Can someone tell me what this sidearm is?
Tis an RK-3 http://starwars.wikia.com/wiki/RK-3
It was also featured in Star Wars Battlefront 2 (2017)
Also known in Legends as Relby-K23 and unofficially as the Cloud City Pistol.
If you look at the following chart taken from the wookieepedia article on blaster pistols, you can see that the Relby-K23 (column 1/row 5) is approximately the same.
You can see both guns have a larger flaring tip and something sticking out of the bottom in front of the trigger.
Additionally this is the correct period for the Stinger Pistol to be in use.
The stinger pistol was a model of blaster pistol used during the Galactic Civil War.
And the legends article says:
The BlasTech Industries Relby-k23 blaster pistol was a sidearm banned from civilian purchase and used by agents of the Galactic Empire.
So allowing for deviation due to animation style and allowing Legends influence on Rebels, I'd identify this gun as a Stinger Pistol. Admittedly, it is very plausible that the show creators had no specific model in mind, and this is just another gun. One that will end up being classified at some point in the future by the next edition of Star Wars: The Ultimate Visual Guide.
Couldn't find a definitive source, but looking through Wookipedia it looks like it might be an SC-4 Blaster pistol http://starwars.wikia.com/wiki/SC-4_blaster_pistol