The North Polar Spur and Aquila Rift

Abstract

Soft X-ray intensity at 0.89 keV along the North Polar Spur (NPS) is shown to follow the extinction law due to the interstellar gas in the Aquila Rift by analysing the ROSAT archival data, which proves that the NPS is located behind the rift. The Aquila-Serpens molecular clouds, where the X-ray optical depth exceeds unity, are shown to have a mean LSR velocity of v =7.33 ± 1.94 km s-1, corresponding to a kinematic distance of r=0.642 ± 0.174 kpc. Assuming a shell structure, a lower limit of the distance to NPS is derived to be 1.01±0.25 kpc, with the shell centre being located farther than 1.1 kpc. Based on the distance estimation, we argue that the NPS is a Galactic halo object.