  Solar flare emissions at Halpha and EUV/UV wavelengths often appear in the
form of two ribbons, which has been regarded as evidence for a typical
configuration of solar magnetic reconnection. However, such a ribbon structure
has rarely been observed in hard X-rays (HXRs), although it is expected as
well. In this letter, we report a ribbon-like HXR source observed with the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) at energies as
high as 25-100 keV during the 2005 May 13 flare. For a qualitative
understanding of this unusual HXR morphology, we also note that the source
active region appeared in a conspicuous sigmoid shape before the eruption and
changed to an arcade structure afterward as observed with the Transition Region
and Coronal Explorer (TRACE) at 171 A. We suggest that the ribbon-like HXR
structure is associated with the sigmoid-to-arcade transformation during this
type of reconnection.
