  We present the interstellar scintillation analysis of fast radio burst (FRB)
20220912A during its extremely active episode in 2022 using data from the
Five-hundred-meter Aperture Spherical Radio Telescope (FAST). We detect a
scintillation arc in the FRB's secondary spectrum, which describes the power in
terms of the scattered FRB signals' time delay and Doppler shift. The arc
indicates that the scintillation is caused by a highly localized region of the
ionized interstellar medium (IISM). Our analysis favors a Milky Way origin for
the localized scattering medium but cannot rule out a host galaxy origin. We
present our method for detecting the scintillation arc, which can be applied
generally to sources with irregularly spaced bursts or pulses. These methods
could help shed light on the complex interstellar environment surrounding the
FRBs and in our Galaxy.
