Catching wind with the HST: novel UV spectroscopy of a bona fide ultraluminous X-ray source
We request 17 orbits to secure for the first time an UV spectrum of a generic ultraluminous X-ray source (ULX). Consisting of an accreting compact object, a neutron star or a black hole, and a non-degenerate donor star, ULXs have highest X-ray luminosities among all types of X-ray binaries. These ultra-high luminosities can be achieved either (1) because of an unusual supercritical regime of accretion, or/and (2) because the black holes have masses exceeding a few tens solar, i.e., in the upper range among those detected by gravitational wave observatories. Advances in X-ray astronomy have already proven that supercritically accreting systems indeed exist among the ULX population. Herewith, we request UV spectroscopy to probe the second scenario and establish the existence of ULXs consisting of heavy black holes in a close orbit around a very massive star. ULXs have a broad spectral energy distribution, from X-rays to UV to optical. They are well studied in X-rays, while optical spectra are secured for about ten ULXs. However, no UV spectrum of an ULX exists yet. We propose to close this gap by obtaining a high-quality UV spectrum of the bona fide ULX, Ho II X-1. Our target is the closest among the generic group of those ULXs which show Wolf-Rayet spectra in the optical. The COS spectra will test whether we observe the donor star or the accretion disk wind. Securing the first UV spectrum of a typical ULX is also necessary to gauge the models of the Universe re-ionization which incorporate ULXs as important feedback agents. The UV spectroscopy of an ULX is a necessary addition to the HST UV legacy.