Recently, Zijinshan Observatory of the Chinese Academy of Sciences discovered a 400 GeV photon from the GRB 221009A direction of the gamma ray burst using the observation data of Fermi Satellite Large Area Gamma Ray Telescope (Fermi LAT), and realized the first inference of the intensity of the intergalactic magnetic field using this observation. The relevant research results were published in Nature Communications under the title A delayed 400 GeV photo from GRB 221009A and implication on the interstitial magnetic field.
The intergalactic magnetic field has important scientific value in exploring the origin and evolution of cosmic magnetic fields, the formation of large-scale structures in the early universe, the formation and evolution of galaxies, and the origin and propagation of extragalactic cosmic rays. The strength of the intergalactic magnetic field is one of the key parameters in modern astronomy, but due to its extremely weak nature, it has not yet been reliably measured.
The team analyzed long-term observation data of Fermi LAT in the direction of GRB 221009A and discovered a photon with an energy of approximately 400 GeV approximately 0.4 days after the outbreak. This photon is the highest energy photon detected by Fermi LAT from a gamma storm to date. Its position is correlated with the observation and positioning altitude of GRB 221009A by the Swift Satellite UV Optical Telescope. The probability of it coming from GRB 221009A is 99.99937%, corresponding to a significance level of 4.4 σ. It is worth noting that no accompanying low-energy radiation has been detected at this moment. The energy spectrum of this characteristic is difficult to explain by the synchronous Compton high-energy afterglow model, but it can naturally be generated by secondary electromagnetic cascades triggered by primary photons with energy of~10 TeV.
The initial TeV photons emitted by gamma bursts will be absorbed by the infrared background light during propagation, generating extremely relativistic positive and negative electron pairs. They can generate secondary GeV photons by scattering cosmic microwave background radiation photons. Due to the influence of the intergalactic magnetic field on the propagation of positive and negative electron pairs, they are deflected, resulting in a delay in the arrival time of the secondary GeV photons observed. Assuming the coherence length of the intergalactic magnetic field is 1Mpc, research has found that the strength of the intergalactic magnetic field is~4 × 10-17 G, which can naturally explain the generation of~400 GeV photons and their delayed arrival time. Furthermore, combining the TeV observations of LHAASO, the study confirmed through Monte Carlo simulation that the origin explanation of the cascade radiation model is superior to the synchronous self Compton afterglow model, and excluded the parameter space of intergalactic magnetic field intensity below 1 × 10-18 G.
The research work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences and Jiangsu Province.
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