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.

The Symbiotic X-Ray Binary IGR J16194-2810: A Window on the Future Evolution of Wide Neutron Star Binaries From Gaia

“…We note that the uncertainties on the predicted stellar parameters are implausibly small, and are likely to be dominated by unaccounted-for systematics in the stellar models. Nevertheless, most of our results are in good agreement with those derivedin the contemporaneous analysis of Hinkle et al (2024).…”

“…As we were preparing this manuscript, an independent analysis of IGR J16194-2810 by Hinkle et al (2024) appeared. The orbital period and most physical parameters we infer in our analysis are in good agreement with their findings.…”

“…We predict A 2 for a given choice of mass ratio, Roche lobe filling factor, inclination, and limb-and gravity-darkening coefficients using the prescription of Morris & Naftilan (1993) (which assumes circular orbits, tidal locking, and linear limbdarkening and gravity-darkening laws) and apply the correction factor of Gomel et al (2021) (which is relevant at large Roche lobe filling factors). Unlike Hinkle et al (2024), we do not assume that the red giant fills its Roche lobe.…”

“…Unlike Hinkle et al (2024), we do not use the lack of eclipses in the optical light curve of IGR 16194-2810 to place a limit on the inclination as a function of mass ratio. This is because-given the small physical size of the NS and accretion flow relative to a red giant-we do not expect to see eclipses in SyXBs in the optical bands.…”

“…While the system has been studied extensively at X-ray wavelengths (Masetti et al 2007;Ratti et al 2010;Kitamura et al 2014;Bozzo et al 2024), the optical counterpart had, until recently, not been studied in detail. Shortly before this paper was completed, Hinkle et al (2024) reported optical and near-infrared observations of the source. Our analysis here is independent of theirs, but we compare to their work throughout the text.…”