On its way to the neutron star-white dwarf binary graveyard, IGR J16194-2810, a first ascent M giant X-ray binary

TL;DR

This paper reports the discovery and analysis of a neutron star-M giant binary system, providing detailed orbital, compositional, and evolutionary insights, and discussing its future as a neutron star-white dwarf binary.

Contribution

It presents the first spectroscopic orbit of the M giant in IGR J16194-2810, confirms the system's classification as a LMXB, and models its evolution into a neutron star-white dwarf binary.

Findings

Orbital period of 192.5 days confirmed as twice the photometric period.

The M giant is a first ascent giant nearing the red giant tip.

The system likely evolves into a neutron star-white dwarf binary.

Abstract

A single-lined spectroscopic orbit for the M giant in the X-ray binary IGR J16194-2810 is determined from a time-series of optical spectra. The spectroscopic orbital period of 192.5 days is twice that of the photometric period, confirming that the M giant in the system is an ellipsoidal variable. The giant is identified as a first ascent giant approaching the red giant tip. The primary is a neutron star (NS) with its M giant companion filling its Roche lobe verifying the system classification as a Low-Mass X-ray binary (LMXB). Stellar C, N, O and Fe abundances are derived for the M giant with the C, N, O values typical for a field giant with [Fe/H] = -0.14. The system does not have a large kick velocity. Models for the evolution of the system into a binary NS-white dwarf (WD) are presented. The X-ray properties are discussed in the context of this model. This binary is a rare example of a luminous, long orbital period, LMXB early in the transient ellipsoidal phase.