Resonant Scattering Effect on the Soft X-Ray Line Emission from the Hot Interstellar Medium. I. Galactic Bulges

Abstract

Diffuse soft X-ray line emission is commonly used to trace the thermal and chemical properties of the hot interstellar medium, as well as its content, in nearby galaxies. Although resonant line scattering complicates the interpretation of the emission, it also offers an opportunity to measure the kinematics of the medium. We have implemented a direct Monte Carlo simulation scheme that enables us to account for resonant scattering effect in the medium, in principle, with arbitrary spatial, thermal, chemical, and kinematic distributions. Here we apply this scheme via dimensionless calculation to an isothermal, chemically uniform, and spherically symmetric medium with a radial density distribution characterized by a β-model. This application simultaneously account for both optical depth-dependent spatial distortion and intensity change of the resonant line emission due to the scattering, consistent with previous calculations. We further apply the modeling scheme to the OVII and OVIII emission line complex observed in the XMM-Newton RGS spectrum of the M31 bulge. This modeling, though with various limitations due to its simplicity, shows that the resonant scattering could indeed account for much of the spatial distortion of the emission, as well as the relative strengths of the lines, especially the large forbidden to resonant line ratio of the OVII Heα triplet. We estimate the isotropic turbulence Mach number of the medium in M31 as ∼ 0.17 for the first time and the line-emitting gas temperature as ∼ 2.3 × 10 6 K. We conclude that the resonant scattering may in general play an important role in shaping the soft X-ray spectra of diffuse hot gas in normal galaxies.

XMM-Newton RGS Spectroscopy of the M31 Bulge. I. Evidence for a Past AGN Half a Million Years Ago

“…Here we examine the possibilities of the two very distinct scenarios proposed in previous works: CX (Liu et al 2010) and RS (Chen et al 2018). But first of all, there are lines of evidence, mostly from the spatial-variation results, suggesting that the plasma, if locally in a CIE state, has a decreasing temperature trend from the inner to the outer bulge region.…”

“…A RS model based on Monte Carlo simulation has been applied to the RGS spectrum of its O vii+O viii complex in the 18-23 Å wavelength range. The best-fit result suggests that a 0.2 keV plasma with a half solar oxygen abundance (Grevesse & Sauval 1998) explains both the G-ratio and the broadening profiles of oxygen lines (Chen et al 2018).…”

“…The RGS observations and data reduction have been detailed in Section 4 of Chen et al (2018). Briefly, there are 36 XMM-Newton observations toward the bulge region of M31, with a total effective exposure of 766 ks, as listed in Table 6 in Appendix A.…”

“…We divide the 36 observations over 12 years (2001Chen et al 2018) into four groups. One group has the instrumental position angles of ∼ 70 • and has a total effective exposure of 112 ks.…”

“…However, the O vii G-ratio of its hot gas was also reported larger than 1.46 (Liu et al 2010). It could be due to the weakening of the r line if resonant scattering (RS) process is important (Chen et al 2018), or to the strengthening of the f line, which may result from such a process as charge exchange (CX) or recombination.…”