Abstract
Context. Supernova remnants interacting with molecular clouds are ideal laboratories to study the acceleration of particles at shock waves and their transport and interactions in the surrounding interstellar medium. Aims. Here, we focus on the supernova remnant W28, which over the years has been observed in all energy domains from radio waves to very-high-energy gamma rays. The bright gamma-ray emission detected from molecular clouds located in its vicinity revealed the presence of accelerated GeV and TeV particles in the region. An enhanced ionization rate has also been measured by means of millimeter observations, but such observations alone cannot tell us whether the enhancement is due to low-energy (MeV) cosmic rays (either protons or electrons) or the X-ray photons emitted by the shocked gas. The goal of this study is to determine the origin of the enhanced ionization rate and to infer from multiwavelength observations the spectrum of cosmic rays accelerated at the supernova remnant shock in an unprecedented range spanning from MeV to multi-TeV particle energies. Methods. We developed a model to describe the transport of X-ray photons into the molecular cloud, and we fitted the radio, millimeter, and gamma-ray data to derive the spectrum of the radiating particles. Results. The contribution from X-ray photons to the enhanced ionization rate is negligible, and therefore the ionization must be due to cosmic rays. Even though we cannot exclude a contribution to the ionization rate coming from cosmic-ray electrons, we show that a scenario where cosmic-ray protons explain both the gamma-ray flux and the enhanced ionization rate provides the most natural fit to multiwavelength data. This strongly suggests that the intensity of CR protons is enhanced in the region for particle energies in a very broad range covering almost six orders of magnitude: from ≲100 MeV up to several tens of TeV.
Highlights
Supernova remnants (SNRs) interacting with molecular clouds (MCs) are ideal laboratories to study the acceleration of particles at astrophysical shocks (Gabici & Montmerle 2015)
Additional constraints on the origin of the enhanced ionization rate can be obtained from hard X-ray observations of W28 performed by Suzaku (Nobukawa et al 2018). These observations revealed the presence of the Fe I Kα line in the X-ray spectrum. This line is produced by interactions between low-energy (MeV domain) cosmic rays (CRs) and cold gas, and it is tempting to propose a common origin for the line emission and the excess in the ionization rate measured in the northeastern MC
The gamma-ray emission from the MCs in the region demonstrates that an excess of CR protons is present there
Summary
Supernova remnants (SNRs) interacting with molecular clouds (MCs) are ideal laboratories to study the acceleration of particles at astrophysical shocks (Gabici & Montmerle 2015). The MCs in the vicinity of W28 are prominent gamma-ray sources (Aharonian et al 2008; Abdo et al 2010) The origin of this emission is due to interactions of GeV and TeV CR protons that were accelerated in the past at the SNR shock and that fill a vast region surrounding the remnant (e.g., Gabici et al.2010). As we see in the following, X-rays are in this case not a viable explanation for the enhanced ionization, and CRs are left as the only possible ionizing agents present inside the cloud This fact opens the possibility to combine high- and low-energy observations of the SNR/MC system (gamma rays and millimeter waves, respectively), and constrain the spectrum of CRs present in the region over an interval of particle energies of unprecedented breadth: from the MeV to the TeV domain.
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