  Supermassive black hole binaries (SMBHBs) are products of galaxy mergers, and
are important in testing Lambda cold dark matter cosmology and locating
gravitational-wave-radiation sources. A unique electromagnetic signature of
SMBHBs in galactic nuclei is essential in identifying the binaries in
observations from the IR band through optical to X-ray. Recently, the flares in
optical, UV, and X-ray caused by supermassive black holes (SMBHs) tidally
disrupting nearby stars have been successfully used to observationally probe
single SMBHs in normal galaxies. In this Letter, we investigate the accretion
of the gaseous debris of a tidally disrupted star by a SMBHB. Using both
stability analysis of three-body systems and numerical scattering experiments,
we show that the accretion of stellar debris gas, which initially decays with
time $\propto t^{-5/3}$, would stop at a time $T_{\rm tr} \simeq \eta T_{\rm
b}$. Here, $\eta \sim0.25$ and $T_{\rm b}$ is the orbital period of the SMBHB.
After a period of interruption, the accretion recurs discretely at time $T_{\rm
r} \simeq \xi T_b$, where $\xi \sim 1$. Both $\eta$ and $\xi$ sensitively
depend on the orbital parameters of the tidally disrupted star at the tidal
radius and the orbit eccentricity of SMBHB. The interrupted accretion of the
stellar debris gas gives rise to an interrupted tidal flare, which could be
used to identify SMBHBs in non-active galaxies in the upcoming transient
surveys.
