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Two X-class solar flares occurred on 2017 September 6 from active region NOAA12673: 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 twoX-class flares occurred in the same position with similar magneticconfigurations, but one is eruptive while the other is not? Here we track thecoronal magnetic field evolution via nonlinear force-free field extrapolationsfrom a time sequence of vector magnetograms with high cadence. A detailedanalysis of the magnetic field shows that a magnetic flux rope (MFR) forms andgrows gradually before the first flare, and shortly afterwards, the MFR'sgrowth is significantly enhanced with a much faster rise in height, from farbelow the threshold of torus instability to above it, while the magnetic twistonly increases mildly. Combining EUV observations and the magnetic fieldextrapolation, we found that overlying the MFR is a null-point magnetictopology, where recurrent brightening is seen after the first flare. We thussuggest a scenario to interpret the occurrence of the two flares. The firstflare occurred since the MFR reached a high enough height to activate the nullpoint, and its continuous expansion forces the null-point reconnectionrecurrently. Such reconnection weakens the overlying field, allowing the MFR torise faster, which eventually crosses the threshold of torus instability andtriggers the second, eruptive flare.

Continuous Null-Point Magnetic Reconnection Builds Up a Torus Unstable Magnetic Flux Rope Triggering the X9.3 Flare in Solar Active Region~12673