Microquasars near the Earth emerge as powerful radiation sources

Modern astronomy has clung to the idea that relativistic outflows or jets, responsible for the presence of particularly high-energy electromagnetic radiation, are located in the cores of active galaxies far from Earth. But the latest data from the HAWC observatory paints a different picture. Jets launched by astrophysical sources from our own “backyard” within our galaxy are also sources of extremely high-energy gamma photons.

Very high-energy electromagnetic radiation is produced not only by jets fired from active nuclei in distant galaxies, but also by jet projectiles called microquasars in the Milky Way. This latest discovery by scientists at the International High Water Cherenkov Gamma-Ray Observatory (HAWC) fundamentally changes previous understanding of the mechanisms involved in the formation of ultra-high-energy cosmic radiation, and indeed represents a revolution in further research. This shows that. .

Ever since Victor Hess discovered cosmic radiation in 1912, astronomers have believed that the objects in our galaxy accelerating these particles to their highest energies are the remains of giant supernova explosions, called supernova remnants. I’m here.

But the latest data from the HAWC observatory reveals a different picture. The very high energy radiation source turned out to be a microquasar. Astrophysicists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow played a key role in this discovery.

The study was published in the journal Nature.

The HAWC observatory was built on the slopes of Mexico’s Sierra Negra volcano with the aim of recording particularly high-energy particles and photons arriving from space. The facility consists of 300 steel water tanks equipped with photomultiplier tubes that detect flashes of light known as Cerenkov radiation. This occurs when particles traveling faster than the speed of light in the water fall into the aquarium.

Typically, HAWC captures gamma photons with energies ranging from a few hundred gigaelectronvolts to a few hundred teraelectronvolts. These are up to a trillion times more energetic than a visible photon and more than 10 times more energetic than a proton accelerated in the Large Hadron Collider (LHC) accelerator.

Supermassive black holes in quasars, the active nuclei of some galaxies (objects with massive masses equivalent to hundreds of millions of solar masses), accelerate and absorb material from the surrounding accretion disk. During this process, very narrow and very long streams of material called jets are ejected from very close to the black hole in both directions along its axis of rotation. These often travel at speeds close to the speed of light, resulting in shock waves. There, extremely high-energy photons are produced, reaching up to several hundred teraelectronvolts.

Quasars, which are located in the cores of other galaxies, are among the objects that are very far from us. The closest one (Markarian 231) is 600 million light years from Earth. This is not the case for microquasars. These are compact binary star systems consisting of massive stars and their material-absorbing black holes, which emit jets hundreds of light-years long. To date, dozens of such objects have been discovered in the Milky Way alone.

“The energies of photons detected from microquasars are typically much lower than photons from quasars. Typically we are talking about values ​​on the order of tens of gigaelectronvolts. On the other hand, the HAWC observatory’s detector They observed something very incredible in the data recorded by “Photons that come from microquasars in our galaxy, but they have an energy tens of thousands of times higher than the normal energy.” Dr. Sabrina Casanova (IFJ PAN) speaks with Dr. Xiaojie Wang and Dr. Deji from Michigan Technological University. Huang of the University of Maryland was the first to observe this anomaly.

The source of photons with energies of up to 200 teraelectronvolts turned out to be the Sagittarius microquasar V4641 (V4641 Sgr). It lies in the background of the constellation Sagittarius, about 20,000 light years from Earth. The main role here is played by a black hole with a mass about 6 times that of the Sun, which draws material from a giant star with a mass 3 times that of the Sun. These celestial bodies orbit around a common center of gravity and complete one orbit around each other in just under three days.

Interestingly, the jet emitted by the V4641 Sgr system is directed towards the solar system. In this configuration, Earth-based observers have a relativistically distorted perception of material time at the beginning and end of the jet. This means that the front of the jet starts to look younger than it actually is. As a result, the jet appears to propagate through space at superluminal speeds, in this case nine times the speed of light.

“Importantly, it turns out that the V4641 Sgr microquasar is not the only one. On the other hand, very energetic photons are found not only in this microquasar, but also in other microquasars detected by the LHAASO observatory. They have also been detected in quasars. Therefore, microquasars are likely to be a major contributor to cosmic ray radiation, which is the highest energy in our galaxy,” Casanova added.

The latest discovery is of interest not only to cosmic ray scientists. This suggests that at relatively short distances from Earth, the mechanisms of jet formation and ultra-high-energy photon production operate similarly to those in the nuclei of distant active galaxies, scaled appropriately to the mass of the black hole. This proves that there must be. These processes in microquasars occur on much more human-friendly timescales, over days rather than hundreds of thousands or millions of years.

Furthermore, the photons emitted by microquasars do not have to travel through the vacuum of space, millions of light years, where they can be scattered or absorbed during interactions with photons of the ubiquitous cosmic background radiation. All of this means that for the first time, astrophysicists have the ability to observe processes important to the evolution of galaxies comprehensively and virtually undisturbed.