I’m assuming that I am missing something obvious, but how did these isotopes of manganese and iron traverse the distance between the earth and this supernova in a timeframe that would make it contemporaneous with the supernova event??
Space dust, micro meteoroids, I would assume fell upon the earth a million years ago, or so.........................
Cosmic rays are produced from massive galactic events like a supernovae and consist of matter particles like Fe or Mn that have additional neutrons making them radioactive.
The explosion accelerates the particles to nearly the speed of light. If the SN was 10,000 light years away it would take about 10,000 years for the particles to get here. In addition because they are traveling at nearly the speed of light they would not appear to “decay” relative to our Earth clocks which are at rest.
Once the particles arrived at the Earth and came to rest they began to decay. From the known half-life of these isotopes that was about 2.6 billion years ago.