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A Massive AGB Donor in Scutum X-1: Identification of the First Mira Variable in an X-Ray Binary
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De_2022_ApJL_928_L8.pdf
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Published version
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Author(s)
De, Kishalay
Chakrabarty, Deepto
Soria, Roberto
Ashley, Michael CB
Conroy, Charlie
Hankins, Matthew J
Kasliwal, Mansi M
Lau, Ryan M
Moore, Anna M
Simcoe, Robert
Date Issued
March 1, 2022
Journal
The Astrophysical Journal Letters
Publisher
American Astronomical Society
Citation
De, Kishalay, Chakrabarty, Deepto, Soria, Roberto, Ashley, Michael CB, Conroy, Charlie et al. 2022. "A Massive AGB Donor in Scutum X-1: Identification of the First Mira Variable in an X-Ray Binary." The Astrophysical Journal Letters, 928 (1).
Version
Final published version
Abstract
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 (AV ≳ 25 mag). Here, we present (i) improved localization of the counterpart using Gaia and Chandra observations, (ii) the first broadband infrared spectrum (≈1–5 μm; R ≈ 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 ≈ 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 (ΔJ ≈ 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 (MK = −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 ≈3–5 M⊙ star at an age of ≈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.
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DOI of Published Version
10.3847/2041-8213/ac5b11
