I believe that is authentic but looks somewhat different than a standard Fisher autograph due to the confined space. Official Pix and Coolwaters usually have a good selection of Carrie Fisher autographs if you're looking.
When Carrie is signing in a tighter area(such as this smaller white rectangle), she changes her signature a bit, especially the F. Her f is usually huge so she changes it to fit in the tighter space. Many of these photos were signed at Celebration Japan and many were also sold on official pix. This is an official pix Fisher signed photo I found on ebay for comparison.