Solars have the following weapon attack on their stat block:
Slaying Longbow. Ranged Weapon Attack: +13 to hit, range 150/600 ft., one target. Hit:Hit: 15 (2d8 + 6) piercing damage plus 27 (6d8) radiant damage. If the target is a creature that has 100 hit points or fewer, it must succeed on a DC 15 Constitution saving throw or die.
If you defeat a solar, can you take its Slaying Longbow and use it?