Astronomers have discovered the first microblazar in the Milky Way, a black hole-powered particle accelerator that is blasting near-light-speed plasma jets at Earth.
The system known as IRAS 18293−0941 is located around 12,00 light-years away and consists of a black hole feeding on material stripped from a massive companion star, which it orbits every 11 Earth days. Some of this material escapes the black hole after being channeled to its poles, from where it is blasted out as twin jets travelling at near light-speed. The difference between a blazar and a microblazar is that the former consists of a supermassive black hole with a mass millions or billions of times that of the sun, feeding on surrounding matter and firing jets directly at Earth. In the latter, and thus in IRAS 18293−0941, the feeding object is a much smaller stellar-mass black hole with a mass of up to a few hundred times that of our star.
If you think such a system should be highly conspicuous, you would be right. Yet, despite microquasars having been theorized to exist within our galaxy for around three decades, this one has avoided detection because it is blocked by a thick wall of interstellar gas and dust.
"Everything about IRAS 18293−0941 was hiding in plain sight," researcher leader Josep Martí of the University of Jaén, Spain, said in a statement. "It sits behind so much dust that it is essentially invisible in ordinary optical images. It was catalogued decades ago and then more or less forgotten."
A flicker of a discovery
Sometimes in science, big discoveries begin with just a tiny hint. That is definitely the case with the unveiling of IRAS 18293−0941.
Astronomers first got a hint of its existence when they noticed the light of the system's star was flickering. This revealed the 11.4-day orbit of a companion black hole, which scientists realized they were observing almost face-on.
Further observations made in radio waves then revealed the existence of a one-sided jet, with the other jet not visible because it isn't pointed almost directly toward us, but directly away from Earth.
High-resolution observations made using the network of radio telescopes called the European VLBI Network (EVN) confirmed the orientation of the jet and that it originated from the system IRAS 18293−0941 and not from a distant galaxy that happens to lie in the same direction as the star the team was studying.
"This was the moment the result became solid," explained Benito Marcote from the Joint Institute for VLBI in the Netherlands. "The resolution achieved by the EVN position together with the known position of the star from the Gaia satellite confirmed it: the jet belongs to the stellar system."
Though the jet travelling away from us isn't observable directly, astronomers did manage to detect its effects using the MeerKAT radio observatory in South Africa. This revealed a 100 light-year-wide bubble where the jet strikes interstellar gas and dust. At one edge of that bubble is a "hotspot" where the collision between the jet and the material is accelerating particles, warming dust, and causing the hydrogen gas to glow.
This point is also the source of high-energy gamma-rays, each photon of which carries ten times the energy of particles accelerated by the Large Hadron Collider (LHC), Earth's largest and most powerful particle accelerator.
The jet carries 500,000 times the energy radiated by the sun and explains the mystery of how feeding black holes become such powerful cosmic particle accelerators and generate such high-energy gamma-ray photons.
The elegance is that the accelerator engine and the target are two different objects, tens of parsecs apart," team member Pedro Luque-Escamilla of the University of Jaén said. "The jet does the accelerating. The cloud does the shining."
The discovery of a microblazar much closer to us than any fully-fledged blazar, found in the heart of distant active galaxies, offers a unique opportunity to study how such systems evolve.
Additionally, this system presents the chance to examine how jets pour energy from feeding black holes to their surrounding galaxies.
The team's research is available as a preprint on the repository site arXiv and has been accepted for publication in the journal Astronomy & Astrophysics.
