The dead Donnager crew's electromagnetic battle record, meant to identify the attackers' engines without asserting a conclusion on their behalf.
Lieutenant Kelly carries the data cube off the doomed Donnager, making its delivery a final act of military urgency. Fred Johnson has Tycho open it, but its contents appear to be only raw electromagnetic sensor information from the pursuit by six unknown ships and the battle itself; neither Tycho nor the Rocinante initially finds a hidden message.
James Holden, Naomi Nagata, and Miller instead infer that the raw form is deliberate. The data lets the recipient independently compare engine radiation and heat signatures against known ship records, rather than accepting an accusation that may be wrong. The Rocinante identifies standard light-cruiser engines built at Bush Shipyards, potentially connecting the attackers to Earth. Holden broadcasts the raw data across the system to prevent it being buried; Miller fears the evidence is another planted attribution designed to expand the war. The cube has supplied a lead, not an answer.
The item is a compact Martian military data cube holding high-resolution electromagnetic and light-spectrum sensor records from the Donnager's final pursuit and battle.
The cube preserves the dead flagship's forensic evidence and becomes a public, dangerous clue in the search for who is provoking the war.