[Paper Review] Interpretation of multi-TeV photons from GRB221009A

Ali Baktash, D. Horns+1 more|arXiv (Cornell University)|Oct 13, 2022

Dark Matter and Cosmic Phenomena19 citations

TL;DR

This paper investigates the origin of a 18 TeV photon detected from GRB221009A, which appears inconsistent with standard gamma-ray propagation due to extreme optical depth from extra-galactic background light (EBL) absorption. It proposes that photon-ALP mixing or Lorentz invariance violation (LIV) could suppress pair production, enabling the photon’s survival, though required coupling strengths or energy scales conflict with existing astrophysical bounds unless mixing occurs near the source or LIV scales are very low.

ABSTRACT

The nearby GRB221009A at redshift $z=0.1505$ has been observed up to a maximum energy of 18 TeV with the LHAASO air shower array. The expected optical depth for a photon with energy $E_γ=18$ TeV varies between 9.4 and 27.1 according to existing models of the extra-galactic background light (EBL) in the relevant mid infra-red range. The resulting suppression of the flux in several (but not all) EBL models makes it for these EBL models unlikely that this photon could have been observed at the claimed energy. If the photon event and its energy are confirmed and possibly even more photons above 10 TeV have been observed, the photon-pair production process would have to be suppressed by mechanisms predicted in extensions of the Standard Model of particle physics. We consider the possibilities of photon mixing with a light pseudo-scalar (e.g., axion-like particles; ALPs) in the magnetic field of the host galaxy and the Milky Way and Lorentz invariance violation (LIV). In the case of photon-ALP mixing, the boost factor would reach values $\sim10^6$ for photon couplings not ruled out by the CAST experiment, but limited by other astrophysical constraints. Viable scenarios would require either very efficient mixing in or near to the GRB or that the largest part of the total luminosity is radiated at TeV energies, different from previous GRB afterglows. In the case of LIV, required boost factors are achievable for a LIV breaking energy scale $\lesssim 2 imes 10^{29}$~eV ($\lesssim 4 imes 10^{21}$~eV) for the linear (quadratic) modification of the dispersion relation. A more simple explanation would be a misidentification of a charged cosmic-ray air shower.

Motivation & Objective

• To explain the detection of a 18 TeV photon from GRB221009A, which exhibits extreme optical depth due to EBL absorption in standard models.
• To assess whether photon-ALP mixing or Lorentz invariance violation (LIV) could suppress pair production and allow such high-energy photons to reach Earth.
• To evaluate whether the observed photon energy and flux are consistent with astrophysical constraints from other observations.
• To test if the high-energy signal could instead be a misidentified charged cosmic-ray air shower, offering a simpler explanation.

Proposed method

• The authors compute the EBL-induced optical depth for 18 TeV photons using multiple EBL models, finding values between 9.4 and 27.1.
• They model photon-ALP mixing in galactic and intergalactic magnetic fields, deriving required coupling strengths $ g_{a\gamma} $ to achieve a boost factor of ~10^6.
• They analyze Lorentz invariance violation (LIV) via modified dispersion relations, calculating required energy scales $ M_1 $ and $ M_2 $ for linear and quadratic LIV terms.
• They compare the expected VHE afterglow flux from GRB221009A to LHAASO observations using the Swift-XRT light curve as a proxy, varying the normalization factor $ \zeta $.
• They evaluate consistency of predicted $ g_{a\gamma} $ and $ M $ values with existing astrophysical bounds from CAST and other sources.
• They consider the possibility that the 18 TeV event is a misidentified cosmic-ray air shower, assessing background expectations.

Experimental results

Research questions

• RQ1 Can the observed 18 TeV photon from GRB221009A be explained by standard EBL absorption models, or is the optical depth too high for such a detection?
• RQ2 What photon-ALP coupling strength $ g_{a\gamma} $ is required to suppress pair production and allow the 18 TeV photon to survive, and is this consistent with existing astrophysical constraints?
• RQ3 What energy scale $ M $ for Lorentz invariance violation (LIV) is needed to reduce gamma-ray opacity, and is this compatible with observational bounds?
• RQ4 Does the temporal behavior of the VHE afterglow, modeled via the X-ray afterglow with normalization $ \zeta $, reproduce the LHAASO photon count above 500 GeV?
• RQ5 Could the 18 TeV event instead be a misidentified charged cosmic-ray air shower, given the background rate?

Key findings

• The optical depth for a 18 TeV photon from GRB221009A ranges from 9.4 to 27.1 across EBL models, implying a detection probability as low as $ 7 \times 10^{-9} $, making the observation highly improbable under standard physics.
• Photon-ALP mixing with $ g_{a\gamma} \gtrsim 6 \times 10^{-12}~\mathrm{GeV}^{-1} $ is required to achieve a boost factor of ~10^6, but this value is in tension with bounds from CAST and other astrophysical observations.
• For ALP mixing to be viable, either very efficient mixing near the source or a dominant TeV luminosity component is required, differing from typical GRB afterglows.
• Lorentz invariance violation with a linear dispersion relation requires $ M_1 \lesssim 2 \times 10^{29}~\mathrm{eV} $, which is consistent with some bounds, while quadratic LIV requires $ M_2 \lesssim 4 \times 10^{21}~\mathrm{eV} $, which is less viable.
• The probability of detecting a misidentified charged cosmic-ray air shower at 18 TeV is consistent with the observed event, offering a simpler, conventional explanation.
• The observation of such a high-energy photon could signal anomalous transparency if new physics like ALP mixing or LIV is at play, potentially leading to new discoveries.