In-depth investigation of the physical origin of the soft X-ray excess in PG 1448+273

We present systematic X-ray spectral and variability analyses for the narrow-line Seyfert 1 galaxy (NLsy1), PG 1448+273, using the data from two long-exposure XMM-Newton observations of the source in the different flux states. These X-ray spectral analyses indicate that the two main-stream models of soft X-ray excess, the relativistically smeared reflection scenario and the warm corona scenario, show a similar statistical fit to the data sets. However, the warm corona scenario requires a black hole mass that is much larger than that estimated by the single-epoch optical spectroscopy and the X-ray variability scaling technology to alleviate the conflict between the X-ray and optical/ultraviolet data. The time-varying analyses reveal that PG 1448+273 has significant X-ray variability, especially in the low state. The features of X-ray spectra and variability show significant differences between the high and low states. At the low state, a highly variable ultra-fast outflow feature appears in the X-ray spectra, and the X-ray spectra show a very significant soft X-ray excess. At the high state, a `dip', or oscillation, feature with ∼3.2σ confidence level emerges in the power spectral density at ν ∼ 10^3.6 Hz, which is a unique time signature of the reflected X-ray reprocessing. Meanwhile, we also detect a tentative soft lag. It is shown that these unique spectral features and variation behaviours can be explained self-consistently in the relativistically smeared reflection scenario, in which the geometric position of the corona changes. Our results suggest that PG 1448-273 can be used as an ideal object to further study the details of the relativistic reflection process.