[Paper Review] Parameters of axion-like particles required to explain high-energy photons from GRB 221009A

S. Troitsky|arXiv (Cornell University)|Oct 17, 2022

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper proposes that axion-like particles (ALPs) mixing with photons in the Milky Way's magnetic field could explain the detection of ultra-high-energy photons (up to 251 TeV) from GRB 221009A, which are otherwise impossible due to pair production with cosmic background radiation. The ALP-photon conversion is favored to occur in the Galaxy rather than intergalactic space, requiring a specific parameter space of ALP mass and coupling to avoid excessive attenuation.

ABSTRACT

Recent astrophysical transient Swift J1913.1+1946 is possibly associated with the gamma-ray burst GRB 221009A at the redshift z=0.151. The transient was accompanied by very high-energy gamma rays up to 18 TeV observed by LHAASO and a photon-like air shower of 251 TeV detected by Carpet-2. These energetic gamma rays cannot reach us from the claimed distance of the source because of the pair production on cosmic background radiation. If the identification and redshift measurements are correct, one would require new physics to explain the data. One possibility invokes axion-like particles (ALPs) which mix with photons but do not attenuate on the background radiation. Here we explore the ALP parameter space and find that the ALP-photon mixing in the Milky Way, and not in the intergalactic space, may help to explain the observations. However, given the low Galactic latitude of the event, misidentification with a Galactic transient remains an undiscarded explanation.

Motivation & Objective

• To explain the detection of ultra-high-energy photons (up to 251 TeV) from GRB 221009A, which cannot propagate unattenuated over cosmological distances due to pair production with cosmic background radiation.
• To investigate whether axion-like particles (ALPs) with photon mixing can provide a viable mechanism for these photons to survive to Earth.
• To determine whether ALP conversion occurs primarily in the intergalactic medium or within the Milky Way's magnetic field.
• To constrain the allowed parameter space of ALP mass and photon coupling that would permit such a scenario.
• To assess the possibility that the high-energy photons originate from a Galactic transient rather than the distant GRB, given the low Galactic latitude of the source.

Proposed method

• Analyzes photon propagation from GRB 221009A at redshift z ≈ 0.151, considering attenuation via e⁺e⁻ pair production on cosmic microwave and infrared background radiation.
• Applies the ALP-photon mixing formalism in external magnetic fields, using the Lagrangian interaction term −(g/4)aFμνF̃μν to model conversion between photons and ALPs.
• Evaluates two scenarios: ALP-photon oscillations in the intergalactic medium (where conversion is inefficient due to weak fields) and in strong local fields (e.g., in the Milky Way or host galaxy).
• Computes the survival probability of high-energy photons under ALP mixing, favoring conversion in the Milky Way to avoid complete attenuation.
• Uses observational constraints from LHAASO (18 TeV photons) and Carpet-2 (251 TeV air shower) to constrain the ALP parameter space.
• Considers alternative explanations, including misidentification with a Galactic transient due to the low Galactic latitude (b ≈ 4°) of the event.

Experimental results

Research questions

• RQ1 Can axion-like particle (ALP)-photon mixing in magnetic fields explain the detection of 251 TeV photons from GRB 221009A?
• RQ2 Is the ALP-photon conversion more likely to occur in the intergalactic medium or within the Milky Way's magnetic field?
• RQ3 What are the allowed ranges of ALP mass and photon coupling that would permit the observed high-energy photons to survive propagation?
• RQ4 Could the high-energy photons instead originate from a nearby Galactic transient, given the low Galactic latitude of GRB 221009A?
• RQ5 How does the observed light curve and angular coincidence of the photons support or challenge the ALP explanation versus a Galactic origin?

Key findings

• The detection of 251 TeV photons from GRB 221009A at z ≈ 0.151 is inconsistent with standard photon propagation due to extreme attenuation via e⁺e⁻ pair production on the cosmic microwave background (optical depth > 3000).
• ALP-photon mixing in the Milky Way's magnetic field is a viable mechanism to explain the observations, as it avoids the strong attenuation seen in intergalactic mixing scenarios.
• Intergalactic ALP mixing is disfavored because the survival probability for 18 TeV photons would be only ∼10⁻⁵ under assumed intergalactic magnetic fields.
• The ALP parameter space allowing for detectable photon fluxes is constrained to specific combinations of mass and coupling, with conversion occurring primarily in the host galaxy and the Milky Way.
• The low Galactic latitude (b ≈ 4°) of GRB 221009A increases the likelihood of misidentification with a Galactic transient, such as 3HWC J1928+178 or LHAASO J1929+1745, which are located near the line of sight.
• Future work is needed to disentangle whether the high-energy photons originate from the GRB or from a coincident Galactic flare, especially given the unusual light curve of the transient.