  The widely adopted ``lamppost'' thermal reprocessing model, in which the
variable UV/optical emission is a result of the accretion disk reprocessing of
the highly fluctuating X-ray emission, can be tested by measuring inter-band
time lags in quasars spanning a range of X-ray power. This work reports the
inter-band time lag in an apparently X-ray weak quasar, SDSS
J153913.47+395423.4. A significant cross-correlation with a time delay of $\sim
33$ 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 power
seems to be too weak to account for the observed inter-band cross-correlation
with time delay. Hence the X-ray weak quasar SDSS J153913.47+395423.4 is either
intrinsically X-ray normal (but observationally X-ray weak), or the X-ray
emission is not the only mechanism to drive UV/optical variability. In the
former case, the required X-ray power is at least 19 times stronger than
observed, which requires either an exceptionally anisotropic corona or
Compton-thick obscuration. Alternatively, the Corona-heated Accretion disk
Reprocessing (CHAR) or the EUV torus models may account for the observed time
lags.
