The receiver was not connected to an antenna.
That was the problem.
At 04:13, the recorder began writing bursts of narrowband noise to disk. Nine bursts. Each separated by exactly forty-nine seconds. Each shorter than the last. The final burst lasted less than a blink and still produced the clearest spectrogram.
The image looked like a staircase descending into a square.
Capture log
04:13:00 — burst one. broad, low, damaged.
04:13:49 — burst two. same envelope, sharper edges.
04:14:38 — burst three. carrier visible at the center.
04:15:27 — burst four. harmonic line appears above carrier.
04:16:16 — burst five. false silence before onset.
04:17:05 — burst six. center line splits.
04:17:54 — burst seven. left channel delayed by one sample.
04:18:43 — burst eight. no audible sound; spectrogram intact.
04:19:32 — burst nine. staircase descending into a square.
Interpretation
There was a tenth interval.
At 04:20:21, the recorder did not write a file. The directory updated anyway. The parent folder changed permissions to read-only and renamed itself black-sigil-receive without a recorded process event.
Member three suggested that the ninth signal was a checksum.
Member one suggested that the tenth was not missing; it was waiting for a listener.
Member two powered down the machine.
Decision
No decoding attempt was made.
There is a difference between an unread message and an invitation. The former asks for interpretation. The latter asks for entry.
We keep the nine files offline.
We do not look for the tenth.