Neural relay
Neural relays, sometimes called Data Ghosts, are a freak of human technology in which an imprint of human consciousness can be captured in a communicator for a brief period of time after death. The Doctor describes it as "footprints on the beach, and the tide is coming in".
The messages conveyed in neural relays tend to be simple and relevant to the decedent's last conscious thoughts, and it's possible for them to interact verbally with those around them.
The length of neural relays can be anywhere from a few minutes to a few days, and towards the end, the decedent tends to repeat themselves over and over before it finally flickers out. The green bars on the communicator seem to be a reliable indicator as to how long the relay will last (i.e. the fewer bars, the shorter time).
