Low-luminosity gamma-ray bursts as the sources of ultrahigh-energy cosmic ray nuclei

Abstract

Recent results from the Pierre Auger Collaboration have shown that the composition of ultrahighenergy cosmic rays (UHECRs) becomes gradually heavier with increasing energy. Although gammaray bursts (GRBs) have been promising sources of UHECRs, it is still unclear whether they can account for the Auger results because of their unknown nuclear composition of ejected UHECRs. In this work, we revisit the possibility that low-luminosity GRBs (LL GRBs) act as the sources of UHECR nuclei, and give new predictions based on the intrajet nuclear composition models considering progenitor dependencies. We find that the nuclear component in the jet can be divided into two groups according to the mass fraction of silicon nuclei, Si-free and Si-rich. Motivated by the connection between LL GRBs and transrelativistic supernovae, we also consider the hypernova ejecta composition. Then, we discuss the survivability of UHECR nuclei in the jet base and internal shocks of the jets, and show that it is easier for nuclei to survive for typical LL GRBs. Finally, we numerically propagate UHECR nuclei ejected from LL GRBs with different composition models and compare the resulting spectra and composition to Auger data. Our results show that both the Sirich progenitor and hypernova ejecta models match the Auger data well, while the Si-free progenitor models have more difficulty in fitting the spectrum. We argue that our model is consistent with the newly reported cross correlation between the UHECRs and starburst galaxies, since both LL GRBs and hypernovae are expected to be tracers of the star-formation activity. LL GRBs have also been suggested as the dominant origin of IceCube neutrinos in the PeV range, and the LL GRB origin of UHECRs can be critically tested by near-future multimessenger observations.

Progress towards characterizing ultrahigh energy cosmic ray sources

“…A pure-proton composition was originally expected for GRBs [55], due to the extreme temperatures in the collapsar which dissolve nuclei depending on the radius at which they are injected into the jet. The possibility that both pure-proton or mixed compositions can follow from different conditions has been discussed by multiple authors, e.g., see [56,57]. Phenomenologically, it is interesting to investigate whether such an additional component may better describe the Auger data and whether such a component can be ruled out by multimessenger data.…”

Ultrahigh-energy cosmic-ray nuclei and neutrinos from engine-driven supernovae

“…1. Heavy nuclei can be extracted from the inner stellar core [13,77] or synthesized during the expansion of the outflow [12,78,79]. In this work, we adopt the nuclear composition model, Si-R I, proposed in Ref.…”

“…x esc E (acc) A,max 10 18.3 x esc,−0.2 eV can escape efficiently, where x esc is the ratio of the escape boundary to the width of the shocked region [13,102]. The maximum escape energy adopted in this work can be achieved assuming the acceleration efficiency is η = 8 for the jet and η = 1 for the transrelativistic ejecta.…”

“…whereκ Aγ is the inelasticity which represents average energy losses of the photodisintegration process [56]. We can see that UHECR nuclei are in the partial survival regime for model Jet-A which satisfy the condition τ Aγ > 1 and f Aγ < 1 [13,56] (see Fig. 5).…”

“…We performed χ 2 analysis as in Ref. [13,51] to show the quality of our fitting. The spectrum energy fitting range is from 10 18.45 eV to 10 21.15 eV, where the uncertainties on the flux are taken from the measurements of Auger [38] and TA [107].…”