GRB 090926A was detected by both the Gamma-ray Burst Monitor and LargeArea Telescope (LAT) instruments on board the Fermi Gamma-ray SpaceTelescope. Swift follow-up observations began ~13 hr after the initialtrigger. The optical afterglow was detected for nearly 23 days posttrigger, placing it in the long-lived category. The afterglow is ofparticular interest due to its brightness at late times, as well as thepresence of optical flares at T0+105 s and later, which mayindicate late-time central engine activity. The LAT has detected a totalof 16 gamma-ray bursts; nine of these bursts, including GRB 090926A,also have been observed by Swift. Of the nine Swift-observed LAT bursts,six were detected by UVOT, with five of the bursts having bright,long-lived optical afterglows. In comparison, Swift has been operatingfor five years and has detected nearly 500 bursts, but has only seen~30% of bursts with optical afterglows that live longer than105 s. We have calculated the predicted gamma-ray fluence, aswould have been seen by the Burst Alert Telescope (BAT) on board Swift,of the LAT bursts to determine whether this high percentage oflong-lived optical afterglows is unique, when compared to BAT-triggeredbursts. We find that, with the exception of the short burst GRB 090510A,the predicted BAT fluences indicate that the LAT bursts are moreenergetic than 88% of all Swift bursts and also have brighter thanaverage X-ray and optical afterglows.

gamma-ray burst; general; gamma-ray burst; individual; GRB 090926A; shock waves

--