Modeling Galactic Synchrotron Absorption Measurements in the HII region behind the Galactic center

We investigate the effect of additional cosmic-ray electron (CRE) sources on the Galactic synchrotron emission distribution along the line of sight (LOS) through the Galaxy. In this way, we try to explain the differences between low-frequency synchrotron measurements and predictions from current Galactic magnetic field and CRE models. We use stochastic differential equations to describe electron CR propagation in the Galaxy. These specific CRE density models are used to simulate synchrotron emissivity at low radio frequencies. We consider individual young supernova remnants as additional sources of CREs, as well as an enhanced CRE density in an extended ring in the outer Galaxy. We compare the observed low-frequency synchrotron emission in the direction of optically thick HII regions with synchrotron emission as predicted by these models. We found that neither a single supernova remnant emitting CRE in addition to the Galactic CRE background, nor additional CREs emitted in a spiral arm, can explain current discrepancies between low-frequency observations of synchrotron emission in the direction of optically thick HII regions. This indicates the need for other solutions to bring synchrotron data in agreement with CRE and GMF models.