On Its Way to the Neutron Star–White Dwarf Binary Graveyard, IGR J16194‑2810, A First Ascent M Giant X-Ray 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, and 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 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.