Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 476, Issue 2, Pages 1889-1908Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/sty284
Keywords
accretion, accretion discs; stars: black holes; stars: neutron; X-rays: binaries
Categories
Funding
- Curtin Strategic International Research Scholarship from Curtin University
- Australian Research Council [FT140101082]
- Packard Foundation
- National Science Foundation [AST-1308124]
- Natural Sciences and Engineering Research Council of Canada
- Curtin University through the Peter Curran Memorial Fellowship
- European Union [664931]
- NASA [NAS 5-26555]
- NASA through Space Telescope Science Institute [HST-GO-14203.002]
- state government of Western Australia
- STFC [ST/R000506/1] Funding Source: UKRI
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To confirm the nature of the donor star in the ultracompact X-ray binary candidate 47 Tuc X9, we obtained optical spectra (3000-10 000 angstrom) with the Hubble Space Telescope / Space Telescope Imaging Spectrograph. We find no strong emission or absorption features in the spectrum of X9. In particular, we place 3 sigma upper limits on the H alpha and He II lambda 4686 emission line equivalent widths -EWH alpha less than or similar to 14 angstrom and -EWHe II less than or similar to 9 angstrom, respectively. This is much lower than seen for typical X-ray binaries at a similar X-ray luminosity (which, for L2-10 keV approximate to 10(33)-10(34) erg s(-1) is typically -EWH alpha similar to 50 angstrom). This supports our previous suggestion, by Bahramian et al., of an H-poor donor in X9. We perform timing analysis on archival far-ultraviolet, V-and I-band data to search for periodicities. In the optical bands, we recover the 7-d superorbital period initially discovered in X-rays, but we do not recover the orbital period. In the far-ultraviolet, we find evidence for a 27.2 min period (shorter than the 28.2 min period seen in X-rays). We find that either a neutron star or black hole could explain the observed properties of X9. We also perform binary evolution calculations, showing that the formation of an initial black hole/He-star binary early in the life of a globular cluster could evolve into a present-day system such as X9 (should the compact object in this system indeed be a black hole) via mass-transfer driven by gravitational wave radiation.
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