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Published March 20, 2022 | Submitted + Published
Journal Article Open

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 ≳ 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 (M_K = −8.6 ± 0.3 mag) at a distance of 3.6^(+0.8)_(−0.7) 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.

Additional Information

© 2022 The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2022 January 24; revised 2022 February 16; accepted 2022 March 1; published 2022 March 21. We thank S. Rappaport, M. Boyer, and D. Kaplan for valuable discussions. Support for this work was provided by NASA through the NASA Hubble Fellowship grant #HST-HF2-51477.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. R.S. acknowledges grant number 12073029 from the National Science Foundation of China. We acknowledge the Visiting Astronomer Facility at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement no. NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. PGIR is generously funded by Caltech, Australian National University, the Mt Cuba Foundation, the Heising-Simons Foundation, the Bi-national Science Foundation. PGIR is a collaborative project among Caltech, Australian National University, University of New South Wales, Columbia University, and the Weizmann Institute of Science. The scientific results reported in this article are based in part on data obtained from the Chandra Data Archive. This research has made use of software provided by the Chandra X-ray Center (CXC) in the application packages CIAO, ChIPS, and Sherpa.

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Submitted - 2201.09906.pdf

Published - De_2022_ApJL_928_L8.pdf


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August 22, 2023
August 22, 2023