A Massive AGB Donor in Scutum X-1: Identification of the First Mira Variable in an X-Ray Binary

The symbiotic X-ray binary Sct X-1 was suggested to be the first known neutron star accreting from a red supergiant companion. Although known for nearly 50 yr, detailed characterization of the donor remains lacking, particularly due to the extremely high reddening toward the source (A (V) greater than or similar to 25 mag). Here, we present (i) improved localization of the counterpart using Gaia and Chandra observations, (ii) the first broadband infrared spectrum (approximate to 1-5 mu m; R approximate to 2000) obtained with SpeX on the NASA Infrared Telescope Facility, and (iii) the J-band light curve from the Palomar Gattini-IR survey. The infrared spectrum is characterized by (i) deep water absorption features (H2O index approximate to 40%), (ii) strong TiO, VO, and CO features, and (iii) weak/absent CN lines. We show that these features are inconsistent with known red supergiants but suggest an M8-9 III-type O-rich Mira donor star. We report the discovery of large-amplitude (Delta J approximate to 3.5 mag) periodic photometric variability, suggesting a pulsation period of 621 +/- 36 (systematic) +/- 8 (statistical) days, which we use to constrain the donor to be a relatively luminous Mira (M (K) = -8.6 +/- 0.3 mag) at a distance of 3.6(-0.7)(+0.8) kpc. Comparing these characteristics to recent models, we find the donor to be consistent with a approximate to 3-5 M-circle dot star at an age of approximate to 0.1-0.3 Gyr. Together, we show that Sct X-1 was previously misclassified as an evolved high-mass X-ray binary; instead, it is an intermediate-mass system with the first confirmed Mira donor in an X-ray binary. We discuss the implications of Mira donors in symbiotic X-ray binaries and highlight the potential of wide-field infrared time-domain surveys and broadband infrared spectroscopy to unveil their demographics.