Probing the Origin of the NPS with Broadband Radio Observations: New Insights into Future X-ray and Gamma-ray Observations

Abstract

The North Polar Spur (NPS) is a large-scale structure that is clearly visible in both radio and X-ray all-sky maps. 22 MHz to 70 GHz broadband radio observations have been used to systematically analyze the thermal/nonthermal radiations associated with the NPS. We show that the radio emissions of NPS comprises (1) synchrotron radiation, (2) free-free radiation, and (3) dust emission; however synchrotron radiation dominate over other emissions, especially at high galactic-latitudes. In addition, the synchrotron spectra exhibit a power-law behavior $N(\gamma)\propto\gamma^{-s}$ ($s\simeq 2.2-3.0$), which is moderated by a turnover at $\nu\simeq1$ GHz, above which the spectral index $s$ decreases by one. Assuming that the turnover is due to the electrons cooled by synchrotron emission before escaping (or advecting) from the emission region, the strength of the magnetic field can be estimated to be $B\simeq8\rm\mu G$ if the NPS radiates from the Galactic Center (GC). However, an unreasonably strong $B\simeq 114 \mu G$ is required if the NPS is near a local supernova remnant (SNR). We estimated the non-thermal energy stored in the NPS to be $E_{\rm n/th}\sim 4.3\times10^{55}$ $\rm erg$ in the GC scenario, whereas $E_{\rm n/th}\sim 4.1\times10^{52}$ $\rm erg$ is difficult to explain with a single local SNR. We also estimated the gamma-ray radiation associated with the NPS, through inverse Comptonization (IC) of the cosmic microwave background (CMB), which peaks at 100-1000 keV with a flux of $\nu F_{\nu}\sim 10^{-9}$ $\rm erg\,cm^{-2}s^{-1}sr^{-1}$ in the GC scenario, which may be a good candidate for detection by future X-ray/gamma-ray observatories.