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The widely adopted ``lamppost'' thermal reprocessing model, in which thevariable UV/optical emission is a result of the accretion disk reprocessing ofthe highly fluctuating X-ray emission, can be tested by measuring inter-bandtime lags in quasars spanning a range of X-ray power. This work reports theinter-band time lag in an apparently X-ray weak quasar, SDSSJ153913.47+395423.4. A significant cross-correlation with a time delay of $\sim33$ days (observed-frame) is detected in the Zwicky Transient Facility (ZTF)$g$ and $r$ light curves of SDSS J153913.47+395423.4. The observed X-ray powerseems to be too weak to account for the observed inter-band cross-correlationwith time delay. Hence the X-ray weak quasar SDSS J153913.47+395423.4 is eitherintrinsically X-ray normal (but observationally X-ray weak), or the X-rayemission is not the only mechanism to drive UV/optical variability. In theformer case, the required X-ray power is at least 19 times stronger thanobserved, which requires either an exceptionally anisotropic corona orCompton-thick obscuration. Alternatively, the Corona-heated Accretion diskReprocessing (CHAR) or the EUV torus models may account for the observed timelags.

The disk reverberation mapping of X-ray weak quasars: a case study of SDSS J153913.47+395423.4