Analysis of the 3C 445 soft X-ray spectrum as observed by Chandra high-energy gratings

We present a detailed analysis of the soft X-ray emission of 3C 445 using an archival Chandra High Energy Transmission Grating (HETG) spectrum. Highly-ionized H- and He-like Mg, Si and S lines, as well as a resolved low-ionized Si Kα 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 G ratios indicate the lines originate from collisionally ionized plasmas. However, the detection of a resolved narrow Ne X radiative recombination continua (RRC) feature in the spectrum seems to prefer a photoionized environment. The spectrum is subsequently modeled with a photoionization model, and the results are compared with those of a collisional model. Through a detailed analysis of 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 $log\xi ={3.3}_{-0.3}^{+0.4}$ erg cm s⁻¹, $n_H=5_{-4.5}^{+15}\times {10}^{10}$ cm⁻³ and $N_H={2.5}_{-1.7}^{+3.8}\times {10}^{20}$ cm⁻². According to the calculated high density for the emitter, the measured velocity widths of the emission lines and the inferred radial distance (6 × 10¹⁴ – 8 × 10¹⁵ cm), we suggest the emission lines originating from matter are located in the broad line region (BLR). (paper)