Pisma v ZhETF INR-TH-2022-021 Parameters of axion-like particles required to explain high-energy

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Pis’ma v ZhETF

INR-TH-2022-021

Parameters of axion-like particles required to explain high-energy

photons from GRB 221009A

S. V. Troitsky1)

Institute for Nuclear Research of the Russian Academy of Sciences,

60th October Anniversary prospect 7A, 117312 Moscow, Russia

Submitted October 17, 2022

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.

1. Observations. An unusual energetic astrophysical

transient Swift J1913.1+1946 has been detected on

October 9, 2022 [1] and soon associated with a gamma-

ray burst GRB 221009A detected by Fermi GBM

[2]. The redshift of the GRB was determined from

absorption lines in the afterglow [3, 4] and from

emission lines in the host galaxy [5] as 𝑧≈0.151.

The main peculiarity of the transient is the presence of

extremely energetic gamma rays, never detected from

a GRB. In particular, LHAASO reported the detection

of thousands of photons with energies up to 18 TeV in

the ﬁrst 2000 s after the GRB trigger [6], and Carpet-

2 reported the detection of a 251-TeV photon-like air

shower 4536 s after the trigger [7]. We assume, for

the main part of the present Letter, that the photons

observed by LHAASO and Carpet-2 indeed arrived

from the GRB at the reported distance. Potential

misidentiﬁcations will be brieﬂy addressed in Sec. 4.

2. Photon attenuation and axion-like particles.

The observations by both LHAASO and Carpet-2

challenge conventional understanding because gamma

rays of that high energies cannot reach us from distant

sources [8]. They should instead produce 𝑒+𝑒−pairs on

the cosmic background radiation. The predicted mean

free path for 18-TeV photons depends on the assumed

extragalactic infrared background which is known with

considerable uncertainties, so the optical depth for a

source at 𝑧= 0.151 is estimated as ∼15 ±5. A photon

1)e-mail: st@ms2.inr.ac.ru

with the energy of 251 TeV produce pairs on much

more abundant, and better known, cosmic microwave

background (CMB), and its mean free path is only of

order 75 kpc, smaller than the virial radius of our own

Milky Way Galaxy; the optical depth for the distance

of GRB 221009A is >3000 for this energy.

The problem of observations of energetic particles,

possibly photons, from very distant sources arose in

various contexts and always required non-standard

physics to be solved or relaxed. Here we concentrate on

mixing of photons with hypothetical axion-like particles

(ALPs) in the external magnetic ﬁeld [9], which was

ﬁrst invoked to solve an astrophysical problem of this

kind in Ref. [10], and for the gamma-ray propagation

in Ref. [11]. For reviews and more references, see e.g.

[12, 13, 14].

A general ALP is characterized by two parameters,

mass 𝑚and photon coupling 𝑔. The latter appears

in the speciﬁc interaction term in the Lagrangian,

−(𝑔/4)𝑎𝐹𝜇𝜈 ˜

𝐹𝜇𝜈 , where 𝑎is the pseudoscalar ALP ﬁeld,

𝐹𝜇𝜈 is the electromagnetic stress tensor and ˜

𝐹𝜇𝜈 =

(1/2)𝜖𝜇𝜈𝜌𝜆𝐹𝜌𝜆 is its dual tensor. Compared to the QCD

axion, ALP is more general because 𝑚and 𝑔are

arbitrary independent parameters, but at the same time

is simpler because it does not necessarily interact with

gluons in the minimal version. The ALP/photon mixing

is described in Ref. [9], while a collection of equations

relevant in the astrophysical context may be found

Письма в ЖЭТФ 1

arXiv:2210.09250v2 [astro-ph.HE] 20 Oct 2022

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Pis’mavZhETFINR-TH-2022-021Parametersofaxion-likeparticlesrequiredtoexplainhigh-energyphotonsfromGRB221009AS.V.Troitsky1)InstituteforNuclearResearchoftheRussianAcademyofSciences,60thOctoberAnniversaryprospect7A,117312Moscow,RussiaSubmittedOctober17,2022RecentastrophysicaltransientSwiftJ1913.1+1946is...

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