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The X-ray variability of active galactic nuclei (AGN) carries crucialinformation about the X-ray radiation mechanism. We performed a systematicstudy of the X-ray short-term (1-100 ks timescale) variability for a largesample of 78 Seyferts with 426 deep XMM-Newton observations. In this paper, wepresent the time-averaged spectra and rms spectra for the entire sample, whichshow a variety of properties. Based on the spectral shape, we divide the rmsspectra into five subtypes and the time-averaged spectra into four subtypes.The most common shape of the rms spectra is concave-down where the rms peaks at$\sim$ 1 keV. We find that different sources can show similar time-averagedspectra and rms spectra. However, there is no one-to-one mapping between thesubtypes of the time-averaged spectra and rms spectra, as similar time-averagedspectra can be accompanied by different rms spectra, and vice versa. This islikely because different physical mechanisms can produce similar rms spectra.For every subtype of the time-averaged spectra, we report its preferredsubtypes of the rms spectra in both low- and high-frequency bands. We alsocompare the statistical properties for different subtypes, such as the blackhole mass and Eddington ratio. Finally, we investigate the rms in the FeK$\alpha$ line regime and find that those with a broad and extended red-wingprofile tend to show stronger variability than those showing a narrow orrelatively symmetric profile. Our results demonstrate the necessity ofperforming joint spectral and variability modeling in order to understand themechanism of the X-ray emission in AGN. All of the rms spectra have been madepublicly available.

A Systematic Study of the Short-Term X-ray Variability of Seyfert Galaxies I. Diversity of the X-ray Rms Spectra