  Filament eruptions and hard X-ray (HXR) source motions are commonly observed
in solar flares, which provides critical information on the coronal magnetic
reconnection. This Letter reports an event on 2005 January 15, in which we
found an asymmetric filament eruption and a subsequent coronal mass ejection
together with complicated motions of HXR sources during the GOES-class X2.6
flare. The HXR sources initially converge to the magnetic polarity inversion
line (PIL), and then move in directions either parallel or perpendicular to the
PIL depending on the local field configuration. We distinguish the evolution of
the HXR source motion in four phases and associate each of them with distinct
regions of coronal magnetic fields as reconstructed using a non-linear
force-free field extrapolation. It is found that the magnetic reconnection
proceeds along the PIL toward the regions where the overlying field decreases
with height more rapidly. It is also found that not only the perpendicular but
the parallel motion of the HXR sources correlates well with the HXR lightcurve.
These results are discussed in favor of the torus instability as an important
factor in the eruptive process.
