The system is a binary: a massive star and a compact black hole that orbit each other every 11.38 days. The black hole continuously pulls gas from its companion. Part of that gas is ejected as two tightly bundled jets moving at roughly 75 percent of the speed of light. One points almost straight at Earth. The other cannot be seen from here because of relativistic effects.

The unseen jet still left a mark. The MeerKAT radio telescope in South Africa detected a shock bubble about 100 light-years wide, attributed to that opposing jet and its interaction with a dense cloud of interstellar gas.

An international team from Spain, the Netherlands and Argentina carried out the work. It is led by Josep Martí of the University of Jaén, the study's lead author, and Benito Marcote of the Joint Institute for VLBI ERIC. The researchers used the European VLBI Network, which works as a virtual telescope with very high resolving power, and combined its data with positional measurements from the European Gaia space telescope. Together, the two datasets let them confirm that the jet originates in the system and is aligned toward Earth.

Optical images show nothing, because large amounts of cosmic dust hide the object. Martí said the object "was hiding right in front of our eyes."

Three gamma-ray observatories, LHAASO in China, HAWC in Mexico and H.E.S.S. in Namibia, detected ultra-high-energy gamma radiation at the site where the jet strikes the gas cloud. Individual photons carry energies above 100 trillion electronvolts, more than ten times the maximum energy of the Large Hadron Collider at CERN in Geneva.

Extreme cosmic phenomena of this kind are usually observed only in distant galaxies. Having one inside the Milky Way gives astronomers a high-energy laboratory in the nearby cosmos for studying how particles are accelerated over time. The study has been accepted by the journal Astronomy & Astrophysics and is available on the preprint server arXiv.