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We report a strong soft X-ray excess in the BL--Lacartae $\gamma$-ray blazarOJ~287 during the long exposure in May 2015, amongst two of the latest \xmm~{}observations performed in May 2015 and 2018. In case of May 2015 observation, alogparabola model fits the EPIC-pn data well while a logparabola plus powerlawdescribes the overall simultaneous optical to X-ray spectra, suggesting theexcess as the synchrotron tail. This interpretation, however, is inconsistentwith the observed spectral break between near-infrared and optical spectra,attributed to standard disk around a supermassive black hole (SMBH). Based onthis, we considered two commonly invoked accretion disk based models in AGNs toexplain the soft excess: the cool Comptonization component in the accretiondisk and the blurred reflection from the partially ionized accretion disk. Wefound that both cool Comptonization and blurred reflection models provideequally good fit to the data and favor a super-heavy SMBH of mass $\sim10^{10}~M_\odot$. Further investigation of about a month long simultaneousX-ray and UV pointing observations revealed a delayed UV emission with respectto the 1.5-10 keV band, favoring X-ray reprocessing phenomenon as the dominantmechanism. The results suggest that the soft excess is probably caused bystrong light bending close to the SMBH. The detected soft excess in 2015 dataand its disappearance in 2018 data is also consistent with the presence ofaccretion disk emission, inferred from the NIR-optical spectral} break betweenMay 2013 to March 2016.

Strong Soft X-ray Excess in 2015 XMM-Newton Observation of BL--Lac OJ~287