  The occurrence of X-class solar flares and their potential impact on the
space weather often receive great attention than other flares. But predicting
when and where an X-class flare will occur is still a challenge. With the
multi-wavelength observation from the Solar Dynamics Observatory and FengYun-
3E satellite, we investigate the triggering of a GOES X1.0 flare occurring in
the NOAA active region (AR) 12887. Our results show that this unique X-class
flare is bred in a relatively small but complex quadrupolar AR. Before the
X-class flare, two filaments (F1 and F2) exist below a null-point topology of
the quadrupolar AR. Magnetic field extrapolation and observation reveal that F1
and F2 correspond to two magnetic flux ropes with the same chirality and their
adjacent feet rooted at nonconjugated opposite polarities, respectively.
Interestingly, these two polarities collide rapidly, accompanied by
photospheric magnetic flux emergence, cancellation and shear motion in the AR
center. Above this site, F1 and F2 subsequently intersect and merge to a longer
filament (F3) via a tether-cutting-like reconnection process. As a result, the
F3 rises and erupts, involving the large-scale arcades overlying filament and
the quadrupolar magnetic field above the AR, and eventually leads to the
eruption of the X-class flare with a quasi-X-shaped flare ribbon and a coronal
mass ejection. It suggests that the rapid collision of nonconjugated opposite
polarities provides a key condition for the triggering of this X-class flare,
and also provides a featured case for flare trigger mechanism and space weather
forecasting.
