Mercenaries

Mercenaries are independent owners and crews of starships. The Head of a House can purchase the service of mercenaries to bolster their fleets in combat, but doing so tends to spoil any element of surprise and alert the owner of a targeted colony that they face an imminent attack.

Codex
"All houses maintain a permanent military fleet, but there is a practical limit to how many vessels they can afford, especially since it is a major financial drain in times of peace. Mercenaries offer a way for houses to temporarily add firepower to their fleet for a short period of time.

Mercenaries come from many sources; defectors from another house, criminals who prefer a dangerous life on the fringes, or even the remnants of houses that have fallen and been evicted from their colonies. These lowlifes travel between systems, hunting for news and signs of brewing conflict, offering their services at a price. It is said that a good mercenary captain knows when a ruler is going to rebel even before he does."