The M Supergiant High Mass X-Ray Binary 4U 1954+31

TL;DR

This paper reclassifies the binary system 4U 1954+31 as a unique high mass X-ray binary containing a supergiant star and a neutron star, providing detailed properties and evolutionary context.

Contribution

It identifies 4U 1954+31 as the only known HMXB with an M supergiant, challenging previous classifications and expanding understanding of stellar evolution in binary systems.

Findings

The system contains a 9 solar mass M supergiant.

The neutron star has an unusually long 5-hour spin period.

The system's age is estimated between 12 and 50 million years.

Abstract

The X-ray binary 4U 1954+31 has been classified as a Low Mass X-ray Binary (LMXB) containing a M giant and a neutron star (NS). It has also been included in the rare class of X-ray symbiotic binaries (SyXB). The Gaia parallax, infrared colors, spectral type, abundances, and orbital properties of the M star demonstrate that the cool star in this system is not a low mass giant but a high mass M supergiant. Thus, 4U 1954+31 is a High Mass X-ray Binary (HMXB) containing a late-type supergiant. It is the only known binary system of this type. The mass of the M I is 9$^{+6}_{-2}$ M$_\odot$ giving an age of this system in the range 12 - 50 Myr with the NS no more than 43 Myr old. The spin period of the NS is one of the longest known, 5 hours. The existence of M I plus NS binary systems is in accord with stellar evolution theory, with this system a more evolved member of the HMXB population.