Very high energy photons in space

[Rev. Cheeseman]

Previously, in my previous thread I talked about the most energetic photons we've ever seen which is the article here https://arxiv.org/abs/2310.10100 it involved PeV protons accelerating through a medium, not vacuum, to generate the highest energy gamma radiations. Using AI, it said [Redacted by the Mentors -- AI is not a valid reference at PF]Please correct me if the AI did wrong regarding the density level.

Can very strong supernova explosions accelerate electrons and protons to reach GeV or more in very dense environments that are much denser than vacuum like the mechanism that happen in thunderstorms where the acceleration of particles happen in denser environments than vacuum that resulting in 10 to 100 MeV bremsstrahlung photons generated in denser environments than vacuum?

 

[Astronuc]

Using AI, it said[Redacted by the Mentors -- AI is not a valid reference at PF]

Simply using AI, which one writes "it said [Redacted by the Mentors -- AI is not a valid reference at PF]" isn't valid since there is no attribution to the source(s) from which the AI took information. Please provide the original source of the data.

The values of 10^-3 to 10^-1 cm^-3 appear to be incorrect.

From the ArXiV paper cited, in the Introduction one finds the statement with respect to Cygnus-X

The 7° × 7° area harbors several Wolf-Rayet stars and hundreds
of O-type stars grouped in powerful OB associations. It also
contains vast HI and molecular gas complexes with masses ex-
ceeding 10⁶ M⊙. The presence of potential particle accelerators,
namely massive stars and supernova remnants, and targets for
γ-ray production (dense gas regions) make several parts of this
region effective γ-ray emitters.

Note the dense gas statement.

One has to consider what part of the Cygnus-X star-forming regions is producing the PeV γ-rays.

In the paper, Filamentary structures of ionized gas in Cygnus X, one finds the statement in the Abstract:

The electron densities of the filamentary structure range between 10 ≲ n_e [cm⁻³] ≲ 400 with a median value of 35 cm⁻³,

https://www.aanda.org/articles/aa/full_html/2022/08/aa42596-21/aa42596-21.html

in the Abstract, one finds a subsequent statement:

More than half of the filamentary structures are likely photoevaporating surfaces flowing into a surrounding diffuse (~5 cm⁻³) medium.

In the second and third paragraphs of the Introduction, one finds various values for densities in various regions

As a result of peculiar motion and/or inhomogeneities in the medium, the star, its photons, and the gas it ionizes enter a surrounding low-density medium (n_e ~ 0.1–100 cm⁻³) within a few megayears (e.g., Mezger 1978).

Within the plane of the Galaxy, photoionized gas is found in a variety of environments (and referred to with a variety of different names). Dense (n_e > 10³ cm⁻³) ionized gas pervades H II regions of a few parsecs in size.

From leaky H II regions, ionizing photons escaping through porous material create (partially) ionized gas (1–100 cm⁻³) in the envelopes of H II regions. This diffuse ionized gas (1–100 cm⁻³) permeates to larger volumes in blister H II regions. Assisted by SN explosions, massive stars create large excavated regions or plasma tunnels that contain fully ionized gas (1–10 cm⁻³).

In the fourth paragraph in the Introduction:

Mezger (1978) estimated that 84% of ionizing photons are emitted by O stars outside of compact H II regions, in gas characterized by densities of n_e ≈ 5–10 cm⁻³ dubbed extended low-density (ELD) H II gas.

Note the aforementioned densities are much greater than the values (10^-3 to 10^-1 cm^-3) stated in the original post.

The ArXiV paper refers to HI regions vs HII regions. See the significance here
http://csep10.phys.utk.edu/OJTA2dev/ojta/c2c/milkyway/interstellar/regions_tl.html
https://en.wikipedia.org/wiki/H_I_region , https://en.wikipedia.org/wiki/H_II_region

In the following paper, Chandra X-ray spectroscopy of focused wind in the Cygnus X-1 system
https://www.aanda.org/articles/aa/full_html/2016/06/aa22490-13/aa22490-13.html

Large density and temperature inhomogeneities are present in the wind, with a fraction of the wind consisting of clumps of matter with higher density and lower temperature embedded in a photoionized gas.

One has to identify the local conditions of the source of PeV or even TeV photons.

Local pressure will be determine the product of particle (atomic, ion, electron) density (n) and temperature (T) by P = nkT, where k is the Boltzmann constant.

 

[berkeman]

Thread is closed for Moderation.

 

[berkeman]

Due to a long history of problems with the OP, they are now gone from PF, so this thread will remain locked. Thanks for the great reply @Astronuc

 

[berkeman]

Upon further review, the thread is reopened for now.

 

[Astronuc]

"Very high energy photons" is an interesting topic. The OP poses an interesting, although awkward, question with respect to PeV or GeV photons. In between there are TeV electrons. I would emphasize from 1 GeV to 1 PeV is 6 orders of magnitude: 10⁹ to 10¹⁵. In terrestrial accelerators, we are fortunate to get into the TeV range.

Consider that the rest masses of nucleons are ~0.938272 GeV/c² for a proton and ~0.939565 GeV/c², these particles are relativistic in the GeV range, and certainly electrons and positrons with rest masses, 0.511 MeV/² or 0.000511 GeV/² are relativistic, TeV and PeV particles have extraordinary energy, and photons of such energy must produce extraordinary reactions.

It worthwhile to discuss the following questions:

What are the sources of GeV, TeV and PeV photons, which implies electron/positron or other charged particles of comparable energy? A related question - what observations have been made that reveal the existence of such high energy photons, including astrophysical observations and terrestrial experiments with particle accelerators?

What are the conditions/reactions required to produce GeV to PeV charged particles and photons? For terrestrial conditions, e.g., in terrestrial particle accelerators, it's straightforward, but in distant astrophysical systems, the conditions are inferred from observation.

What are the interactions of GeV to PeV photons between the source (production) and observers (on Earth)?

=============================================================================

Reviewing the literature on high energy photons, electrons/positrons and other charged particles, one find a variety of terrestrial experiments and observations of astrophysical sources. One find various articles on high energy electrons, such as:

Scientists (from H.E.S.S = High Energy Steroescopic System) find highest energy cosmic ray electrons ever seen
https://www.space.com/the-universe/scientists-find-highest-energy-cosmic-electrons-ever-seen

"Cosmic rays are a century-old mystery," Mathieu de Naurois, a researcher at the French National Centre for Scientific Research and deputy director of the H.E.S.S. collaboration, told Space.com.

First reported in 1912 by Austrian physicist Victor Hess, cosmic rays were discovered after a series of balloon ascents meant to explore ionizing radiation that was first detected on an electroscope. However, after reaching an altitude of 5,300 meters, Hess unveiled a natural source of high-energy particles from space. Today, we call those particles cosmic rays.

Now, H.E.S.S. scientists are excited because they’ve detected the highest energy electrons and positrons to date (a positron is like the "opposite" of an electron because it has the mass of an electron, but is positively charged like a proton), which make up one component of high-energy cosmic rays. The finding is exciting because it provides tangible evidence of extreme cosmic processes unleashing colossal amounts of energy.

"Understanding these cosmic rays allows us to unveil big particle accelerators in the universe that are often associated with the most violent phenomena: the explosion of stars; very compact objects with huge gravitational and electromagnetic fields, such as neutron stars and pulsars; cataclysmic mergers; and black holes," said de Naurois.

Because electrons at this energy lose energy quickly, the team believes they must be coming from relatively nearby. "In the vicinity of our solar system, there [are] very efficient cosmic accelerators of electrons," de Naurois said. "Within a few hundred light-years, there are many stars, with the nearest ones typically lying two light-years from the Earth. We would therefore also expect to have a few ‘dead stars’ in this region, such as pulsars or supernova remnants, which could be the sources of these electrons."

About H.E.S.S - https://www.mpi-hd.mpg.de/HESS/pages/about/

H.E.S.S. is a system of Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma rays in the energy range from 10s of GeV to 10s of TeV. The name H.E.S.S. stands for High EnergyStereoscopic System, and is also intended to pay homage to Victor Hess, who received the Nobel Prize in Physics in 1936 for his discovery of cosmic radiation. The instrument allows scientists to explore gamma-ray sources with intensities at a level of a few thousandths of the flux of the Crab nebula (the brightest steady source of gamma rays in the sky). H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality. The first of the four telescopes of Phase I of the H.E.S.S. project went into operation inSummer 2002; all four were operational in December 2003, and were officially inaugurated on September 28, 2004. A much larger fifth telescope - H.E.S.S. II - is operational since July 2012, extending the energy coverage towards lower energies and further improving sensitivity.

https://www.mpi-hd.mpg.de/HESS/pages/about/telescopes/
H.E.S.S.: The High Energy Stereoscopic System
https://arxiv.org/abs/2405.11104

In 2005, it was announced that H.E.S.S. had detected eight new high-energy gamma ray sources, doubling the known number of such sources. As of 2014, more than 90 sources of teraelectronvolt gamma rays were discovered by H.E.S.S.

In 2016, the HESS collaboration reported deep gamma ray observations which show the presence of petaelectronvolt-protons originating from Sagittarius A*, the supermassive black hole at the centre of the Milky Way, . . .

Ref: https://en.wikipedia.org/wiki/High_Energy_Stereoscopic_System
https://www.tevcat.org/, https://www.tevcat.org/reviews.html

Acceleration of petaelectronvolt protons in the Galactic Centre​

https://arxiv.org/abs/1603.07730

While there is mention of Sagittarius A*, there are also observations from the Cygnus X-complex concerning PeV photons.

Meanwhile, in terrestrial experiments with collding accelerators:

Scientists recreate 'cosmic fireballs' in CERN particle accelerator to hunt for missing gamma-rays
https://www.space.com/astronomy/sci...le-accelerator-to-hunt-for-missing-gamma-rays

"By reproducing relativistic plasma conditions in the lab, we can measure processes that shape the evolution of cosmic jets and better understand the origin of magnetic fields in intergalactic space."

. . .

Scientists from the University of Oxford and the Science and Technology Facilities Council’s (STFC) Central Laser Facility (CLF) teamed up and turned to the Super Proton Synchrotron based at CERN’s HiRadMat (High-Radiation to Materials) facility to generate electron–positron pairs. They then blasted these matter-antimatter counterpart pairs through 3.3 feet (1 meter) of plasma, recreating conditions in the jets of feeding supermassive black holes known as blazars. This enabled them to simulate some of the universe's most extreme physics.

"These experiments demonstrate how laboratory astrophysics can test theories of the high-energy universe," Bob Bingham, team member and researcher at the University of Strathclyde, said in a statement. "By reproducing relativistic plasma conditions in the lab, we can measure processes that shape the evolution of cosmic jets and better understand the origin of magnetic fields in intergalactic space."

I don't find the space.com article informative in the sense of the details of how CERN generated to electron–positron pairs to what energies. The article is fairly basic for public consumption.

A corresonding article by Phys.org is better
https://phys.org/news/2025-11-scientists-recreate-cosmic-fireballs-probe.html

As these TeV gamma rays propagate across intergalactic space, they scatter off the dim background light from stars, creating cascades of electron–positron pairs. The pairs should then scatter on the cosmic microwave background to generate lower-energy gamma rays—yet these have not been captured by gamma-ray space telescopes, such as the Fermi satellite. Up to now, the reason for this has been a mystery.

The implication here is that high energy photons interact with lower energy photons creating electron-positron pairs, which brings us into the realm of photon-photon (γ-γ) physics. Normally, one is concerned with γ interactions with matter, i.e., electrons/positrons, protons, nuclei, etc.


One has to go to with sites of the institutions, or published article from those institutions.

Astronomers measure electrons from space at record energies (>10 TeV)​

https://www.mpg.de/23743719/highest-electron-energies-measured-from-space
From the figure it looks like energies up to 40 to 50 TeV.

Compare to "LEP collided electrons with positrons at energies that reached 209 GeV." at CERN.
https://en.wikipedia.org/wiki/Large_Electron–Positron_Collider

From PNAS - https://www.pnas.org/doi/10.1073/pnas.2513365122
ArXiV - https://arxiv.org/abs/2509.09040

The generation of dense electron-positron pair beams in the laboratory can enable direct tests of theoretical models of

-ray bursts and active galactic nuclei. We have successfully achieved this using ultra-relativistic protons accelerated by the Super Proton Synchrotron at CERN. In the first application of this experimental platform, the stability of the pair beam is studied as it propagates through a metre-length plasma, analogous to TeV

-ray induced pair cascades in the intergalactic medium. It has been argued that pair beam instabilities disrupt the cascade, thus accounting for the observed lack of reprocessed GeV emission from TeV blazars. If true this would remove the need for a moderate strength intergalactic magnetic field to explain the observations. We find that the pair beam instability is suppressed if the beam is not perfectly collimated or monochromatic, hence the lower limit to the intergalactic magnetic field inferred from

-ray observations of blazars is robust.

I'll tackle PeV photons in a subsequent post. One can search on the term PeVatron for articles on PeV photons.