Equivalent width (EW) ratios for characteristic absorption lines of O-like Fe xix. The two lines with λ =13.521 Å and 13.492 Å are from the ground level. The line with λ =13.420 Å is from the first excited level. Dashed (ξ = 2.3), solid (ξ = 2.4, where ion concentration reaches its maximum), and dotted (ξ = 2.5) lines indicate different ionization parameters of the photoionized plasma.