2021A&A...652A..15H

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 2keV between these two observations, the soft X-ray emission in 0.3-2keV varies by a factor of ∼5. The luminosity of this soft component fades from ∼1.8x10⁴¹ to ∼3.7x10⁴⁰erg/s 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 10⁴⁴erg/s 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 ∼10⁵M_☉ 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.