Gamma rays and neutrinos from mellow supermassive black holes

Date:
September 24, 2021
Source:
Tohoku University
Summary:
The Universe is filled with energetic particles, such as X rays,
gamma rays, and neutrinos. However, most of the high-energy cosmic
particles' origins remain unexplained.



FULL STORY ==========================================================================
The Universe is filled with energetic particles, such as X rays, gamma
rays, and neutrinos. However, most of the high-energy cosmic particles'
origins remain unexplained.


==========================================================================
Now, an international research team has proposed a scenario that explains these; black holes with low activity act as major factories of high-energy cosmic particles.

Details of their research were published in the journal Nature
Communications.

Gamma rays are high-energy photons that are many orders of magnitude
more energetic than visible light. Space satellites have detected cosmic
gamma rays with energies of megaelectron to gigaelectron volts.

Neutrinos are subatomic particles whose mass is nearly zero. They rarely interact with ordinary matter. Researchers at the IceCube Neutrino
Observatory have also measured high-energy cosmic neutrinos.

Both gamma rays and neutrinos should be created by powerful cosmic-ray accelerators or surrounding environments in the Universe. However, their origins are still unknown. It is widely believed that active supermassive
black holes (so-called active galactic nuclei), especially those with
powerful jets, are the most promising emitters of high-energy gamma rays
and neutrinos.

However, recent studies have revealed that they do not explain the
observed gamma rays and neutrinos, suggesting that other source classes
are necessary.

The new model shows that not only active black holes but also non-active, "mellow" ones are important, acting as gamma-ray and neutrino factories.

All galaxies are expected to contain supermassive black holes at their
centers.

When matter falls into a black hole, a huge amount of gravitational
energy is released. This process heats the gas, forming high-temperature plasma. The temperature can reach as high as tens of billions of Celsius degrees for low- accreting black holes because of inefficient cooling,
and the plasma can generate gamma rays in the megaelectron volt range.

Such mellow black holes are dim as individual objects, but they are
numerous in the Universe. The research team found that the resulting
gamma rays from low- accreting supermassive black holes may contribute significantly to the observed gamma rays in the megaelectron volt range.

In the plasma, protons can be accelerated to energies roughly 10,000
times higher than those achieved by the Large Hadron Collider -- the
largest human- made particle accelerator. The sped-up protons produce high-energy neutrinos through interactions with matter and radiation,
which can account for the higher-energy part of the cosmic neutrino
data. This picture can be applied to active black holes as demonstrated by previous research. The supermassive black holes including both active and non-active galactic nuclei can explain a large fraction of the observed
IceCube neutrinos in a wide energy range.

Future multi-messenger observational programs are crucial to identify the origin of cosmic high-energy particles. The proposed scenario predicts
gamma- ray counterparts in the megaelectron volt range to the neutrino
sources. Most of the existing gamma-ray detectors are not tuned to detect
them; but future gamma-ray experiments, together with next-generation
neutrino experiments, will be able to detect the multi-messenger signals.

========================================================================== Story Source: Materials provided by Tohoku_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Shigeo S. Kimura, Kohta Murase, Pe'ter Me'sza'ros. Soft gamma
rays from
low accreting supermassive black holes and connection to
energetic neutrinos. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467-021- 25111-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210924104306.htm

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