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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 di ﬀ erent 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 di ﬀ erences 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 . 2 σ 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.