HST spectrum and timing of the ultracompact X-ray binary candidate 47 Tuc X9
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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 Å) 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 3σ upper limits on the H α and He ii λ4686 emission line equivalent widths − EWH α ≲ 14 Å and −EW HeII ≲9 −EWHeII≲9 Å, respectively. This is much lower than seen for typical X-ray binaries at a similar X-ray luminosity (which, for L 2--10keV ≈10 33 --10 34 L2--10keV≈1033--1034 erg s−1 is typically − EWH α ∼ 50 Å). 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.
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.
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Bahramian, A.; Heinke, C.; Tudor, Vlad; Miller-Jones, James; Bogdanov, S.; Maccarone, T.; Knigge, C.; Sivakoff, G.; Chomiuk, L.; Strader, J.; Garcia, J.; Kallman, T. (2017)47 Tuc X9 is a low-mass X-ray binary (LMXB) in the globular cluster 47 Tucanae, and was previously thought to be a cataclysmic variable. However, Miller-Jones et al. recently identified a radio counterpart to X9 (inferring ...
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Kennea, J.; Romano, P.; Mangano, V.; Beardmore, A.; Evans, P.; Curran, Peter; Krimm, H.; Markwardt, C.; Yamaoka, K. (2011)We report on the detection and follow-up high-cadence monitoring observations of MAXI J1659-152, a brightGalactic X-ray binary transient with a likely black hole accretor, by Swift over a 27 day period after its initialoutburst ...