Analysis of the 3C445 Soft X-ray Spectrum as Observed by Chandra high-energy gratings

Abstract: We present a detailed analysis of the soft X-ray emission of 3C445 using an archival Chandra HETG spectrum. Highly-ionized H- and He-like Mg, Si and S lines, as well as a resolved low-ionized Si K$\alpha$ line, are detected in the high resolution spectrum. The He-like triplets of Mg and Si are resolved into individual lines, and the calculated R ratios indicate a high density for the emitter. The low values of the G ratios indicate the lines originate from collisionally ionized plasmas. However, the detection of a resolved narrow Ne X RRC feature in the spectrum seems to prefer to a photoionized environment. The spectrum is subsequently modelled with a photoionization model, and the results are compared with that of a collisional model. Through a detailed analysis on the spectrum, we exclude a collisional origin for these emission lines. A one-component photoionization model provides a great fit to the emission features. The best-fit parameters are log$\xi$ = $3.3^{+0.4}_{-0.3}$ erg cm s$^{-1}$, $n_{H}$ = $5^{+15}_{-4.5}\times10^{10}$ cm$^{-3}$ and $N_{H}$ = $2.5^{+3.8}_{-1.7}\times10^{20}$ cm$^{-2}$. According to the calculated high density for the emitter, the measured velocity widths of the emission lines and the inferred the radial distance (6 $\times$ $10^{14}$ - 8 $\times$ $10^{15}$ cm), we suggest the emission lines originating from matter locate in the broad line region (BLR).