Merger Precursor: Year-long Transients Preceding Mergers of Low-mass Stripped Stars with Compact Objects

TL;DR

This paper models year-long precursor transients caused by mass transfer in low-mass helium star-neutron star binaries, predicting observable signatures before merger and linking them to Type Ibn supernovae.

Contribution

It introduces a model for long-duration transients preceding mergers of low-mass helium stars with neutron stars, connecting pre-merger activity to observed supernova features.

Findings

Systems can produce transients lasting years with luminosities of 10^{40}-10^{41} erg s^{-1}.

Precursor activity can be observed years before the merger or explosion.

The model explains properties of Type Ibn supernovae like SN 2023fyq.

Abstract

Binary mass transfer can occur at high rates due to rapid expansion of the donor's envelope. In the case where mass transfer is unstable, the binary can rapidly shrink its orbit and lead to a merger. In this work we consider the appearance of the system preceding merger, specifically for the case of a low-mass ($\approx 2.5$-$3~M_\odot$) helium star with a neutron star (NS) companion. Modeling the mass transfer history as well as the wind launched by super-Eddington accretion onto the NS, we find that such systems can power slowly rising transients with timescales as long as years, and luminosities of $\sim 10^{40}$-$10^{41}$ erg s$^{-1}$ from optical to UV. The final explosion following the merger (or core-collapse of the helium star in some cases) leads to an interaction-powered transient with properties resembling Type Ibn supernovae (SNe), possibly with a bright early peak powered by shock cooling emission for merger-powered explosions. We apply our model to the Type Ibn SN 2023fyq, that displayed a long-term precursor activity from years before the terminal explosion.