HST spectrum and timing of the ultra-compact X-ray binary candidate 47 Tuc X9

To confirm the nature of the donor star in the ultra-compact X-ray binary candidate 47 Tuc X9, we obtained optical spectra (3,000$-$10,000 {AA}) 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 $3sigma$ upper limits on the H$alpha$ and HeII $lambda 4686$ emission line equivalent widths $-$EW$_{mathrm{Halpha}} lesssim 14$ {AA} and $-$EW$_{mathrm{HeII}} lesssim 9$ {AA}, respectively. This is much lower than seen for typical X-ray binaries at a similar X-ray luminosity (which, for $L_{mathrm{2-10 keV}} approx 10^{33}-10^{34}$ erg s$^{-1}$ is typically $-$EW$_{mathrm{Halpha}} sim 50$ {AA}). 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 seven-day 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.