Suzaku Observations of the North Polar Spur: Evidence for Nitrogen Enhancement

Abstract

Abstract We present observations of the North Polar Spur (NPS) using the X-ray Imaging Spectrometer (XIS) aboard the Suzaku X-ray satellite. The NPS is a large region of enhanced soft X-ray and radio emission projected above the plane of the Galaxy, likely produced by a series of supernovae and stellar winds from the nearby Sco-Cen OB association. The exceptional sensitivity and spectral resolution of the XIS below 1keV allow unprecedented probing of low-energy spectral lines, including CVI (0.37keV) and NVII (0.50keV), and we have detected highly-ionized nitrogen toward the NPS for the first time. For this single pointing toward the brightest 3/4keV emission ($l =$ 26°.8, $b =$$+$22°.0), the best-fit NPS emission model implies a hot ($kT \approx 0.3$keV), collisional ionization equilibrium (CIE) plasma with depleted C, O, Ne, Mg, and Fe abundances of less than 0.5solar, but an enhanced N abundance, with N$/$O $=$$(4.0^{+0.4}_{-0.5})$ times solar. The temperature and total thermal energy of the gas suggest heating by one or more supernovae, while the enhanced nitrogen abundance is best explained by enrichment from stellar material that has been processed by the CNO cycle. Due to the time required to develop AGB stars, we conclude that this N$/$O enhancement cannot be caused by the Sco-Cen OB association, but may result from a previous enrichment episode in the solar neighborhood.