Even the Quiet Supermassive Black Holes are Blasting out Neutrinos and Gamma Rays

Is radiation possible anywhere else in the Universe? It is not possible to escape radiation from Earth. Not in space, which is filled by diffuse radiation in the form gamma-rays and neutrinos. Scientists have been trying to figure out where all the gamma radiation and neutrinos came from. In a new paper, a trio of researchers proposes a source for all this radiation: Resting black holes.
The center of any large galaxy, such as the Milky Way, is likely to be home to Supermassive Black Holes. When SMBHs are active accreting matter, they can emit a lot radiation across the spectrum from radio waves to Gamma Rays. They are called active galactic nuclear nuclei when this happens. What about SMBHs which are still?

New research has shown that even quiescent SMBHs emit gamma rays, and neutrinos. This discovery helps to explain why the Universe is so full of energetic particles.

The paper's title is Soft gamma radiations from supermassive black hole low accretion and connection to energetic neutrinos.

Radiation from active galaxy nuclei such as this one is quite high. Astronomers believe that some of the Universe's diffuse radiation is emitted by more quiet black holes. Credit: NASA/Dana Berry SkyWorks Digital

Neutrinos are almost massless subatomic particles that are electrically neutral. This is how they got their name. Their gravitational interaction with the strong nucleus is almost zero. They are extremely hard to detect and are currently passing through your body.

Gamma radiations are easy to spot. These photons are the most energetic in the Universe, and you don't want them passing through your body. They are released by atomic bomb explosions, among others. Space-based detectors have detected gamma radiations with voltages in excess of the gigaelectron. If electron volts are not your thing, you can think of them as being more energetic than visible sunlight by several orders of magnitude.

Scientists know a lot about neutrinos, but they aren't sure where they came from. This research may have the answer. The Universe is filled by a diffuse background consisting of MeV gamma rays and PEV neutrinos. Their origins are not known. The authors propose a scenario which can simultaneously account for both backgrounds.

Scientists believe they have a clue as to where the powerful background gamma rays in the gigaelectron Volt (GeV), and teraelectron Volt (TeV), volt ranges originate. These gamma-rays are likely to be from AGNs or star-forming galaxies. However, the source of the softer gamma-rays (e.g. those in the megaelectronvolt (MeV), ranges is unknown. Similar with many neutrinos.

This paper shows how low luminosity galactic nuclear nuclei can be responsible for both neutrinos as well as gamma radiations.

Computer simulation of plasma close to a black hole. Credit: Hotaka Shiokawa / EHT

Black holes have a huge mass and pull that draws matter towards them. The matter eventually falls into the black hole, creating an accretion disc of swirling matter. This is when a lot of gravitational energy gets released. This energy heats up the gas surrounding the hole, creating plasma. The plasma's temperature can reach tens to billions of Celsius because the black hole is low-accreting.

The plasma then energizes protons to a very high degree. These protons can be up to 10,000 times more energetic that what the Large Hadron Collider can achieve. The LHC is the most powerful particle accelerator. These high-speed protons react with radiation and matter to produce neutrinos. This could explain the higher energy neutrinos found in space.

A similar mechanism produces gamma-rays. Comptonization is a process that makes electrons more efficient at producing gamma-rays at extremely high temperatures.

This image shows how mellow SMBHs produce diffuse neutrinos that flood the Universe. Image Credit: Shigeo S. Kimura.

The high-temperature plasma surrounding quiet black holes can also produce neutrinos or gamma rays. These black holes can be dimmed and hard to see but there are many of them. They could be responsible for background radiation, such as gamma-rays or neutrinos.

This is just one possible mechanism. Yet, there is no definitive proof. Is that where it will come from?

Our gamma-ray detectors don't tune to the MeV frequency. They are tuned for higher energy levels. A multimessenger detector is needed, according to the authors. This detector detects both neutrinos and gamma radiation simultaneously, at the appropriate energy ranges. Proposed missions such as e-ASTROGAM and the All-sky Medium Energy Gamma-ray Observatory, (AMEGO), and Gamma-Ray and AntiMatter Survey(GRAMS) could help.

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