A new IISS Strategic Comment—“North Korea’s Nuclear Test: Continuing Reverberations”—confirms statements by US and South Korean officials that the NORKs gave the Chinese all of twenty minutes notice before the nuclear test:
Chinese sources confirm that North Korea informed Beijing 20 minutes in advance of the test, enabling China to notify other governments. But this offered little solace to Chinese officials, who were deeply angered by Kim’s actions, and by the major setback to China’s efforts to fashion a multilateral settlement.
North Korea purportedly informed China that the anticipated yield from the test would be approximately four kilotons, i.e. 4,000 tons of TNT equivalent. A test of this magnitude, though smaller than the explosive power in the first tests of other new nuclear entrants, would have represented a significant achievement. However, following analysis of radioactive debris, the Office of the US Director of National Intelligence announced that the explosive yield was ‘less than a kiloton’; additional official disclosures and private estimates estimated the yield as low as 200 tons of TNT equivalent. US intelligence sources also disclosed that the fissile material used in the test was plutonium reprocessed from spent fuel removed from the North’s reactor at Yongbyon, not highly enriched uranium. The test was not an outright failure, but it was far from a full success.
And, no, IISS never identifies the author. You just have to make an educated guess.