Identification of Three Weak Lines and Their Influences on Diagnostics of Electron Temperature and Density in Astrophysical Plasmas

By comparing of the spectra from the Chandra LETGs observation with the synthetic spectra of argon, we identify two weak emission lines, that is, the Ar XV 27.470Å line with transition 2s3p ¹P₁-2s^{2 1}S₀ and the Ar XVI 27.872Å line with transition 3d ²D_3/2-2p²P_1/2, which blend with the N VII Lyα emission line. Several second-order spectral lines are also identified, one of which is the second-order Fe XVIII 14.534Å line with transition 2p⁴(³P)3d²F_5/2-2p^{5 2}P_3/2, which blends with the inter-combination line of He-like N VI. In order to study the effects of these new identified weak lines on the electron temperature and density, we diagnose the two parameters using the intensity ratio of triplets of the He-like N VI and that of resonance lines of H- and He-like N. In the literature, there is a discrepancy of the temperatures deduced from N VI from other He-like ions, such as C V and O VII. The discrepancy disappears after the contribution of these weak lines is taken into account.