Firstly, the photon accelerator is a +3 keen undead-bane light crossbow. It also has a "flashlight" ability (120-foot cone of light, turned on or off as a swift action), which can also be used to dazzle opponents (-1 to hit). This effect can be boosted using the three "daylight capacitors" (the yellow tubes visible in the image to the right), which effectively allow the spell daylight to be cast thrice per day as a swift action (100-foot range, 3d6 damage to undead, 3d8 if sun-sensitive). There is also an ability to cast searing light (10d6 damage to undead, 10d8 if sun-sensitive) once per day, although this takes a standard action.
There is also a hidden 5-bolt magazine, in which specialised magic bolts can be stored that can then be retrieved as a free action. Finally, the crossbow creates a 5x5 area centred on the wielder (can be turned on or off as a free action) in which cloud effects are automatically disipated within one round (effect on gaseous forms unknown) and the creatures gain +4 to save against cloud-based effects.
Originally the photon accelerator also contained a greater truedeath weapon crystal (ghost-touch, +1d6 against undead, can crit undead), but this was removed and given to Angel.