What is core government debt? In fixed income, the yield on the 10-year US Treasury fell 0.07
  percentage points to 0.56 per cent as investors retreated to the
  safety of core government debt


From : https://www.ft.com/content/26ea5ef9-0619-4e50-b605-58e36d3fc4d9

What is 'core government debt' and how is it raised, since the quoted statement seems to imply it is different from US Treasury bonds.
There’s no formal meaning, but it would be the perceived safest government bonds. The quote indicates thar US Treasury bonds are part of that core. The 10-year yield fell, which meant that price went up.