A near-Earth microquasar appears as a source of powerful radiation

Modern astronomy has clung to the belief that relativistic jets or jets, responsible for the existence of electromagnetic radiation of particularly high energies, are located in the cores of active galaxies far from Earth. However, a different picture of reality emerges from the latest data from the HAWC observatory: jets launched by astrophysical sources from our own intra-galactic “backyard” are also sources of extremely high energy gamma photons.

Extremely high-energy electromagnetic radiation is produced not only by jets launched from active nuclei of distant galaxies, but also by jet-throwing objects in the Milky Way, called microquasars. This latest discovery by scientists at the International High Altitude Water Cherenkov Gamma-ray Observatory (HAWC) radically changes previous understanding of the mechanisms responsible for the formation of ultra-high energy cosmic radiation and marks a practical revolution in its subsequent study. .

Since the discovery of cosmic radiation by Victor Hess in 1912, astronomers have believed that the celestial bodies responsible in our galaxy for accelerating these particles to the highest energies are the remains of gigantic supernova explosions, called supernova remnants.

However, the latest data from the HAWC observatory give a different picture: the sources of radiation with extremely high energies turn out to be microquasars. 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 is published in the journal Nature.

The HAWC observatory was erected on the slope of the Sierra Negra volcano in Mexico with the aim of recording particles and photons arriving from space at particularly high energies. The facility consists of 300 steel water tanks equipped with photomultipliers sensitive to fleeting flashes of light, known as Cherenkov radiation. This appears in the tank when a particle moving faster than the speed of light in the water falls into it.

Typically, HAWC captures gamma photons with energies ranging from hundreds of gigaelectronvolts to hundreds of teraelectronvolts. These are energies up to a trillion times greater than the energy of visible light photons and more than a dozen times greater than the energy of protons accelerated by the Large Hadron Collider accelerator (LHC).

The supermassive black holes of quasars, that is to say the active nuclei of certain galaxies (objects of enormous mass, numbering in hundreds of millions of solar masses) accelerate and absorb matter from the accretion disk which surrounds them. During this process, very narrow and very long streams of matter, called jets, are projected near the poles of the black hole, in both directions along its axis of rotation. These travel at speeds often close to the speed of light, causing shock waves. This is where extremely high energy photons are produced, reaching hundreds of teraelectronvolts.

Located in the nuclei of other galaxies, quasars are among the objects very distant from us. The closest (Markarian 231) is 600 million light years from Earth. This is not the case for microquasars. These are compact binary systems, made up of a massive star and its black hole absorbing matter, which emit jets several hundred light years long. Several dozen objects of this type have so far been discovered in our galaxy alone.

“The photons detected by microquasars generally have energies much lower than those of quasars. Usually, we are talking about values ​​of the order of tens of gigaelectronvolts. In the meantime, we observed something quite incredible in the data recorded by the detectors of the HAWC observatory: photons coming from a microquasar located in our galaxy, and yet carrying energies tens of thousands of times higher than normal”, declares Dr Sabrina Casanova (IFJ PAN), who, with Dr. Xiaojie Wang of Michigan Tech University and Dr. Dezhi Huang of the University of Maryland were the first to observe the anomaly.

The source of photons with energies of up to 200 teraelectronvolts turned out to be the microquasar V4641 Sagittarii (V4641 Sgr). It is located deep in the constellation Sagittarius, approximately 20,000 light years from Earth. The main role here is played by a black hole with a mass of about six solar masses, attracting matter from the stellar giant with a mass three times that of the Sun. Objects orbit around a common center of mass, rotating around each other once every three days.

Interestingly, the jet emitted by the V4641 Sgr system is directed towards the solar system. In this configuration, an Earth observer has a relativistic and distorted perception of the time of matter at the beginning and end of the jet: his forehead begins to appear younger than it actually is. As a result, the jet appears to propagate through space at superluminal speed, in this case up to nine times the speed of light.

“Significantly, the V4641 Sgr microquasar turns out not to be unique. Meanwhile, extremely energetic photons are detected not only from it, but also from other microquasars, detected by the observatory LHAASO It therefore seems likely that microquasars contribute significantly to cosmic ray radiation at the highest energies in our galaxy,” adds Dr Casanova.

The latest discovery isn’t just of interest to cosmic ray scientists. This proves that at a relatively small distance from Earth, mechanisms of jet formation and production of ultra-energetic photons must be at work in a manner analogous to those of the nuclei of active and distant galaxies, on the scale appropriate to the mass of the black hole. These processes in microquasars occur on a much more human-friendly time scale: over several days and not over hundreds of thousands or millions of years.

Additionally, photons emitted by microquasars do not need to cross the millions of light-years of the cosmic vacuum, where they can be scattered or absorbed during interactions with photons of the ubiquitous cosmic background radiation. All this means that astrophysicists have, for the first time, acquired the ability to make complete and virtually untouched observations of processes crucial to the evolution of galaxies.