The X-ray spectrum of the North Polar Spur

Abstract

An analysis is presented of the soft X-ray background spectrum measured by the EPIC-MOS cameras on XMM-Newton in three observations targeted on the North Polar Spur (NPS). Three distinct Galactic plasma components are identified: a cool local hot bubble (LHB) component, Tlo ∼ 0.1 keV; a cool Galactic halo component, at a similar temperature; and a hotter component, Thi ∼ 0.26 keV, associated with the NPS itself. Using the new data in combination with the ROSAT All-Sky Survey count rates measured in the 0.1-0.4 keV band, we estimate the emission measure of the LHB material to be 0.0040-0.0052 cm-6 pc, which implies an electron density of 0.008-0.011 cm-3 and pressure of ∼22 000 cm-3 K. The halo and NPS components lie behind at least 50 per cent of the line-of-sight cold gas for which the total Galactic column density is in the range (2-8) × 1020 cm-2. Modelling the X-ray emitting superbubble as a sphere at distance 210 pc, radius 140 pc and centre l II = 352°, bII = 15°, the implied electron density in the NPS is ∼0.03 cm-3 with pressure ∼150000 cm-3 K. The observed spectral line complexes from O VII, O VIII, Fe XVII, Ne IX, Ne X and Mg XI provide constraints on the composition of the plasma. The hot component in the NPS is depleted in oxygen, neon and, to some extent, magnesium and iron. Assuming that the effective line of sight across the halo emission is 1 kpc, the electron density in the halo is 0.007-0.011 cm-3 and the pressure is ∼16500 cm-3 K, conditions very similar to those in the LHB.