Optical Variability of the Dwarf AGN NGC 4395 from the Transiting
  Exoplanet Survey Satellite
  We present optical light curves from the Transiting Exoplanet Survey
Satellite (TESS) for the archetypical dwarf active galactic nucleus (AGN) in
the nearby galaxy NGC 4395 hosting a $\sim 10^5\,M_\odot$ supermassive black
hole (SMBH). Significant variability is detected on timescales from weeks to
hours before reaching the background noise level. The $\sim$month-long, 30
minute-cadence, high-precision TESS light curve can be well fit by a simple
damped random walk (DRW) model, with the damping timescale $\tau_{\rm DRW}$
constrained to be $2.3_{-0.7}^{+1.8}$~days ($1\sigma$). NGC 4395 lies almost
exactly on the extrapolation of the $\tau_{\rm DRW}-M_{\rm BH}$ relation
measured for AGNs with BH masses that are more than three orders of magnitude
larger. The optical variability periodogram can be well fit by a broken power
law with the high-frequency slope ($-1.88\pm0.15$) and the characteristic
timescale ($\tau_{\rm br}\equiv 1/(2\pi f_{\rm br})=1.4_{-0.5}^{+1.9}\,$days)
consistent with the DRW model within 1$\sigma$. This work demonstrates the
power of TESS light curves in identifying low-mass accreting SMBHs with optical
variability, and a potential global $\tau_{\rm DRW}-M_{\rm BH}$ relation that
can be used to estimate SMBH masses with optical variability measurements.
