This is most likely up to GM Discretion
There are no specific rules regarding the interaction of solar flares and weapon accessories and this has not yet been addressed by an official FAQ, leaving this particular combination within a GM’s purview to adjudicate. That said, I would personally be inclined to allow it.
In terms of game balance, there are no particular concerns with allowing weapon accessories to work with solar flares. The solar flare functions as a small arm with damage closer to a long arm once the bonus damage of a solarian weapon crystal is included. Since weapon accessories function equivalently for small and long arms, the designers clearly don’t see an issue with higher damage ranged weapons gaining the benefit of weapon accessories.
In terms of narrative, the solar flare is described as:
This weapon appears to be made of either glowing light or solid darkness, as determined by the appearance of your solar manifestation, but it can take any general shape you desire. The shape of normal ranged weapons—such as pistols, crossbows, and handcannons are fairly common—but more esoteric manifestations such as a shimmering crystal that launches fragments of itself, a dark aura that forms directly onto your foes, or an orb of stellar energy that releases beams of pure light are also possible. Your solar flare’s general design has no impact on its function, and it doesn’t give the weapon any weapon special properties.
Although the text states that the shape of the flare has no impact on its function, narratively the more esoteric manifestations lack a physical form that you could mount an accessory on. On the other hand, a manifestation of “solid darkness” in the shape of a conventional weapon could easily have the right shape and solidity to carry an accessory. As a result, I would be inclined to allow more conventional manifestations to function with weapon accessories.
However, a significant concern with accessories is what happens when the solar flare is dismissed back to mote form. For the solarian weapon crystals, they explicitly remain within the mote until the next time the weapon is created. For accessories, I would be inclined to rule that they detach and drop off, and need to be reattached next time.
See the following Paizo forum thread for a brief discussion of whether this is possible.