Abstract
<title>Abstract</title> <p>We investigate the magnetic topology and trigger mechanism of the X1.5-class flare hosted by NOAA Active region 13006 on 10 May 2022, combining multi-wavelength observations with a Non-Force-Free-Field (NFFF) extrapolation. Observations show a circular brightening and a remote brightening that evolve into corresponding circular and remote flare ribbons during the impulsive phase. GONG H\(({\alpha})\) data reveal a pre-existing arc-shaped filament within the flaring region that stays stable throughout the flare and subsequently shows signs of eruption shortly afterward. The NFFF extrapolation of the active region depicts the presence of a three dimensional (3D) null exhibiting a fan-spine structure, a filament located beneath the fan dome within the flaring kernel. The extrapolated field accurately reproduces the observed coronal morphology and demonstrates a close spatial relationship between the fan footprints and the circular ribbon. The analysis also reveals a circular quasi-separatrix layer (QSL) surrounding the fan footprints, while enhanced \((|\mathbf{J}|/|\mathbf{B}|)\) near the null point indicates favorable conditions for current-sheet formation and reconnection. Measurements of the photospheric magnetic flux further indicate flux cancellation beneath the fan structure during the flare, implying a gradual buildup of magnetic free energy. The flux evolution implies that the sheared arcade beneath the fan may progressively transform into a flux rope, eventually leading to eruption. We propose that the flare is triggered by slipping magnetic reconnection within the circular QSL, leading to the initial circular brightening, followed by reconnection at the 3D null that amplifies the circular ribbon and produces the remote ribbon.</p>