GRB 221009A a potential source of ultra-high-energy cosmic rays Rafael Alves Batista12 1Instituto de F sica Te orica UAM-CSIC Universidad Aut onoma de Madrid

2025-05-06 0 0 671.46KB 7 页 10玖币
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GRB 221009A: a potential source of ultra-high-energy cosmic rays
Rafael Alves Batista1,2∗
1Instituto de F´ısica Te´orica UAM-CSIC, Universidad Aut´onoma de Madrid,
C/ Nicol´as Cabrera 13-15, 28049 Madrid, Spain
2Departamento de F´ısica Te´orica, Universidad Aut´onoma de Madrid, M-15, 28049 Madrid, Spain
Recently an extraordinarily bright gamma-ray burst, GRB 221009A, was observed by several
facilities covering the whole electromagnetic spectrum. Gamma rays with energies up to 18 TeV
were detected, as well as a possible photon with 251 TeV. Such energetic events are not expected
because they would be attenuated by pair-production interactions with the extragalactic background
light. This tension is, however, only apparent, and does not call for any unconventional explanation.
Here I show that these observations can be interpreted as the result of ultra-high-energy cosmic rays
(UHECRs) interacting with cosmological radiation fields during their journey to Earth, provided
that intergalactic magnetic fields are reasonably weak. If this hypothesis is correct, it would establish
bursts like GRB 221009A as UHECR sources.
I. INTRODUCTION
On 9 October 2022 the Burst Alert Telescope (BAT)
aboard the Swift satellite observed an astonishingly
bright gamma-ray burst (GRB), GRB 221009A [1]. This
burst had triggered Fermi’s Gamma-ray Burst Monitor
(GBM) about one hour before [2], but was only confirmed
as a GRB later [3]. It was also detected by the Large Area
Telescope (Fermi LAT) [4]. This object is located at red-
shift z= 0.151, having an estimated isotropic-equivalent
energy of Etot '2×1047 J, based on the fluence mea-
sured by Fermi GBM [5]. The Large High Altitude Air
Shower Observatory (LHAASO) observed thousands of
gamma rays from this same direction within ∼2000 s
of the burst [6]. The observed photons have energies
extending up to E'18 TeV. Interestingly, the array
Carpet-2 at the Baksan Neutrino Observatory reported a
photon-like air shower triggering the detectors within the
same time window, with a (pre-trial) significance of 3.8σ,
which points to a primary photon of E'251 TeV [7].
Gamma rays from GRB 221009A with energies E&
10 TeV should be strongly suppressed due to pair pro-
duction (γ+γbg →e++e−) with backgrounds photons
(γbg), mostly from the extragalactic background light
(EBL). Notwithstanding EBL uncertainties, LHAASO
and Carpet-2 would likely not observe primary gamma
rays from the burst, even for weaker EBL models. For
this reason, non-conventional physics such as Lorentz in-
variance violation (LIV) [8–10] and axion-like particles
(ALPs) [8, 11–13] have been quickly invoked to inter-
pret the observations. A more conventional explanation
would be the misidentification of a cosmic-ray shower by
LHAASO as a genuine gamma ray detection [8, 14] –
which is less likely if the Carpet-2 detection is true.
Here I suggest a plausible interpretation of these obser-
vations that does not require any unconventional physics.
If the GRB emitted ultra-high-energy cosmic rays (UHE-
CRs) with energies E&1 EeV (1 EeV ≡1018 eV) to-
∗rafael.alvesbatista@uam.es
wards Earth, then it is possible that they interacted with
background photons, ultimately accounting for at least
part of the observed signal. The necessary conditions for
the validity of this hypothesis are the following:
(i) UHECRs should not be significantly deflected by
magnetic fields before they produce the particles re-
sponsible for the observed gamma rays.
(ii) Part of these gamma rays should arrive within the
same time window as the observations, retaining
temporal and angular correlation with the burst.
(iii) Neutrinos are also produced through the very same
interactions that produce gamma rays. Therefore,
this model has to be compared with observational
neutrino limits [15].
Conditions (i) and (ii) essentially depend on the proper-
ties of intergalactic magnetic fields (IGMFs), discussed
in section IV. Condition (iii) is carefully analysed using
the numerical simulations described in section III.
As a prelude to this work, one must first establish
that GRBs are suitable UHECR sources (for reviews, see
refs. [16, 17]). Indeed, they have long been deemed to be
adequate UHE sources (e.g., [18–23]). They satisfy the
Hillas criterion [24], according to which charged particles
of charge qcan be accelerated by shocks of velocity vsh
up to energies Emax ∼η−1vshcqBR, where Band Rde-
note, respectively, the magnetic field and characteristic
scale of the site wherein acceleration takes place, and η
is an efficiency factor. The actual details of how particle
acceleration occurs are not trivial and vary depending on
the properties of the burst. GRB 221009A is thought to
be a collapsar [25], i.e., the result of exceedingly powerful
supernova. It is beyond the scope of this work to delve
into the details of how CRs are accelerated in GRBs; for
that, the reader is referred to refs. [26, 27]. At this stage,
it suffices to know that many models posit that GRBs are
sources of UHECRs. Once they escape the environment
of the GRB, however that happens, they travel to Earth
and their interactions with cosmological photons can lead
to appreciable fluxes of cosmogenic particles along the
line of sight, including neutrinos and photons. The de-
tails of how this occurs are described in section II.
arXiv:2210.12855v2 [astro-ph.HE] 24 Dec 2022
2
II. PROPAGATION OF COSMIC RAYS AND
GAMMA RAYS
UHECRs can interact with cosmological photon fields
such as the EBL, as well as the cosmic microwave back-
ground (CMB) and the cosmic radio background (CRB)
during their journey to Earth. For a given cosmic-ray
nucleus A
ZX of atomic number Zand mass A, the main
photonuclear processes that affect them are: photopion
production (e.g., p+γbg →p+π0,p+γbg →p+π+),
Bethe-Heitler pair production (A
ZX + γbg →A
ZX + e++
e−), and photodisintegration (A
ZX + γbg →A−1
ZX + n,
A
ZX + γbg →A−1
ZX + p, etc). Unstable nuclei undergo
alpha, beta, and gamma decays during the photodis-
integration chain. An additional subdominant energy-
loss channel for photon production is elastic scattering
(A
ZX + γbg →A
ZX + γ). The by-products of these inter-
actions are the still-undetected [28] cosmogenic particles,
whose existence has been predicted long ago [29, 30].
Electrons and photons, too, interact during intergalac-
tic propagation. The main ones are pair production
(γ+γbg →e++e−) and inverse Compton scattering
(e±+γbg →e±+γ), although higher-order processes such
as double pair production (γ+γbg →e++e−+e++e−)
and triplet pair production (e±+γbg →e±+e++e−) may
also contribute at some specific energy ranges. These
processes feed one another, thus constituting an electro-
magnetic cascade (see ref. [31] for a detailed review).
In addition to the aforementioned interactions, all par-
ticles lose energy losses due to the adiabatic expansion of
the universe. Moreover, charged particles can emit syn-
chrotron radiation in the presence of magnetic fields.
Cosmogenic particles can be produced approximately
along the line of sight, leading to interesting obser-
vational signatures. Such model has been invoked,
for example, to explain gamma-ray observations from
blazars [32–35], and also GRB 221009A [36–38]. This di-
rectional correlation with sources can only occur if mag-
netic fields are not exceedingly strong. For a magnetic
field of coherence length LB, the deflection of a charged
particle after travelling a distance `is ∆δB'arcsin RL/`
if `LBand √2`LB/3RLotherwise [39], with Bdenot-
ing the strength of the magnetic field and RLthe Larmor
radius of the particle. The associated time delay is [40]:
∆tB'
`
c[1 −cos(∆δB)] if `LB,
`∆δ2
B
12cif `LB.
(1)
The main source of uncertainty in ∆tBare the proper-
ties of magnetic fields (see, e.g., refs. [41–43] for reviews).
UHECRs deflections cannot be properly estimated be-
cause IGMFs are unknown, especially in cosmic voids.
Nevertheless, even in the most extreme scenarios they
can be bounded [44].
III. SIMULATIONS
To interpret the observations, one-dimensional Monte
Carlo simulations are performed using the CRPropa
code [45, 46]. UHECRs are assumed to be emitted by the
GRB with spectrum dN/dE∝E−α, with an exponen-
tial suppression factor exp(−E/ZRmax) for E≥ZRmax.
Here Rmax denotes the maximal rigidity of the emitted
cosmic rays, given by Rmax ≡Emax/Z. A fraction ηCR
of the isotropic-equivalent energy of the GRB is assumed
to be converted into cosmic rays (of all energies). To en-
sure that the secondary fluxes arrive within .1 day of
the burst, magnetic deflections ought to be small. Since
IGMFs are poorly know, a conservative and an optimistic
scenario are considered, depending on the rigidity of the
UHECRs. This established a lower bound on the rigidity
of the primary UHECRs that will be considered.
The interaction processes described in section II are
included in the analysis. As a benchmark for the simula-
tions, the EBL model by Gilmore et al. [47] is chosen, as
well as the CRB model by Protheroe & Biermann [48].
Naturally, these choices have a considerable impact on
the results of the simulation, together with other factors
like photonuclear cross sections, for example [49, 50]. It
is beyond the scope of the present Letter to discuss these
uncertainties in detail. It is already enough to prove that
the proposed interpretation of GRB 221009A is justified
considering at least one realistic model.
IV. ANALYSIS AND INTERPRETATION OF
THE OBSERVATIONS
GRB 221009A drew much attention because the events
detected by LHAASO and Carpet-2, in principle, should
not have arrived at Earth due to pair production with
the EBL. If primary gamma rays were emitted, to first
order, this flux (Φ0) would be exponentially suppressed
by a factor corresponding to the optical depth (τ):
Φ(E, z) = Φ0(E, z) exp [−τ(E, z)] (2)
where zis the redshift. It is understandable that a spec-
trum extending up to ≈18 TeV from a source at z'0.15
could be seen as a signature of New Physics (see ref. [51]
for a discussion of some models), especially with a pos-
sible coincident event with E≈251 TeV. But many
plausible hypotheses remain within the realm of conven-
tional explanations. An UHECR origin is one of them,
provided that the requirements (i), (ii), (iii) are fulfilled.
To obtain a rough estimate for the time delay of pho-
tons from GRB 221009A for the scenario here proposed,
it is important to know its precise location within the
large-scale structure of the universe. The contribution
of the host galaxy of the GRB can be ignored given the
jet’s extension. For this reason, it is a good approxima-
tion to consider only the contributions of galaxy clusters,
摘要：

GRB221009A:apotentialsourceofultra-high-energycosmicraysRafaelAlvesBatista1;21InstitutodeFsicaTeoricaUAM-CSIC,UniversidadAutonomadeMadrid,C/NicolasCabrera13-15,28049Madrid,Spain2DepartamentodeFsicaTeorica,UniversidadAutonomadeMadrid,M-15,28049Madrid,SpainRecentlyanextraordinarilybrightgamm...

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