Highly collimated microquasar jets as efficient cosmic-ray sources

G.J. ESCOBAR; L.J. PELLIZZA; ROMERO, GUSTAVO E

ASTRONOMY & ASTROPHYSICS. SUPPLEMENT SERIES (PRINT)

ESO 2022

Año: 2022

0365-0138

Context. Supernova remnants are currently believed to be the main sites where Galactic cosmic rays originate. This scenario, however,fails to explain some of the features observed in the cosmic-ray spectrum. Microquasars have been proposed as additional candidates,because their non-thermal emission indicates the existence of efficient particle acceleration mechanisms in their jets. Only a few partialattempts have been made so far to quantify the contribution of microquasars to the Galactic cosmic-ray population. A promisingscenario envisages the production of relativistic neutrons in the jets, that decay outside the system injecting relativistic protons to thesurroundings. The first investigations of this scenario suggest that microquasars might be fairly alternative cosmic-ray sources.Aims. We aim at assessing the role played by the degree of collimation of the jet on the cosmic-ray energetics in the neutron-carrierscenario, as well as the location and size of the emission region, and the interactions of protons with photon fields. Our goals areto explain the Galactic component of the observed proton cosmic-ray spectrum at energies higher than ∼ 10 GeV and to relate thementioned jet properties with the power and spectral index of the produced cosmic rays.Methods. We improve previous analytical models of relativistic particle transport in microquasar jets by including prescriptions forthe jet geometry and convection within it. We introduce the neutron component through catastrophic terms that couple the proton andneutron transport equations, and then compute the escape and decay of these neutrons. Finally, we follow the propagation of the decayproducts and obtain the proton cosmic-ray spectrum once the particles reach the interstellar medium.Results. We find that collimated jets, with compact acceleration regions close to the jet base, are very efficient sources that coulddeliver a fraction of up to ∼ 0.01 of their relativistic proton luminosity into cosmic rays. Collimation is the most significant featureregarding efficiency; a well collimated jet might be ∼ 4 orders of magnitude more efficient than a poorly collimated one. These sourcesproduce a steep spectral index of ∼ 2.3 at energies up to ∼ 10 TeV.Conclusions. Single microquasars may rival supernova remnants regarding the power injected to the interstellar medium in cosmicrays. The main advantage of the former is the production of a steeper spectrum than the latter, closer to what is observed. Thepredictions of our model may be used to infer the total contribution of the population of Galactic microquasars to the cosmic raypopulation, and therefore to quantitatively assess their significance as cosmic-ray sources.