Hint at an axion-like particle from GRB 221009A (2412.21175v1)

Abstract: The detection by the LHAASO Collaboration of the gamma-ray burst GRB 221009A at redshift $z = 0.151$ with energies up to $(13-18) \, \rm TeV$ challenges conventional physics. Photons emitted with energies above $10 \, \rm TeV$ at this redshift can hardly be observed on Earth due to their interaction with the extragalactic background light (EBL). We show that indeed the LHAASO Collaboration should not have observed photons with energies above $10 \, \rm TeV$ if the state-of-the-art EBL model by Saldana-Lopez et al. is taken into account. A problem therefore arises: the Universe should be more transparent than currently believed. We also show that the issue is solved if we introduce the interaction of photons with axion-like particles (ALPs). ALPs are predicted by String Theory, are among the best candidates for dark matter and can produce spectral and polarization effects on astrophysical sources in the presence of external magnetic fields. In particular, for GRB 221009A, photon-ALP oscillations occur within the crossed magnetized media, i.e. the host galaxy, the extragalactic space, the Milky Way, partially reducing the EBL absorption to a level that explains the LHAASO detection of GRB 221009A and its observed spectrum without the need of contrived choices of parameter values, which are instead compulsory within proposed emission models within conventional physics. This fact regarding GRB 221009A represents a strong hint at the ALP existence, which adds to two other indications coming from blazars, a class of active galactic nuclei.

• The paper demonstrates that photon-ALP conversions can explain the unexpected detection of TeV photons from GRB 221009A despite heavy EBL absorption forecasts.
• It employs a comprehensive EBL model and simulates photon-ALP oscillations across multiple magnetic fields to reconcile observational data with theoretical predictions.
• The results imply that axion-like particles could be viable dark matter candidates, prompting a reevaluation of high-energy photon propagation in cosmological environments.

Analysis of the Detection of GRB 221009A and Implications for Axion-like Particles

The paper "Hint at an axion-like particle from GRB 221009A" explores the unusual detection of high-energy photons from gamma-ray burst GRB 221009A. Detected by the LHAASO Collaboration, this gamma-ray burst exhibited photon energies up to approximately 13-18 TeV at a redshift $z = 0.151$. This observation is particularly intriguing because standard physics, via interactions with the extragalactic background light (EBL), predicts heavy absorption of photons above 10 TeV, contrasting with empirical observations.

Theoretical Background and Methodology

The authors employ the EBL model by Saldana-Lopez et al., widely regarded for its use of comprehensive galaxy data and minimization of foreground interference, to model photon propagation. According to this model, the survival probability of photons with energies exceeding 10 TeV would be markedly lower than observed.

To reconcile this discrepancy, the paper introduces the interaction of photons with axion-like particles (ALPs), anticipated by several theoretical constructs, including string theory. ALPs are posited as candidates for dark matter and engage with photons in the presence of magnetic fields, facilitating photon-ALP oscillations that effectively extend the mean free path of high-energy photons.

In examining the GRB event, the authors postulate that the conversion of photons to ALPs occurs throughout several magnetic environments: the galaxy hosting the burst, extragalactic space, and the Milky Way. They analyze the photon-ALP beam transfer through these magnetic regions, suggesting that ALPs would evade the EBL absorption, accounting for the observed photon energies at Earth.

Results

The central claim is that the ALP model, with specific parameters ($m_a$ and $g_{a\gamma\gamma}$), successfully explains the detection of GRB 221009A at TeV energies while adhering to existing constraints from ALP bounds. The results demonstrate a significantly higher survival probability of photons when ALP interactions are included, compared to conventional physics paradigms.

Implications and Future Perspectives

This paper posits a robust case for ALP interactions in explaining anomalies in high-energy astrophysical observations. The implications extend to potential confirmation of ALPs as constituents of dark matter, where their interaction with photons manifests in detectable spectral phenomena. Furthermore, the research suggests a need for re-examining EBL constraints and models when considering high-energy photon propogation across cosmological distances.

While alternative theories like the Lorentz invariance violation were considered, they failed to account feasibly for the observational data, further solidifying the proposed significance of photon-ALP oscillations.

Future investigations could focus on cross-verifying these results with additional high-energy astrophysical phenomena, refining models for ALP interactions in various cosmic environments, and synchronizing with observational data from other gamma-ray bursts and astrophysical sources.

This work is a compelling contribution to the ongoing exploration into the nature of dark matter and high-energy astrophysical processes, with axion-like particles offering a promising avenue for resolving longstanding theoretical challenges in particle astrophysics.