On the distance to the North Polar Spur and the local CO-H₂ factor

Most models identify the X-ray bright North Polar Spur (NPS) with a hot interstellar (IS) bubble in the Sco-Cen star-forming region at ≃130 pc. An opposite view considers the NPS as a distant structure associated with Galactic nuclear outflows. Constraints on the NPS distance can be obtained by comparing the foreground IS gas column inferred from X-ray absorption to the distribution of gas and dust along the line of sight. Absorbing columns towards shadowing molecular clouds simultaneously constrain the CO-H₂ conversion factor.

We derived the columns of X-ray absorbing matter N from spectral fitting of dedicated XMM-Newton observations towards the NPS southern terminus (l ≃ 29°, b ≃ +5 to +11°). The distribution of the IS matter was obtained from absorption lines in stellar spectra, 3D dust maps and emission data, including high spatial resolution CO measurements recorded for this purpose.

N varies from ≃ 4.3 to ≃ 1.3 × 10²¹ cm^-2 along the 19 fields. Relationships between X-ray brightness, absorbing column and hardness ratio demonstrate a brightness decrease with latitude governed by increasing absorption. The comparison with absorption data, local and large-scale dust maps rules out a NPS near side closer than 300 pc. The correlation between N and the reddening increases with the sightline length from 300 pc to 4 kpc and is the tightest with Planck τ ₃₅₃ -based reddening, suggesting a much larger distance. N(H)/E(B-V) ≃ 4.1 × 10²¹ cm^-2 mag^-1, close to Fermi-Planck determinations. N absolute values are compatible with HI-CO clouds at -5 ≤ V ≤ +25 to +45 km s^-1 and a NPS potentially far beyond the Local Arm. A shadow cast by a b=+9° molecular cloud constrains X in that direction to ≤ 1.0 × 10²⁰ cm^-2 K^-1 km^-1 s. The average X over the fields is ≤ 0.75 × 10²⁰ cm^-2 K^-1 km^-1 s.