Sudden radio black-out on low bands

click above to enlarge view

Yesterday morning during the 3740 kHz QSO with W4UOA, N4NR and K4JPM, we noticed a sudden, extreme drop in signal level on 75 m. We switched to 60 m. Carl and I could not hear each other, and Dennis could barely hear us. We were keeping in touch on the chat room. A brief visit to 40 m had similar results. We suspected a sudden solar event, and Dennis pulled up the graph shown above and shared the link with us via the chat room. The time stamp on the page bottom was just after we noticed the big signal drop. Dennis later added the red arrows pointing to the big spike.
He reported later spikes in an update to his Facebook post and included this link to the flare photos published by NASA.

The current X-ray Flux graphs are available here.