Possible connection between the asymmetry of North Polar Spur and Loop I with Fermi Bubbles

TL;DR

This paper proposes that the asymmetry of the North Polar Spur and Loop I can be explained by a slight difference in circumgalactic medium density, supporting a common Galactic center origin for these structures and Fermi Bubbles.

Contribution

It demonstrates that small density asymmetries and projection effects can account for observed asymmetries, linking NPS, Loop I, and Fermi Bubbles to a star formation driven wind from the Galactic center.

Findings

Asymmetries explained by 20% lower CGM density in the south.

Estimated FB age of approximately 28 million years.

Consistent mechanical luminosity with star formation driven wind scenario.

Abstract

The origin of North Polar Spur (NPS) and Loop-I has been debated over almost half a century and is 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). I show in this paper, that such asymmetries of NPS and Loop-I in both X-rays and $\gamma$-rays can be easily produced if the circumgalactic medium (CGM) density in the southern hemisphere is only smaller by $\approx 20\%$ than the northern counterpart in case of a star formation driven wind scenario. The required mechanical luminosity, $\mathcal{L} \approx 4-5\times 10^{40} $ erg s$^{-1}$ (reduces to $\approx 0.3$ M$_\odot$ yr$^{-1}$ including the non-thermal pressure) and the age of the FBs, $t_{\rm age} \approx 28$ Myr, are consistent with previous estimations in case of 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 $\approx 5^\circ$ bigger than the northern one. The results, therefore, indicate towards a possibility for a common origin of the NPS, Loop-I and FBs from the Galactic centre (GC). I also estimate the average sky brightness in X-ray towards the south Galactic pole and North Galactic pole in the ROSAT-R67 band and find that the error in average brightness is far too large to have any estimation of the deficiency in the southern hemisphere.