From kiloparsecs to fermi: deciphering the messages of cosmic gamma rays

Roland Diehl got us started today with a review on astronomical observations of gamma rays from rare isotopes.

In the last few decades there has been an impressing advance in astronomical instruments and techniques that have provided us a variety of probes to look into the guts of our galaxy, and beyond. He showed us spectra taken with the INTEGRAL observatory, as well as neat simulations of supernovae (SNe) occurring within a 1 kiloparsec* square.

There are plenty of gamma lines out there and we need to understand their messages. They come from radioactive rare isotopes, which in turn are nucleosynthesis byproducts. By combining experimental nuclear data with observations and theoretical predictions we can infer the gamma ray source dilution time, yields, production mechanism, etc.

For example, 26Al has been used to characterize our galaxy: by calculating how much mass of 26Al is there we can then estimate how many SNe occur per century (about 2). One interesting open question is the correlation between the 511keV line from positron annihilation coming from the beta+ decay of 26Al and its 1809keV line.

Other important gamma emitters and their sources are:

56Ni, 57Ni, 44Ti, 22Na(?) – Interstellar Medium, SNe

44Ti – Inner SNe ejecta

26Al, 60Fe – Cosmic nucleosynthesis, massive stars

*For nuclear physicists: 1 kiloparsec = 30.86×10¹⁵ km = 3.086×10³⁴ fermi.