The Origin of the Vanishing Soft X-Ray Excess in the Changing-look Active Galactic Nucleus Mrk 590

We have studied the nature and origin of the soft X-ray excess detected in the interesting changing-look active galactic nucleus (CLAGN) Mrk 590 using two decades of multiwavelength observations from XMM-Newton, Suzaku, Swift, and NuSTAR. In light of vanishing soft excess in this CLAGN, we test two models, "the warm Comptonization" and "the ionized disk reflection," using extensive UV/X-ray spectral analysis. Our main findings are as follows: (1) the soft X-ray excess emission, last observed in 2004, vanished in 2011 and never reappeared in any of the later observations; (2) we detected a significant variability (~300%) in the observed optical-UV and power-law flux between observations with the lowest state (L _bol = 4.4 × 10⁴³ erg s^-1, in 2016) and those with the highest state (L _bol = 1.2 × 10⁴⁴ erg s^-1, in 2018); (3) the UV and power-law fluxes follow the same temporal pattern; (4) the photon index showed a significant variation ( ${\rm{\Gamma }}={1.88}_{-0.08}^{+0.02}$ and ${\rm{\Gamma }}={1.58}_{-0.03}^{+0.02}$ in 2002 and 2021, respectively) between observations; (5) no Compton hump was detected in the source spectra, but a narrow Fe Kα line is present in all observations; (6) we detected a high-energy cutoff in power-law continuum ( ${92}_{-25}^{+55}\,\mathrm{keV}$ and ${60}_{-08}^{+10}\,\mathrm{keV}$ ) with the latest NuSTAR observations; (7) the warm Comptonization model needs an additional diskbb component to describe the source UV bump; and (8) there is no correlation between the Eddington rate and the soft excess as found in other CLAGNs. We conclude that, given the spectral variability in UV/X-rays, the ionized disk reflection or the warm Comptonization models may not be adequate to describe the vanishing soft excess feature observed in Mrk 590.