Investigating Multiband Variability Characteristics of the Typical BLSy1 Mrk 1220 through Intensive Swift Monitoring

Abstract Broad-line Seyfert 1 galaxies (BLSy1s), characterized by high black hole masses and low accretion rates, represent a crucial subclass of active galactic nuclei. We present a systematic analysis of the variability and X-ray spectral properties of the BLSy1 Mrk 1220 using 225 high-quality Swift observations from 2016 to 2018. Mrk 1220 exhibits significant multiwavelength variability across X-ray, ultraviolet (UV), and optical bands, with amplitude increasing with energy but remaining weaker than narrow-line Seyfert 1 galaxies (NLSy1s) with high accretion rates. Cross-correlation analysis reveals no significant interband time delays. The UV/optical–X-ray correlation weakens with increasing wavelength, with a maximum correlation coefficient of only 0.54. The relatively weak UV/X-ray connection may be attributed to the dynamic evolution of the coronal structure. Structure function (SF) analysis reveals a distinctive double “S” structure in optical/UV bands. Short-timescale SF parameters resemble those of NLSy1s, while X-ray bands show significantly longer decorrelation timescales, indicating an extended X-ray corona. SF spectral coefficients ( β ) across bands generally follow the damped random walk model, with only the U -band long-timescale β slightly elevated, potentially influenced by broad-line region radiation. X-ray spectral characteristics remain consistent between high and low flux states, showing weak soft excess below 1 keV. The stability of the soft excess parameters and power-law spectral index suggests unchanged coronal geometry across states, with X-ray variations primarily driven by intrinsic electron density fluctuations. These results provide valuable observational constraints for understanding the variability mechanisms and spectral characteristics of typical BLSy1s.