  Two X-class solar flares occurred on 2017 September 6 from active region NOAA
12673: the first one is a confined X2.2 flare, and it is followed only $\sim 3$
hours later by the second one, which is the strongest flare in solar cycle 24,
reaching X9.3 class and accompanied with a coronal mass ejection. Why these two
X-class flares occurred in the same position with similar magnetic
configurations, but one is eruptive while the other is not? Here we track the
coronal magnetic field evolution via nonlinear force-free field extrapolations
from a time sequence of vector magnetograms with high cadence. A detailed
analysis of the magnetic field shows that a magnetic flux rope (MFR) forms and
grows gradually before the first flare, and shortly afterwards, the MFR's
growth is significantly enhanced with a much faster rise in height, from far
below the threshold of torus instability to above it, while the magnetic twist
only increases mildly. Combining EUV observations and the magnetic field
extrapolation, we found that overlying the MFR is a null-point magnetic
topology, where recurrent brightening is seen after the first flare. We thus
suggest a scenario to interpret the occurrence of the two flares. The first
flare occurred since the MFR reached a high enough height to activate the null
point, and its continuous expansion forces the null-point reconnection
recurrently. Such reconnection weakens the overlying field, allowing the MFR to
rise faster, which eventually crosses the threshold of torus instability and
triggers the second, eruptive flare.
