Small amounts of longer-lived isotopes used in radioimmunoassays -- such as cobalt-57
tritium, and carbon-14 -- enter the sewage system, she says, though they are "in such a diluted state that they pose no health risk.
The flatter curve matches the slower decay of another isotope, cobalt-57, which the supernova produced in smaller amounts, the group notes.
One way to explain the greater emissions, note Suntzeff and his colleagues, is to assume that the supernova produced a ratio of cobalt-57 to cobalt-56 five times the ratio typical in our solar system.
GRO measured the spectra of gamma rays from 1987A, which should allow researchers to calculate the amount of cobalt-57 produced by the supernova.