You might be reading this because you've noticed that some validator on mainnet are earning extra rewards.
This is due to a new feature of the beacon chain added in the Altair fork called sync committees. Sync committees are groups of 512 validators which are randomly selected every 256 epochs (~27 hours) to perform additional duties. They sign a message called a
SyncCommitteeMessage every slot, which is similar to an attestation. For more information, you can read Vitalik's write-up in the annotated spec:
So far none of the explorers show the sync committees explicitly, so they are chronicled here in the meantime.
Some of the early sync committees are included below as
sync_committee_$epoch.txt files, where
$epoch is the epoch at which that committee begins performing duties. Each line of the file is a validator index (512 total). You'll notice that the first two committees are identical: this is intentional (it si