Long-term X-ray evolution of SDSS J134244.4+053056.1. A more than 18 year-old, long-lived IMBH-TDE candidate

SDSS J134244.4+053056 is a tidal disruption event candidate with strong temporal coronal line emitters and a long fading, mid-infrared dust echo. We present detailed analyses of X-ray emission from a Swift/XRT observation in 2009 and the most recent XMM-Newton/pn observation in 2020. The two spectra can be modeled with hard and soft components. While no significant variability is detected in the hard component above 2 keV between these two observations, the soft X-ray emission in 0.3-2 keV varies by a factor of ∼5. The luminosity of this soft component fades from ∼1.8 × 10⁴¹ to ∼3.7 × 10⁴⁰ erg s⁻¹ from the observation in Swift to that of XMM-Newton, which are 8 and 19 years after the outburst occurred, respectively. The evolution of luminosity matches with the t^−5/3decline law well; there is a soft X-ray peak luminosity of 10⁴⁴ erg s⁻¹ at the time of the optical flare. Furthermore, the spectra of the soft component harden slightly in the decay phase, in which the photon index Γ varies from 4.8_−0.9^+1.2 to 3.7 ± 0.5, although they are consistent with each other if we consider the uncertainties. Additionally, by comparing the BH mass estimate between the M − σ correlation, the broad Hα emission, and the fundamental plane relation of BH accretion, we find that a value of ∼10⁵ M_⊙ is favored. If so, taking its X-ray spectral variation, luminosity evolution, and further support from theory into account, we suggest that SDSS J134244.4+053056 is a long-lived tidal disruption event candidate lasting more than 18 years with an intermediate-mass black hole.