Observational constraints on accelerators of ultra-high energy cosmic rays Sullivan Maracoa Jonathan Biteaua Antonio Condorellia

2025-05-02 0 0 917.59KB 6 页 10玖币

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Observational constraints on accelerators of

ultra-high energy cosmic rays

Sullivan Maraﬁcoa, Jonathan Biteaua, Antonio Condorellia,

Olivier Delignya,*, Quentin Luceb

aUniversit´e Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France

bKarlsruhe Institute of Technology, Institute for Experimental Particle

Physics (ETP), Karlsruhe, Germany

E-mail: *deligny@ijclab.in2p3.fr

September 2022

Abstract. We explore two generic hypotheses for tracing the sources of ultra-

high energy cosmic rays (UHECRs) in the Universe: star formation rate density or

stellar mass density. For each scenario, we infer a set of constraints for the emission

mechanisms in the accelerators, for their energetics and for the abundances of elements

at escape from their environments. From these constraints, we generate sky maps

above 40 EeV expected from a catalog that comprises 410,761 galaxies out to 350 Mpc

and provides a near-infrared ﬂux-limited sample to map both stellar mass and star

formation rate over the full sky. Considering a scenario of intermittent sources hosted in

every galaxy, we show that the main features observed in arrival directions of UHECRs

can in turn constrain the burst rate of the sources provided that magnetic-horizon

eﬀects are at play in clusters of galaxies.

Introduction. A correlation between arrival directions of ultra-high energy cosmic

rays (UHECRs) and the ﬂux patterns of star-forming galaxies within a few Mpc provides

currently the most promising evidence for anisotropy above 40 EeV [1, 2, 3]. Its meaning,

however, remains to be uncovered. In particular, the relatively low-signal strength, on

the order of 10–15%, prevents one from drawing ﬁrm statements about an origin of

UHECRs exclusively from star-forming galaxies. The aim of this contribution, based

on [4, 5], is to constrain a benchmark scenario that could accommodate the observed

correlation.

Starburst galaxies are responsible for '15% of the total star formation rate (SFR)

for redshifts z < 2 [6, 7]. If UHECR sources are related to cataclysmic events associated

with the deaths of short-lived, massive stars, the rate of which is traced by the SFR, the

signal strength is then suggestive of sources being hosted in every star-forming galaxy

in the Universe, with starburst galaxies harboring more events than main-sequence

galaxies. We explore such a benchmark scenario by considering that the density of

sources is fairly traced by the SFR density (SFRD) or, alternatively, by the stellar

mass density (SMD). In both cases, we ﬁrst infer constraints on the emission processes,

arXiv:2210.09633v1 [astro-ph.HE] 18 Oct 2022

Constraints on UHECR accelerators 2

10 19

10 20

[eV]EDetected energy,

]

-1

-2

[eV kmJ(E)

EEnergy flux,

]

-1

dex

-3

[erg Mpc(E)

Energy density,

All-particle spectrum

Protons (EPOS-LHC)

det

4≤

det

A≤2

22≤

det

A≤5 38≤

det

A≤23

Figure 1. UHECR energy ﬂux on Earth as modeled by the best-ﬁt parameters for the

benchmark scenario, for ﬁve detected mass groups and for all particles. Left: SMD [4].

Right: SFRD [5].

energetics, and abundances of elements in the source environments by ﬁtting, to the

energy spectrum and mass-composition data reported in [8, 9], a benchmark scenario

in which the intensity of each individual nuclear components is assumed to drop oﬀ

at the same magnetic rigidity. This is consistent with the basic expectation that

electromagnetic processes accelerate particles up to a maximum energy proportional

to their electric charge Z. Based on these results, we map the UHECR intensity in

the sky above 40 EeV that would result from sources following the SFRD or the SMD

by using the most complete ﬂux-limited catalog of galaxies over the entire sky [10, 11].

Considering an intermittent nature for the UHECR sources then enables us to match

the arrival directions of UHECRs for some viable range of rate density.

Source properties required to match observations. The benchmark model follows

here from [4], in which the energy spectrum of the ejected particles as well as the amount

of ejected nuclei Amay diﬀer strongly from those injected into the electromagnetic ﬁeld

due to in-source interactions, see e.g. [12, 13]. This has two important consequences:

a) the ejected spectrum of the charged nuclei in E−γAcan be much harder than that

injected, due to the escape mechanism; b) the interactions can produce a copious ﬂux

of secondary neutrons of energy En=E/A. These neutrons can escape freely from the

magnetic conﬁnement zones, with an ejection spectrum E−γpmuch softer than that of

nuclei, to decay into protons on their way to the Earth. Consequently, the ejection rate

of the sources per comoving volume unit and per energy unit of elements is modeled by

an exponentially-suppressed power law, the spectral index of which is possibly diﬀerent

for protons (γp) and nuclei (γA).

For reproducing simultaneously the energy spectrum and mass-composition data

above '0.6 EeV, the modelled abundance of nuclear elements is found to be dominated

by intermediate-mass ones, ranging from He to Si, accelerated to EZ

max '5ZEeV and

escaping from the source environments with a very hard spectral index γA, consistent

with previous studies above 5 EeV [14]. Such a hard spectral index is called for to

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Observationalconstraintsonacceleratorsofultra-highenergycosmicraysSullivanMaracoa,JonathanBiteaua,AntonioCondorellia,OlivierDelignya,*,QuentinLucebaUniversiteParis-Saclay,CNRS/IN2P3,IJCLab,91405Orsay,FrancebKarlsruheInstituteofTechnology,InstituteforExperimentalParticlePhysics(ETP),Karlsruhe,Germa...

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Observational constraints on accelerators of ultra-high energy cosmic rays Sullivan Maracoa Jonathan Biteaua Antonio Condorellia

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