4. Summary and outlook

Figure 11. γ-ray emission from the inner 70 pc of the Galactic Center by [50] and by [49].

114 Cosmic Rays

Figure 12. <sup>γ</sup>-ray luminosity as function of the distance from SgrA <sup>∗</sup> for <sup>E</sup>cut <sup>¼</sup> 1 PeV.

Star-forming regions are of prime importance for investigating the origin of CRs, as they can be considered as CR birthplaces. During the past decades, a large variety of telescopes went into operation, starting to shed light on the nonthermal multi-messenger picture from star-forming regions in the Milky Way. Of particular interest are three concrete regions in the Milky Way:

1. The Cygnus complex is the most interesting star-forming region in the northern sky. It reveals nonthermal emission from diffuse regions and point sources up to > TeV energy, in particular detected by the Milagro detector. This makes it a prime candidate to search for hadronic interaction signatures. The northern location is beneficial for the IceCube experiment, which can detect neutrinos in the TeV range with a spatial resolution of below 1<sup>∘</sup> from the northern sky. The neutrino flux from Cygnus X approaches the IceCube's upper limit and past already the 2 σ deviations from background could be detected in the past. Model predictions show that a detection of the region could already be possible with IceCube, certainly with the next-generation array IceCube-Gen2 for a detection threshold in the TeV range.

2. The Eta Carinae region is of particular interest because of the binary system η Carinae, detected at energies from radio to gamma rays. It is one of the most energetic binary systems in the galaxy and could have the potential to accelerate hadrons at the shock front forming from the collision of the winds of the two massive stars. Gamma-ray fluxes for such binary systems are in general quite low, i.e., apart from η Carinae they are so far below the detection threshold of Fermi. Thus, even the expected neutrino flux is relatively low, with one more disadvantage of an energy cutoff at relatively low energy. A window of opportunity here could be the detection of astrophysical sources in the 25 GeV neutrino energy range, as atmospheric muon neutrinos have an oscillation minimum here, which results in a highly reduced background.

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3. The Galactic Center itself is no classical star-forming region, as—despite of the high molecular densities—the star formation rate is not increased in the amount expected for a correlation with the gas. It is, nevertheless, a region of high nonthermal emission. The most striking evidence for the existence of PeV CRs comes from H.E.S.S. measurements of the Galactic Center, where the gamma emission suggests a CR spectrum extending up to the knee. The question now is if a large part of the CR energy budget can come from that region or source or if this is a smaller part of the total energy budget. Neutrino measurements in the future by KM3NeT and IceCube-Gen2 will help to disentangle hadronic from leptonic signatures and thereby to quantify the hadronic contribution.

Future developments of telescopes will help to use star-forming regions in our galaxy in order to understand the birthplaces of CRs, i.e., the construction instruments like SKA, CTA, KM3NeT, IceCube-Gen2, and more. At least equally important is the proper modeling of these regions. Here, both plasma aspects, present in the transport equation via the diffusive and convective terms, and particle aspects, defined through the loss terms of leptons and hadrons, need to be considered in detail. Recent developments of numerical tools like CRPropa [25], DRAGON [51], GALPROP [52], and PICARD [53] represent important steps for a proper modeling, in which a diffusion tensor can be applied as well as hadronic and leptonic interactions with state-of-the-art cross sections that combine forward measurements with highenergy collision results at LHC. Thus, the next decade is very promising, as we will now be able to learn from this combination of precision astro- and astroparticle measurements and detailed theoretical modeling of the physics processes involved.
