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SB 12.6.61

vaiśampāyana-śiṣyā vai
 carakādhvaryavo ’bhavan
yac cerur brahma-hatyāṁhaḥ
 kṣapaṇaṁ sva-guror vratam
Synonyms: 
vaiśampāyana-śiṣyāḥ — the disciples of Vaiśampāyana; vai — indeed; caraka — named the Carakas; adhvaryavaḥ — authorities of the Atharva Veda; abhavan — became; yat — because; ceruḥ — they executed; brahma-hatyā — due to the killing of a brāhmaṇa; aṁhaḥ — of the sin; kṣapaṇam — the expiation; sva-guroḥ — for their own guru; vratam — the vow.
Translation: 
The disciples of Vaiśampāyana became authorities in the Atharva Veda. They were known as the Carakas because they executed strict vows to free their guru from his sin of killing a brāhmaṇa.