Black holes can attract matter from their surroundings, which powers jets of matter and radiation escaping their poles with extremely high energies. When such a jet is pointed towards Earth, it makes the object appear much brighter. In case of supermassive black holes – which are found at the centre of many galaxies – such so-called blazars have been studied for a long time. Now, for the first time, astronomers have identified a stellar sized black hole in the Milky Way, whose jet is pointed towards Earth. It is the first ‘microblazar’ found.
Radio imaging has revealed not only the jet but also the interaction of the jet with its surroundings. First the jet crosses a region of about 100 light-years where it has already ‘cleared’ the so-called interstellar medium. After that, it hits a relatively dense molecular cloud, which consists mainly of molecular hydrogen and dust. The interaction creates a bright spot on the cloud where the interstellar material is ionized and the dust is heated. Here, particles are accelerated to nearly the speed of light, with energies up to petaelectronvolts. This makes the microblazar one of the most powerful particle accelerators in the Milky Way, about 100 times stronger when compared to the Large Hadron Collider, the strongest particle accelerator on Earth.
Microblazars have been predicted for about thirty years. This discovery does not only shed new light on the origins of the most energetic particles in the Milky Way, it also teaches astronomers about similar but larger systems already found in other galaxies. “This discovery allows us to study remote blazars, created by distant supermassive black holes”, says Benito Marcote, senior support scientist of ASTRON and JIVE, and one of the authors of the paper. “Those blazars are too remote to be resolved in our images. Having an analogue object in our galaxy allows for detailed study of blazar physics.”
Black hole and massive star
The object – IRAS 18293−0941 – consists of a black hole, of about ten times the mass of our Sun, and a hot massive star orbiting each other every eleven days. The jets are produced by matter that the black hole attracts from the star. First, this matter is ‘collected’ in a disk surrounding the black hole. Just before it finally falls into the black hole, part of the matter is ejected through powerful jets at both poles of the black hole.
IRAS 18293−0941 was already catalogued in the 1983 by the Dutch-American IRAS satellite. The attention of the astronomers was drawn to the object because in many radio observations it appeared to have a bright and compact core and radio emission on one side only, hinting on the existence of such a jet.
An extensive observational campaign was launched, using radio telescopes (for high-resolution images and showing jet interactions), optical telescopes (for spectra and measuring velocities in the system), X-ray and gamma-ray telescopes (for probing the hot plasma around the black hole) and infrared images (showing the warm dust around the system). “This was a genuinely multi-wavelength observational campaign”, says Marcote.
Elusive particles
Cosmic particles with energies reaching up to petaelectronvolt have been detected by cosmic ray observatories on Earth, but for over a century it has remained unclear where these particles get accelerated to such high energies. One candidate accelerator is the shockwave created by a powerful black hole jet hitting the interstellar medium, as seen in the microblazar. In this study, the scientist found that the bright spot in the molecular cloud coincides with a gamma-ray signature of high energy particles.
Marcote is ecxited that the microblazar’s jet can be associated with this elusive class of particles. “We are planning more observations on the area of the hotspot where the jet hits the interstellar medium”, says Marcote. “Better characterizing this region and how the material gets heated and ionized would have strong implications for the galactic formation of stars, that happens in molecular clouds. It can also teach us how microblazars impact the structure and evolution of galaxies like the Milky Way.”
“This discovery highlights the power of studying the Universe with different kind of telescopes at the same time, because none of the individual telescopes could have told the entire story”, says coauthor Jakob van den Eijnden (University of Amsterdam). “It is a great showcase of the international team efforts that are at the core of modern astronomy.”