Cosmic rays are called high-energy elementary particles and the nuclei of light elements that arrive to us from deep space. They carry information about the most energetic events and active objects in the Universe: galactic nuclei, supernova explosions, relativistic jets of matter and much more. Perhaps someday they will shed light on the secrets of dark matter and the absence of antimatter in the universe.

One of the main problems of the theory of the origin of cosmic rays is the mechanism of their acceleration. In the 60s of the last century, theoretical physicists Vitaly Lazarevich Ginzburg and Sergey Ivanovich Syrovatsky (FIAN) suggested that cosmic rays may occur during supernova explosions. A specific mechanism for the acceleration of charged particles on the shock waves arising in this case was proposed by Germogen Filippovich Krymsky and his colleagues (Institute of Cosmophysical Research and Aeronomy, Yank) However, the time of existence of such shock waves is not enough to give cosmic rays energy above 1014–1015 eV (electron volt). For comparison: the energy of accelerated protons in the Large Hadron Collider is also within these limits. The question of the nature of particles with energies above 1015 eV remained open.

Researchers from Institute of Physics, together with Chinese colleagues, developed a model to explain the nature of cosmic rays in our Galaxy in the energy range from 3×1015 to 1018 eV, which is two to three orders of magnitude higher than the energy of particles generated by supernova explosions.

The model connects high-energy cosmic rays with the Fermi bubbles discovered in November 2010, two huge regions in the central region of our Galaxy emitting radiation in the gamma and x-ray ranges. These areas, symmetrical with respect to the galactic plane, extend over a distance of 50 thousand light years, which is comparable with the size of the Milky Way itself. Later, the Planck telescope team detected the radiation of these structures in the microwave range. Similar structures are observed in other galactic systems with active nuclei.

Fermi bubbles

The nature of the Fermi bubbles is not completely clear. Fermi bubbles location clearly indicates the connection of bubbles with the activity of the center of our Galaxy, where giant energy is released as a result of the tidal destruction of stars during a fall (accretion) on a central black hole with a mass of 106 solar masses.
Dmitry Chernyshev, Vladimir Dogel and their colleagues from Hong Kong and Taiwan in previous papers showed that radiation in Fermi bubbles is generated by various processes involving relativistic electrons accelerated by shock waves, which are formed when stellar matter falls on a black hole. In this case, the shock waves should also accelerate the protons and light nuclei that make up the cosmic rays. In a new paper published in the journal EPJ Web of Conferences, the authors suggested that the giant shock fronts of Fermi bubbles could additionally accelerate protons emitted by supernovae to energies substantially higher than 1015 eV.

In the proposed mechanism, particles born with explosions of supernovae with energies lower than 3×1015 eV, moving from the galaxy disk to the galactic halo, are additionally accelerated by the processes occurring in Fermi bubbles. The spectral distribution of cosmic rays calculated from the developed model is fully consistent with that observed in the energy range from 3×1015 to 1018 eV, which indicates that the model is working.

The spectrum in this case is the dependence of the energy brightness, which characterizes the intensity (power) of cosmic rays, depending on the energy of the particles making up them. The boundary energy value of 3×1015 is chosen because with this value a change in the slope of the dependence is observed (the so-called “knee”), which indicates some changes in the generation mechanism for higher energies.

Giant bridges found in the center of the Milky Way

X-ray observations of the Milky Way center have led to the discovery of two high-temperature flows, originating in a supermassive black hole and extending hundreds of light years above and below the plane of our Galaxy. They may be the suppliers of energy and matter from the heart of the Milky Way to the Fermi bubbles – huge formations shining in the gamma range.

We know that the flows of matter and energy emanating from the center of the galaxy are crucial for its evolution and form, but the study of these processes is extremely difficult.

-Gabriel Ponti, lead author of the study from the Institute for Extraterrestrial Physics. Max Planck (Germany) and the National Institute of Astrophysics (Italy)

In 2010, near the center of the Milky Way, NASA’s Fermi spacecraft observed two bubbles of colossal size, which together form an hourglass-like shape and cover a distance of 50 thousand light years, about half the diameter of our entire Galaxy. Astronomers believe that they are formed by material thrown away in the distant past by the supermassive black hole Sagittarius A *, but their exact nature is still not revealed, which does not allow to trace in detail the evolution of our star house.

Fortunately, the Milky Way Center is the closest laboratory to us to study in detail the outflow of material from galaxies into their surrounding space, and thanks to data collected by the ESA satellite XMM-Newton between 2016 and 2018, we were able to build the most extensive X-ray map, ever made for the core of our galaxy.

-Gabriel Ponti

On this map, scientists have identified long channels of superheated gas, possibly acting as a pair of exhaust pipes, through which energy and matter are transferred from the heart of the Milky Way to the base of Fermi bubbles, adding new material to them. If this is so, then it clarifies how the processes occurring in the center of our Galaxy in the present and the past are connected with the existence of large structures around it.

Researchers believe that two “bridges” of gas may be the remnant of the past of the Milky Way, from the period when its activity was much higher. It also shows that even “calm” galaxies, which today contain a relatively quiet supermassive black hole, and the star formation in them proceeds at a moderate rhythm, can boast enormous flows of material.

The Milky Way is seen as a kind of prototype standard spiral galaxy. In a sense, this discovery sheds light on how all typical spiral galaxies and their contents can behave throughout the universe.

-Mark Morris, co-author of the study at the University of California (USA)

And although on a cosmic scale of galactic activity, the center of the Milky Way is classified as stationary and calm, previous data from XMM-Newton showed that it is still quite turbulent and chaotic, with constant supernova explosions, collisions of stars and absorption of matter by a supermassive black hole with an ejection radiation.

We still have a lot to do with XMM-Newton – the telescope is able to scan a significant area of ​​the Milky Way core, which will help us to display the bubbles and hot gas surrounding our Galaxy, as well as to trace their connection with its other components.

-Gabriel Ponty

Cosmic rays, energy and Fermi bubbles
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