Gamma-ray burst measurements might expose a flaw in high-energy radiation theory

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Artist impression of a relativistic jet of a gamma-ray burst, breaking out of a collapsing star and emitting very-high-energy photons. (DESY, Science Communication Lab)

In 2019, astronomers detected one of the most energetically extreme cosmological events ever observed: a faraway exploding star that produced jets of radiation in a gamma-ray burst. A new analysis of the event has raised questions about prevailing theories on gamma-ray bursts and other violent occurrences in the universe.

An international collaboration of 240 scientists found that in the hours following the initial detection of the burst, very-high-energy gamma rays were observed for much longer than expected, according to a study being published Friday in Science.

According to the paper, the observations cast doubt on current emission models of gamma-ray burst "afterglows," and the authors suggest potential changes to assumptions that they say are inconsistent with the findings.

"It puts in jeopardy some of the well-established theory, basically, based on new high-resolution observations that we didn't have before," said Fabian Schüssler, a lead author of the new paper and an astrophysics researcher at the Institute of Research into the Fundamental Laws of the Universe, which is part of a French government research organization.

Gamma-ray bursts are very rare, extremely energetic explosions that most commonly occur during supernovas, when a massive star expels enormous quantities of mass and energy before collapsing into a black hole or neutron star. During a gamma-ray burst, two jets of particles traveling near the speed of light are emitted in opposite directions, and the particles then generate gamma rays, X-rays and other electromagnetic radiation.

A 40-second gamma-ray burst detected in 2017 released about as much energy as the sun will in its entire lifetime.

At the focus of the new study is GRB 190829A, a gamma-ray burst first detected Aug. 29, 2019. It was unusually close to Earth for a gamma-ray burst; while they occur 20 billion light-years away on average, according to Schüssler, this one was "only" 1 billion light-years away, or about 1% of the observable universe's diameter.

This proximity helped make the burst and its subsequent afterglow — a weaker, slowly fading wave of radiation — observable over three nights, much longer than many previous bursts. Schüssler and his colleagues measured the event's afterglow using the High Energy Stereoscopic System, a system of five gamma-ray telescopes in Namibia.

It was also only the third known burst with an afterglow that released photons with energy levels in the teraelectronvolt domain, which are at least 1 billion times more energetic than photons of visible light. These very-high-energy gamma-ray photons allowed the researchers to test scientific models of how the jets of radiation in gamma-ray bursts are created.

Specifically, the existing model provided two ways for high-energy photons to be generated.

The first is when electrons are accelerated in a magnetic field around the dying star and create photons in a process known as synchrotron emission, but it is thought to have an upper limit and be unable to produce very-high-energy gamma rays like the teraelectronvolt photons observed in the 2019 gamma-ray burst.

Instead, these photons would emerge from self-synchrotron Compton emission, when X-ray photons generated from synchrotron emission are given even more energy by the same pool of electrons that initially created them.

But the authors of the new study argue that this model does not align with their own findings. They could observe teraelectronvolt photons from GRB 190829A over all three nights, a decay rate that is slower than expected by the model. The photons' decay and spectral properties were also very similar to that of the X-ray photons observed in the same burst, suggesting that both kinds of photons were created in the same process, rather than two different ones.

"We had quite some issues trying to make sense of our observations in that standard theory," Schüssler said. "In the end, we didn't succeed and had to give up on parts of the standard theory."

Instead, they dropped the model's upper limit for synchrotron emission and found that their updated theory matched the observations. According to the authors, this is a sign that the emissions model for gamma-ray bursts may need fixing, which Schüssler hopes will be addressed by theorists in future research.

The researchers could not determine a definite fix themselves but speculated on one possibility: Perhaps the electrons in the gamma-ray burst passed through two distinct magnetic fields rather than one, getting accelerated in one field and releasing photons in another. This would bypass the upper limit and allow the very-high-energy gamma rays to be emitted through synchrotron emission, the team said.

Such a change in scientists' understanding of synchrotron emissions would impact how they interpret other cosmological events to which the model is applied, according to Schüssler. They are currently employed to describe other instances when energetic jets are emitted, such as when a black hole is consuming matter from other stars, either with stellar black holes or supermassive black holes at a galaxy's center.

The first two bursts that contained teraelectronvolt photons did not conflict with the previous model, but Schüssler said the new study had unprecedentedly high resolution and observation length.

"We can draw stronger conclusions than we could before and highlight potential problems in the standard theory," the astrophysicist said.

Looking toward the future, Schüssler said the observations of GRB 190829A will inform observations of gamma-ray bursts and possibly encourage longer measurements of the extreme events. He is also preparing for the 2022 opening of the Cherenkov Telescope Array, which will include gamma-ray telescopes from around the world and enable much more sensitive measurements.

The study, "Revealing X-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow," published June 4 in Science, was authored by H. Abdalla, I.D. Davids, E. Kasai, J.N.S. Shapopi, K. Shiningayamwe, R. Steenkamp and C. van Rensburg, University of Namibia; F. Aharonian, Dublin Institute for Advanced Studies, Max-Planck-Institut für Kernphysik and Russian-Armenian University; F. Ait Benkhali, K. Bernlöhr, M. Breuhaus, C. Deil, A. Donath, J. Hahn, G. Hermann, J.A. Hinton, W. Hofmann, A. Jardin-Blicq, V. Marandon, A. Mitchell, L. Olivera-Nieto, M. Panter, G. Peron, Q. Remy, F. Rieger, C. Romoli, E. Ruiz-Velasco, S. Sailer, S. Steinmassl, M. Tsirou, R. Tuffs, H.J. Völk, F. Werner, R. White, R. Zanin and J. Zorn, Max-Planck-Institut für Kernphysik; E.O. Angüner and J.-P. Ernenwein, Centre de Physique des Particules de Marseille; C. Arcaro, M. Böttcher, T. Chand, S. Chandra, L. Dreyer, M. Kreter, H. Ndiyavala, H.M. Schutte, A.S. Seyffert, J. H.E. Thiersen, D.J. van der Walt, C. Venter, Z. Wadiasingh and N. ̇Zywucka, North-West University; C. Armand, M. Barnard, M. de Bony de Lavergne, S. Caroff, A. Carosi, A. Fiasson, G. Lamanna, G. Maurin, Q. Piel, V. Poireau and D.A. Sanchez, Laboratoire d'Annecy de Physique des Particules; T. Armstrong, G. Cotter, J. Davies, M. Hörbe, P. Morris, S. Spencer and  J. Watson, University of Oxford; H. Ashkar, F. Brun, P. Brun, J.F. Glicenstein, K. Kosack, A. Montanari, E. Moulin, B. Peyau, P. Reichherzer, L. Rinchiuso, F. Schüssler, M. Seglar-Arroyo and T. Tavernier, Institut de Recherche sur les Lois Fondamentales de l'Univers; M. Backes, University of Namibia and North-West University; V. Baghmanyan, A. Nayerhoda and J. Niemiec, Instytut Fizyki Jądrowej Polskiej Akademii Nauk; V. Barbosa Martins, D. Berge, J. Damascene Mbarubucyeye, M. F ̈ußling, G. Giavitto, M. Haupt, T. L. Holch, S. Klepser, R. Konno, D. Kostunin, I. Lypova, J. Majumdar, T. Murach, S. Ohm, E. de Ona Wilhelmi, H. Prokoph, A.M. Taylor and S.J. Zhu, Deutsches Elektronen-Synchrotron; A. Barnacka, M. Jamrozy, M. Ostrowski, A. Priyana Noel and Ł. Stawarz, Uniwersytet Jagiellónski; Y. Becherini, T. Bylund and M. Senniappan, Linnaeus University; B. Bi, V. Doroshenko, F. Leuschner, D. Malyshev, G. Pühlhofer, H. Salzmann and A. Santangelo and M. Scalici, Universität Tübingen; E. Bissaldi, Politecnico di Bari and Istituto Nazionale di Fisica Nucleare, Sezione di Bari; C. Boisson, A. Dmytriiev, G. Fichet de Clairfontaine, H. Sol and A. Zech, Laboratoire Univers et Théories; J. Bolmont, G. Emery, J.-P. Lenain, C. Levy and P. Vincent, Laboratoire de Physique Nucléaire et de Hautes Energies; M. Bryan, D.A. Prokhorov, R. Simoni, L. Sun and J. Vink, University of Amsterdam; M. Büchele, F. Eichhorn, S. Funk, D. Glawion, D. Jankowsky, V. Joshi, I. Jung-Richardt, U. Katz, D. Malyshev, M. Meyer, L. Mohrmann, K. Nakashima, S. Raab, M. Sasaki, J. Schäfer, A. Specovius, D. Tiziani, L. Tomankova, C. van Eldik, J. Veh, Yu Wun Wong and A. Yusafzai, Friedrich-Alexander-Universität Erlangen-Nürnberg; T. Bulik and M. Curyło, The University of Warsaw, S. Casanova, Max-Planck-Institut für Kernphysik and Instytut Fizyki Jądrowej Polskiej Akademii Nauk; A. Chen, Nu. Komin, P. Marchegiani and N. Shafi, University of the Witwatersrand; J. Devin, A. Djannati-Ataï, S. Gabici, B. Khelifi, A. Lemière, S. Pita, A. Sinha, M. Spir-Jacob, R. Terrier and S. Zouari, Astroparticule et Cosmologie; L. Dirson, D. Horns, M. A. Kastendieck and M. Tluczykont, Universität Hamburg; C. Duffy, C. Moore, P. O'Brien, P. Evans and K. Page, The University of Leicester; J. Dyks, W. Kluźniak, R. Moderski, B. Rudak and A.A. Zdziarski, Polish Academy of Sciences; K. Egberts, C. Hoischen and C. Steppa, Universität Potsdam; S. Einecke, K. Feijen and G. Rowell, University of Adelaide; S. Fegan, G. Fontaine, J. Muller and M. de Naurois, Laboratoire Leprince-Ringuet; Y.A. Gallant, A. Marcowith, M. Renaud and G. Vasileiadis, Laboratoire Univers et Particules de Montpellier; L. Giunti, Institut de Recherche sur les Lois Fondamentales de l'Univers and Astroparticule et Cosmologie; M.-H. Grondin, M. Lemoine-Goumard and A. Mares, Centre d'Études Nucléaires de Bordeaux Gradignan; M. Holler, D. Huber, G. Martí-Devesa, S. Panny, R. Rauth, A. Reimer and O. Reimer, Leopold-Franzens-Universität Innsbruck; F. Jankowsky, A. Quirrenbach and S.J. Wagner, Universität Heidelberg; K. Katarzyński, Nicolaus Copernicus University; D. Khangulyan and Y. Uchiyama, Rikkyo University; T. Lohse, L. Oakes, R.D. Parsons and U. Schwanke, Humboldt-Universität zu Berlin; J. Mackey, Dublin Institute for Advanced Studies; R. Marx, Universität Heidelberg and Max-Planck-Institut für Kernphysik; P.J. Meintjes and B. van Soelen, University of the Free State; H. Odaka, The University of Tokyo; M. Punch, Linnaeus University and Astroparticule et Cosmologie; V. Sahakian, Yerevan Physics Institute; C. Stegmann, Universität Potsdam and Deutsches Elektronen-Synchrotron; T. Takahashi, World Premier International Research Center Initiative; T. Tam, Sun Yat Sen University; A. Wierzcholska, Instytut Fizyki Jądrowej Polskiej Akademii Nauk and Universität Heidelberg; M. Zacharias, North-West University and Laboratoire Univers et Théories; and D. Zargaryan, Dublin Institute for Advanced Studies and Russian-Armenian University.

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