A massive AGB donor in Scutum X-1: Identification of the first Mira variable in an X-ray binary

Abstract

The symbiotic X-ray binary Sct X-1 was suggested as the first known neutron star accreting from a red supergiant companion. Although known for nearly 50 years, detailed characterization of the donor remains lacking, particularly due to the extremely high reddening towards the source ($A_V\gtrsim 25$ mag). Here, we present i) improved localization of the counterpart using Gaia and Chandra observations, ii) the first broadband infrared spectrum ($\approx 1-5\mu$m; $R\approx 2000$) obtained with SpeX on the NASA Infrared Telescope Facility and iii) $J$-band light curve from the Palomar Gattini-IR survey. The infrared spectrum is characterized by i) deep water absorption features (H${}_2$O index $\approx 40$%), 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 a M8-9 III type O-rich Mira donor star. We report the discovery of large amplitude ($\Delta J\approx 3.5$ mag) periodic photometric variability suggesting a pulsation period of $621\pm 36(\mathrm{systematic})\pm 8(\mathrm{statistical})$ days, which we use to constrain the donor to be a relatively luminous Mira ($M_K=-8.6\pm 0.3$ 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 $\approx 3-5$ M${}_{\odot }$ star at an age of $\approx 0.1-0.3$ Gyr. Together, we show that Sct X-1 was previously mis-classified 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.

10 pages, 4 figures. Accepted for publication in ApJ Letters

References in corpus (13)

• Towards a unified view of inhomogeneous stellar winds in isolated supergiant stars and supergiant high mass X-ray binaries
• A census of symbiotic stars in the 2MASS, WISE and Gaia surveys
• M-type giants as optical counterparts of X-ray sources 4U 1700+24 and 4U 1954+319
• The X-Shooter Spectral Library (XSL): Data Release 2
• Palomar Gattini-IR: Survey overview, data processing system, on-sky performance and first results
• A machine learning approach for identification and classification of symbiotic stars using 2MASS and WISE
• Wind-accreting Symbiotic X-ray Binaries
• Modelling Long-Period Variables -- II. Fundamental mode pulsation in the nonlinear regime
• Infrared Spectroscopy of Symbiotic Stars. XII. The Neutron Star SyXB System 4U 1700+24 = V934 Herculis
• The period-age relation of long-period variables
• Period estimation for sparsely-sampled quasi-periodic light curves applied to Miras
• New infrared spectral indices of luminous cold stars: from early K to M-types
• A Rapidly Varying Red Supergiant X-Ray Binary in the Galactic Center

Cited by in corpus (5)

• SRGA J181414.6-225604: A new Galactic symbiotic X-ray binary outburst triggered by an intense mass loss episode of a heavily obscured Mira variable
• The Symbiotic X-ray Binary IGR J16194-2810: A Window on the Future Evolution of Wide Neutron Star Binaries From Gaia
• The first Palomar Gattini-IR catalog of J-band light curves: construction and public data release
• Betelgeuse's Buddy: X-Ray Constraints on the Nature of $\alpha$ Ori B
• Multiwavelength study of 1eRASS J085039.9-421151 with eROSITA NuSTAR and X-shooter