2019MNRAS.482.4813S

The origins of the North Polar Spur (NPS) and Loop I have been debated for almost half a century, and are still unresolved. Most of the confusion is caused by the absence of any prominent counterparts of these structures in the Southern Galactic hemisphere (SGH). This has also led to doubts over the claimed connection between the NPS and Fermi bubbles (FBs). In this paper, I show that such asymmetries of the NPS and Loop I in both X-rays and γ-rays can be easily produced if the density of the circumgalactic medium in the SGH is only ≃20 per cent smaller than its northern counterpart for a star-formation-driven wind scenario. The required mechanical luminosity, L ~4¡-¡5×10⁴⁰ erg s^–1 (which reduces to ≃0.3 M_☉ yr^–1 when including non-thermal pressure), and the age of the FBs, t_age ≃ 28 Myr, are consistent with previous estimations for a star-formation-driven wind scenario. One of the main reasons for the asymmetry is the projection effects at the solar location. Such a proposition is also consistent with the fact that the southern FB is ≃5° bigger than the northern FB. Therefore, the results indicate a possible common origin of the NPS, Loop I and FBs from the Galactic Centre. I also estimate the average sky brightness in X-rays towards the South Galactic Pole and North Galactic Pole in the ROSAT-R67 band, and I find that the error in average brightness is far too large to provide any estimation of the deficiency in the SGH.