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Preprint predicts the Penrose process turns neutrons near Sgr A* into PeV protons
Marina Cermeño's team estimates that neutrons decaying near Sgr A* could produce petaelectronvolt protons using energy from the black hole's spin. The gamma-ray and neutrino signals it predicts are too faint for today's detectors and will need upgraded HAWC and IceCube instruments.
The Scientist · Science desk
Drafted by a language model from the sources cited here and checked against its claim ledger before publication. How we use AISend a correction
What happened
- In the model, a neutron in Sgr A*'s ergosphere decays into a proton, an electron and a neutrino, and the two charged particles feel the magnetic field there.
- Protons leaving at nearly the speed of light would hit gas around the black hole and produce intense gamma rays with a specific spectral signature.
- Because each decay also emits a neutrino, the process should produce high-energy neutrinos too, giving a combined signal of light and neutrinos.
- The study is a 2026 arXiv preprint titled 'Sgr A* as a Galactic PeVatron: Multimessenger Signatures of the Magnetic Penrose Process'.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- decision One decay yields both messengers, so a gamma-ray excess at the galactic centre without matching neutrinos would weigh against this model, and the two kinds of result have to be read together.
- capability A matched detection would give astronomers a way to measure energy being drawn from a black hole's rotation at Sgr A*, a process so far known only from theory.
- constraint The PeV estimate rests on assumed neutron trajectories into the ergosphere, so revising those assumptions would change the predicted proton energies and the signal built on them.
Most of the energy supermassive black holes produce comes from the matter around them, as superheated plasma in the accretion disk interacts with strong magnetic fields [12]. The Penrose process takes energy from the rotating black hole itself and lowers its total mass [1]. Roger Penrose proposed it in 1969, 57 years before this study appeared [1][13]. It depends on the ergosphere, a region close to a spinning hole where space is dragged around too strongly for anything to resist [2]. If mass is discarded there, the hole absorbs negative energy and loses mass, and whatever escapes leaves with more energy than it brought in [11].
The neutron is a clever choice of particle for this. It has no net charge, yet its decay takes the place of the discarded mass in Penrose's thought experiment [5]. The electron is shed and the proton gets the kick [5]. The magnetic step matters. According to phys.org's account, the simple version of the process is not particularly efficient, while variations that bring in magnetic fields can be quite efficient [3].
A petaelectronvolt is about a thousand times the energy of the most energetic protons in the Large Hadron Collider, according to the same account [7]. That is an energy per proton. The thing this doesn't tell you is how many neutrons reach the ergosphere and decay there. The account does not report a predicted flux, explain where the neutrons come from, or compare the model with measurements already made toward the galactic centre. On that evidence, the "Galactic PeVatron" in the paper's title is something the model predicts Sgr A\* can do [4].
The phys.org write-up closes on the hope that "in the near future we might discover evidence of the Penrose process and proof that black holes can lose mass after all" [15]. I would not use the word proof until both predicted spectra have been measured and agree with the model.
What to watch
- Whether the Cermeno preprint comes through peer review with a predicted gamma-ray and neutrino flux that detector teams can set against their sensitivity.
- Galactic-centre spectra from an upgraded HAWC, compared with the spectral signature the model predicts for protons striking gas.
- Neutrino arrivals from the direction of Sgr A* in the new IceCube, and whether their energies line up with any gamma-ray signal.