An international team of astronomers has accurately measured the distance from Earth to a black hole for the first time. Without needing to rely on mathematical models the astronomers came up with a distance of 7800 light years, much closer than had been assumed until now. The researchers achieved this breakthrough by measuring the radio emissions from the black hole and its associated dying star. Due to the much lower error margin (<6%), astronomers can now gain a better picture of how black holes evolve, for example. Moreover, an exact distance is important for measurements of the black hole´s spin.

Astronomical distances are most easily measured using the so-called trigonometric parallax, in which astronomers make use of the annual shift in the star’s position as a consequence of the earth’s orbit around the sun (parallax shift). Peter Jonker from SRON Netherlands Institute for Space Research and his colleagues have now applied this method for the first time to a relatively near black hole and its associated star, V404 Cygni, in the Cygnus constellation. The outermost layers of the star are being drawn into the black hole. This gas first of all accumulates in a plasma disc around the black hole before it disappears into it, a process during which a lot of X-rays and radio waves are emitted. Jonker and his colleagues could accurately measure the parallax shift of this binary system using a combination of telescopes spread throughout the world, the High Sensitivity Array.

Using this approach the astronomers could establish that the black hole of V404 Cygni is 7800 light years from Earth, slightly more than half the distance that was previously assumed. The researchers believe that the previous overestimation of this distance was due to an underestimation of the absorption and diffraction of interstellar dust that can give an error margin of about 50 percent. The error margin of the new measurement is less than 6 percent.

Supernova
From their measurements the researchers could work out that the black hole developed from a supernova explosion, and that it moves through space at a rate of about 40 km per second. The binary-star system acquired this velocity during the explosion. Jonker: ‘With this information we have gained a better idea about how back holes evolve. For example, we hope to be able to answer the question as to whether there is a difference between black holes that evolve directly from the collapse of a star without a supernova and black holes that evolve via a supernova and a temporary intermediate star, a proto-neutron star. We expect that the black holes in the last group can get a kick. Black holes formed in this way could then move through space faster.’ Interestingly, V404 Cygni belongs to this second group but has not received a big kick. Fellow researcher James Miller-Jones: ‘We are now trying to apply the same measurement method to several other black holes.’

Publication
The authors published their research results on 1 December in The Astrophysical Journal, under the title The first accurate parallax distance to a black hole. In the same week the research results appeared as a research highlight in Nature. The authors are:
J.C.A. Miller-Jones (NRAO), P.G. Jonker (SRON), V. Dhawan (NRAO), W. Brisken (NRAO), M.P. Rupen (NRAO), G. Nelemans (Radboud University Nijmegen) and E. Gallo (MIT).