Science1 publisher2 min readPublished
Radio images catch a Milky Way black hole pointing its jet at Earth
IRAS 18293-0941 is the first microblazar found in our own galaxy. The authors say the spot where its jet strikes a molecular cloud accelerates particles to petaelectronvolt energies, matching a gamma-ray signal there.
The Scientist · Science desk
What happened
- An international team from ASTRON, JIVE, the University of Amsterdam and other institutions reports in Astronomy & Astrophysics the first microblazar found in the Milky Way, a stellar-mass black hole whose jet points toward Earth.
- The system, IRAS 18293-0941, pairs a black hole of about 10 solar masses with a hot massive star on an 11-day orbit, and the jets are fed by star material that collects in a disk before falling in.
- Radio imaging shows the jet crossing about 100 light-years of interstellar medium it has already cleared, then striking a relatively dense molecular cloud of molecular hydrogen and dust.
- At the impact point the interstellar material is ionized and the dust heated, and the authors report particles accelerated to nearly the speed of light at energies up to petaelectronvolts.
- The bright spot in the cloud falls at the same place on the sky as a gamma-ray signature of high-energy particles.
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Why it matters
- constraint The accelerator identification stands on a positional match between a radio hot spot and a gamma-ray signal, so confirming it will take a measured particle spectrum from that spot.
- capability Where the galaxy's petaelectronvolt cosmic rays are accelerated has been open for more than a century, and observers now have a specific, resolvable place inside the Milky Way to test.
- precedent A black hole jet shocking the interstellar medium was already on the candidate list for those particles, and a nearby example of the geometry gives gamma-ray surveys a template to match against.
Position is most of the evidence behind calling the impact site an accelerator [6]. The report does not say which gamma-ray telescope was involved, and it gives no significance for the match. A shared location on the sky is a good reason for follow-up, and it is not the same as showing that this particular shock accelerated those particles.
The published account also scales the result against terrestrial hardware, calling the microblazar about 100 times stronger than the Large Hadron Collider, the strongest particle accelerator on Earth [7]. That comparison is about the energy a single particle reaches. How many particles the shock sends out, and the shape of their energy distribution, are separate measurements.
What makes the system look extreme is where its jet points, since a jet aimed at Earth makes the object appear much brighter [11]. The early clue was one-sided. In many radio observations the object showed a bright, compact core with radio emission on only one side, a hint of a jet [13].
The identification came out of an observing campaign spread across wavelengths: radio for high-resolution images and the jet's interaction with its surroundings, optical for spectra and velocities in the system, X-ray and gamma-ray for the hot plasma around the black hole, and infrared for the warm dust [14]. "This was a genuinely multiwavelength observational campaign," Marcote said [15]. Benito Marcote, a senior support scientist at ASTRON and JIVE and one of the paper's authors, said of the blazars powered by distant supermassive black holes: "Those blazars are too remote to be resolved in our images. Having an analog object in our galaxy allows for detailed study of blazar physics" [16].
IRAS 18293-0941 has been in the catalogs for a long time. The Dutch-American IRAS satellite logged it in 1983 [12]. Microblazars have been predicted for about 30 years [18]. The catalog entry predates the prediction of the class by about a decade [20].
What to watch
- Marcote said more observations of the hot spot where the jet hits the interstellar medium are planned; a measured particle spectrum there would test the petaelectronvolt inference.
- An independent gamma-ray analysis that resolves the hot spot and publishes a significance for the positional match.
- Whether any other Milky Way system is identified as a microblazar, giving the class more than one member.