutron star or black hole — accretes gas from a stellar companion and glows in X-rays; when the donor is a massive O- or B-type star the system is a high-mass X-ray binary (HMXB), and when that donor is a rapidly rotating B star with an equatorial decretion disk (a Be star) it is a Be X-ray binary. When the iron core of a massive star collapses, the newborn neutron star is commonly flung out by a natal kick of hundreds of km/s; the kick, together with the mass ejected in the supernova, remodels or disrupts any binary. Tauris et al. (2015) predicted that in a very tight binary a neutron-star companion can strip the exploding helium star down to a nearly naked core before an ultra-stripped supernova — an explosion ejecting less than roughly 0.2 M⊙0.2\,M_\odot, delivering little or no kick, and leaving the binary bound; a second such event can then produce a double neutron star (DNS), the source class of GW170817 — the gravitational-wave event from a binary neutron star merger, which also produced a kilonova, an optical/infrared transient powered by radioactive decay of the neutron-rich matter ejected in the merger (Abbott et al. 2017; see also Tauris et al. 2017 on DNS formation). A related issue is the failed supernova: a massive star whose core collapses to a black hole with little or no visible explosion. Richardson et al. (2023) identified CPD −29 2176 (= CD −29 5159, 2SXPS J075542.5−293353, and the unconfirmed magnetar candidate SGR 0755−2933; a magnetar is a neutron star with an extremely strong magnetic field) — a Be X-ray binary with a B0Ve star of about 18 M⊙18\,M_\odot, a neutron star assumed at 1.44 M⊙1.44\,M_\odot, an orbital period of 59.7 d and eccentricity near 0.06 — as the first HMXB descended from an ultra-stripped supernova: its low eccentricity, low peculiar velocity (15.3±3.015.3 \pm 3.0 km/s relative to the local stellar neighborhood), and matches to BPASS (Binary Population and Spectral Synthesis) binary-evolution models pointed to a low-ejecta, low-kick explosion preceded only by stable (non-common-envelope) mass transfer, with future Roche-lobe overflow — mass transfer that begins when the Be star expands to fill its Roche lobe, the region around it within which gas remains gravitationally bound to it — onto the neutron star leading, after a second supernova, to a DNS. However, Larsen et al. (2024; including Tauris) reanalyzed the same observed parameters with Monte Carlo supernova kinematics and found that 92.4% of admissible progenitor solutions eject more than 0.2 M⊙0.2\,M_\odot (mean about 1.5 M⊙1.5\,M_\odot), i.e. a normal Type Ib/c supernova (the explosion of a star stripped of its hydrogen — and, for Type Ic, also its helium — envelope) with a low or favorably directed kick; they further argued on binary-evolution grounds that ultra-stripping requires mass transfer onto a compact object (Case BB Roche-lobe overflow — transfer from a helium star onto a neutron star), which cannot have operated before the first supernova in a system whose current companion is a massive non-degenerate star, and that the system is unlikely to survive the common-envelope phase (a spiral-in of the neutron star inside a shared gas envelope expelled by the expanding companion, which shrinks the orbit) needed to reach a DNS. Richardson & Eldridge (2024) replied that the dynamical solutions are degenerate and the outcome depends on the assumed pre-supernova binary evolution and kick distribution. How this neutron star actually formed — and what the system implies for the ultra-stripped/low-kick channel invoked to produce double neutron stars and gravitational-wave sources — therefore remains open and actively contested. No direct measurement of the neutron star is yet in hand: its spin period and spin-down (measurable in pulsation searches), its magnetic field and age (constrained through X-ray luminosity and spectral/thermal properties), and refinements to the systemic kinematics all remain pending, and joint kinematic-plus-binary-evolution modeling of the observed orbit leaves the kick magnitude, ejecta mass and kick direction degenerate (Doroshenko et al. 2021; Richardson et al. 2023; Larsen et al. 2024; Richardson & Eldridge 2024).

Problem Statement

Determine the formation channel and future fate of the neutron star in the Be X-ray binary CPD −29 2176 (SGR 0755−2933), and through it calibrate the low-ejecta, low-kick core-collapse channel invoked to produce double neutron stars. Concretely: (1) establish the nature of the core collapse that formed this neutron star — an ultra-stripped iron-core-collapse supernova, an electron-capture collapse (collapse triggered by electron captures onto nuclei in an oxygen–neon–magnesium core, expected to yield a small kick), a direct collapse to a neutron star without a bright explosion, or a normal Type Ib/c supernova whose kick happened to be small and/or favorably directed; (2) constrain the efficiency of the earlier non-conservative mass-transfer phase(s) that set the present component masses and orbital separation; (3) establish whether the system will evolve through Roche-lobe overflow from the Be star, a common-envelope phase and a second supernova into a bound double neutron star, with the dependence on the mass-transfer and common-envelope prescriptions quantified; and (4) determine the size, diversity and formation rate of the ultra-stripped/low-kick binary population from an expanded sample of candidate descendant systems and transients, gaining insight into how calm some stellar deaths may be and whether such stars can also die without supernovae. These four questions are coupled facets of one objective: they jointly test whether near-circular, low-peculiar-velocity Be X-ray binaries such as CPD −29 2176 are the observed descendants of ultra-stripped supernovae and genuine double-neutron-star progenitors, as required for this channel to contribute to gravitational-wave source populations.

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