Non-thermal emission from cosmic rays accelerated in H II regions

Abstract

Context. Radio observations at metre-centimetre wavelengths shed light on the nature of the emission of Hii regions. Usually this category of objects is dominated by thermal radiation produced by ionised hydrogen, namely protons and electrons. However, a number of observational studies have revealed the existence of Hii regions with a mixture of thermal and non-thermal radiation. The latter represents a clue as to the presence of relativistic electrons. However, neither the interstellar cosmic-ray electron flux nor the flux of secondary electrons, produced by primary cosmic rays through ionisation processes, is high enough to explain the observed flux densities. Aims. We investigate the possibility of accelerating local thermal electrons up to relativistic energies in Hii region shocks. Methods. We assumed that relativistic electrons can be accelerated through the first-order Fermi acceleration mechanism and we estimated the emerging electron fluxes, the corresponding flux densities, and the spectral indexes. Results. We find flux densities of the same order of magnitude of those observed. In particular, we applied our model to the 'deep south' (DS) region of Sagittarius B2 and we succeeded in reproducing the observed flux densities with an accuracy of less than 20% as well as the spectral indexes. The model also gives constraints on magnetic field strength (0.3 − 4 mG), density (1 − 9 × 10 4 cm −3 ), and flow velocity in the shock reference frame (33 − 50 km s −1 ) expected in DS. Conclusions. We suggest a mechanism able to accelerate thermal electrons inside Hii regions through the first-order Fermi acceleration. The existence of a local source of relativistic electrons can explain the origin of both the observed non-thermal emission and the corresponding spectral indexes.

New Insights into the H ii Region G18.88–0.49: Hub–Filament System and Accreting Filaments

“…The required value of U NT greatly exceeds the Galactic cosmic ray energy density (U GCR ≈ 1 eV cm −3 ). We conclude that the observed synchrotron flux is produced by relativistic particles locally accelerated in shocked regions of gas, consistent with recent non-thermal emission models (Padovani et al 2019). However, we still need more observations in frequency ranges below 1 GHz and above 2 GHz to make more robust estimates of the magnetic field and energy particle distribution in this H ii region.…”

The physical and chemical structure of Sagittarius B2

“…Therefore, the possibility that DS is an extragalactic radio source is excluded. Thus, the only remaining A73, page 8 of 14 3) by the First-order Fermi acceleration model through a χ 2 test using the model described in Padovani et al (2019).…”

“…In a companion paper (Padovani et al 2019), we present an extension to this theory applied to HII regions. We demonstrate that electrons can be efficiently accelerated under the hypothesis that the flow velocity in the reference frame of an expanding HII region hitting denser material is sufficiently high (>35 km s −1 ) to switch on particle acceleration.…”

Interstellar anatomy of the TeV gamma-ray peak in the IC443 supernova remnant

“…Our second aim is to identify all possible sources of ongoing acceleration of "fresh" CRs additional to the "old" injection of CRs previously accelerated by the SNR. There can be two kinds of these sources: ionized regions in which kinetic energy is deposited and the magnetic field structure and ionization fraction make the acceleration possible (such as [H II] regions; see Padovani et al 2019); or protostellar jets and outflows, where these conditions can be naturally combined. Our work can thus provide support for further studies of CR-related questions only if the study of local star formation is performed simultaneously.…”