Improved He-like Kα Line Diagnostics for Photoionized Plasmas

The helium-like Kα line complexes are valuable diagnostics of the electron density, temperature, and ionization conditions of astrophysical plasmas. The ratios of these lines can be altered by the presence of lithium-like ions through both inner-shell ionization and resonant Auger–Meitner destruction (RAD), in which the He-like intercombination lines are diminished by line absorption from near-coincident Li-like (1s2s2p → 1s²2s) lines. We modeled the effect of an increased Li-like presence on the He-like line ratios R ≡ z / (x + y) and G ≡ (x + y + z)/ w for astrophysically important ions in a photoionized plasma by solving the steady-state rate equations as a function of Li-like column density NLi, He-like column density NHe, velocity dispersion σv, electron density nₑ, and UV photoexcitation rate Φ. We also define and analytically solve for a ratio Q ≡ (q + r)/w , which is a diagnostic of the Li-like column density. We find that a high Li-like column density can significantly modify R and G through both enhancement of z and suppression of x and y, and that the effects of RAD are strongly dependent on the energy difference between the emitting and absorption transitions for a given element. Due to this dependence, we stress the importance of a carefully curated database as a reference for transition energies. Our diagnostic framework is suitable for application to spectra of active galactic nuclei and X-ray binaries observed by the XRISM Resolve, XMM-Newton RGS, and Chandra HETGS. : subscript