New discovery in'universe,
intercepted a photon that should never have come to earth. It is a very high energy photon associated with the gamma-ray burst most powerful recorded so far and has undermined the current model that describes these very violent celestial events. Inaf and Infn have announced that an all-Italian group – made up of researchers from the National Institute of Astrophysics and the National Institute of Nuclear Physics – has tried to shed light on this photon by proposing an interpretation that contemplates the presence of a oscillation between photons and ALP, hypothetical particles predicted by string theory and “optimal candidate” particles to constitute cold dark matter. The article Observability of the very-high-energy emission from GRB 221009A by Giorgio Galanti, Lara Nava, Marco Roncadelli, Fabrizio Tavecchio, Giacomo Bonnoli was published today in the journal Physical Review Letters.
It is a single photon but so energetic that it undermines current astrophysical models on the propagation of gamma rays. The event in which it was observed, called BOAT, or the brightest of all time, is the gamma ray burst GRB 221009A, emitted by a galaxy over two billion light years from us and detected – from the ground and in space – 9 October 2022. Scientists explain that Among the very high energy gamma photons intercepted by the Chinese Lhaaso detector on the occasion of this event, there was, in fact, one of even 18 TeV: the highest energy ever recorded by a GRB. An interesting interpretation of this unexpected observation is provided by an entirely Italian study, coordinated by Inaf-National Institute of Astrophysics together with Infn-National Institute of Nuclear Physics, with authors Giorgio Galanti, Lara Nava, Marco Roncadelli, Fabrizio Tavecchio and Giacomo Bonnoli , published today, December 18, in Physical Review Letters.
“The Gamma Ray Burst that the Chinese Lhaaso collaboration observed is the brightest ever discovered, and this is already an event for science, but then the photon is also the most energetic ever observed from a Gamma Ray Burst and the photon changes the cards on the table because it should never have been observed according to the laws of physics known so far, according to standard physics” Giorgio Galanti of INAF, first author of the article, explains to Adnkronos.
“A few minutes after receiving news of the explosion – Galanti also recalls – we understood that this GRB could not only be an extraordinary astrophysical event but could also represent a unique opportunity for fundamental physics studies, in particular regarding axion-like particles , which are excellent candidate particles to constitute cold dark matter.” According to the hypothesis put forward by the research group, that very energetic photon could be a 'transformative photon', capable of changing its nature, oscillating from one 'personality' to another while traveling at the speed of light. And the ALPs – the axion-like particles, hypothetical particles predicted by string theory, are similar to other equally hypothetical particles, the axions, and the ALPs would be one of these 'personalities' assumed by the 'transforming photon' which “is sometimes photon and sometimes it is Alp” explains the astrophysicist Galanti.
In short, a bit like Mr. Hyde, an ALP is in fact capable of carrying out actions that a photon, the Dr. Jekyll of this strange story, would never be able to complete: crossing the so-called EBL – the extragalactic background light – unscathed , the extragalactic background light, i.e. the light emitted by all stars during the entire evolution of the universe.
The scientists further explain that when a high-energy photon — say above 100 GeV — collides with an EBL photon, there is a probability that an electron-positron pair will form, causing the high-energy photon to disappear. And this effect becomes progressively more important as both energy and distance increase. Returning, therefore, to GRB 221009A, according to conventional physics, photons of energy higher than about 10 TeV would be completely absorbed. Considering the redshift of the source, and therefore the enormous distance traveled by the gamma ray burst, photons at higher energies would theoretically never have been able to reach us.
How is it then possible that Lhaaso, the only instrument for the detection of gamma-ray bursts that did not go into saturation on 9 October a year ago, observed photons from GRB 221009A at energies between 500 GeV and 18 TeV? This is precisely where ALPs come into play. “According to our hypothesis, in the presence of magnetic fields, photons transform into ALP and vice versa – adds Marco Roncadelli, researcher associated with the Infn and the Inaf – thus making it possible for a greater number of photons to reach the Earth, because the ALP are invisible to photons from the extragalactic background.”
Going into a little more detail, ALPs couple to two photons, but not to a single photon. This fact implies that in the presence of an external magnetic field – which, as is well known, is made up of photons – 'photon-ALP oscillations' can occur. These are very similar to the oscillations of massive neutrinos of different types, with the only difference that for ALPs the existence of the magnetic field is essential in order to guarantee the conservation of the angular momentum, as the photon has spin 1 while ALPs have spin 0: the missing or excess spin is compensated by the external magnetic field.
The oscillation between photons and ALP to circumvent the opacity of the extragalactic background to high energy photons is not a new idea: it is a solution proposed for the first time in 2007 by Alessandro De Angelis, Oriana Mansutti and Marco Roncadelli. And it's a solution to a more general problem than the one posed by this gamma-ray burst. In addition to gamma ray bursts, there are in fact other distant sources that emit photons at very high energies yet capable of reaching us, in defiance of standard physics.
Scientists underline that sources such as FSRQ (flat spectrum radio quasar) type quasars, where the 'opaque' component that hinders the travel of high energy photons, to the point of making their escape theoretically impossible, is not the ELB but something very similar: a field of ultraviolet radiation within the source itself. Or the BL Lac type blazars, whose spectrum – as shown by a study published in 2020 by Galanti, Roncadelli and De Angelis together with Giovanni F. Bignami – would in some cases be inexplicable without resorting to a mechanism that allows to increase the 'cosmic transparency', thus reducing the absorption produced by the EBL.
Photons from FSRQ quasars, photons from BL Lac blazars and now photons from this BOAT gamma-ray burst, then. All three apparently inconceivable within the scope of standard physics. But all three could be explained if instead of 'simple' photons there were “Jekyll-Hyde” particles that oscillate from photon to ALP and vice versa. Finally, the researchers indicate that “to give solidity to this hypothesis, other observations will be needed, and for this reason the new high energy astrophysical observatories – first of all CTA and the Italian ASTRI – will be of great help, ready to come into operation in the coming years “. (by Andreana d'Aquino)
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