Thermal Structure and Chemical Enrichment of the North and South Polar Spurs: Supersolar N/O and Ne/O in the X-ray Plasma

Abstract

The North Polar Spur (NPS) is a prominent diffuse X-ray feature whose origin has remained uncertain for decades. Using a uniform analysis of archival \textit{Suzaku} and \textit{XMM--Newton} data with new \textit{Chandra} observations, we constrain its thermal and chemical properties. The NPS emission is fully absorbed by the neutral interstellar medium, demonstrating that the plasma lies beyond the Galactic disk and is not a local supernova remnant or nearby superbubble. The spectra require a two-temperature model with a warm--hot component ($kT\approx 0.2$ keV) and a hotter component ($kT=0.4$--$0.5$ keV), with emission measures of $(41.8\pm 4.9)\times 10^{-3}$ and $(12.9\pm 2.2)\times 10^{-3}\mathrm{c}{\mathrm{m}}^{-6}\mathrm{pc}$, respectively. A key result is the detection of super-solar abundance ratios in the warm--hot phase, with N/O $=3.6\pm 0.3$ and Ne/O $=1.9\pm 0.1$ solar. A Suzaku observation of the outer South Polar Spur (SPS) shows similar absorption, temperatures, and enhanced abundances (N/O $=2.9\pm 0.4$, Ne/O $=1.6\pm 0.2$), though with lower emission measures. The similar super-solar abundance ratios suggest a common enrichment history. These properties are consistent with those measured along other sightlines through the X-ray--bright shells of the Galactic bubbles. Together, these results support that the NPS and SPS trace opposite limbs of the Galactic bubbles. The chemical properties suggest a strong contribution from stellar feedback in shaping the Galactic bubbles.

Accepted for publication in ApJ