Spatially Resolved X-Ray Spectra of NGC 4258

Abstract

We report a spatially resolved, X-ray spectral analysis of NGC 4258 using archival Chandra and XMM-Newton observations. The XMM-Newton spectrum of the nuclear region is well described by two power-law components, a soft (0.57 keV) thermal component and an Fe Kα line with EW=40+/-33 eV. The properties of the second, weaker power-law component are similar to those of an off-nuclear source 2.5" southwest of the nucleus. The spectrum of the extended emission of the entire galaxy is well described by two thermal components (MEKAL) models with temperatures ~=0.60 and 0.22 keV. The Chandra and XMM-Newton spectra along the anomalous arms show that the absorbing column density to the southeast anomalous arm is consistent with absorption by gas in our Galaxy, while the absorbing column to the northwest anomalous arm is higher, indicating that the northwest arm is partially on the far side of the galactic disk. The combined Chandra data clearly detect the X-ray emission from the hot spots at the end of the approximately north-south radio jets. By assuming the hot spots represent shocked thermal gas at the ends of the jets, we estimate shock powers of ~=3×10³⁹f^-1/2 ergs s^-1 (f is the filling factor), similar to the radiative power in the inner anomalous arms, consistent with the notion that the jets could be responsible for heating the gas in the anomalous arms.