"It sits behind so much dust that it is essentially invisible in ordinary optical images."
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 12,000
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," team leader Roi Rahin of the University of Maryland, Baltimore
County and NASA Goddard Space Flight Center said in a statement.
Josep Martà of the University of Jaén, Spain, added:
"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."
Astronomical images of IRAS 18293-0941. Very-high-resolution
radio observations with the EVN, and the jet interaction with the interstellar
medium, producing a bubble and a hotspot is visible to the MeerKAT radio
telescope. (Image credit: B. Marcote (JIVE/ASTRON)) |
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.