Supernova remnants

The Fermi Large Area Telescope (LAT) looks at gamma rays. A lot of them come from radiation produced when high energy electrons move in strong magnetic fields, but some come from the decays of pi-zero mesons. The latter tend to have a "minimum energy" and so their spectrum has some characteristic features that differ from the former.

According to a paper accepted by Science magazine (I can't link to it, but I saw an early copy), the Fermi LAT team was able to

1. Tell where a gamma ray came from
   
2. Tell what energy it had
   
3. Accumulate enough statistics in their sky map to be able to tell that
   
   • Nearby supernova remnants are bright in the gamma ray spectrum
     
   • Their spectrum of gamma rays is consistent with having a lot of gamma rays from pi-zero decays
     

The MAGIC and VERITAS experiments showed that the supernova remnants produce pretty high energy (TeV) gamma rays, but couldn't tell the difference between "bremstrahlung" radiation and pi-zero decays. LAT can, and so we have proof for the first time that supernova remnants generate high energy cosmic rays. That's not a huge surprise, but nobody has a good model for how it works yet--there's always some detail that doesn't fit. Notice that this is the remnant, not the initial explosion. Therefore the high energy particles are coming from something else, likely the shock fronts colliding.

Pi-zero's come from interactions of protons and neutrons (not so much from electrons), and tell of high energy cosmic rays (mostly protons) coming from the area. Where you have pi-zero's you also have pi-plus and pi-minus, and therefore you also have neutrinos.

So, does IceCube see neutrinos coming from supernova remnants? Neither gamma rays nor neutrinos are bent by galactic magnetic fields...

Well, Naoko was just allowed(*) to unblind part of her analysis yesterday (certain important details are left out until we're sure the analysis isn't biased to look for whatever happened to pop up first). The results haven't been approved for publication yet. But I can say that our statistics are far lower than LAT's. Like less than 3 dozen events. True, a couple of them are doozies--PeV level energies--but you just can't do anywhere nearly as clean a sky map with that few events. But it is interesting, and results should be public shortly.

Disclaimer: I had nothing to do with either of the analyses. Congratulations to both.

---

(*) That conference call discussion was long. Someone else had a not-quite-mature analysis that overlapped, and which would be much less valuable if the whole of Naoko's analysis was unblinded. The discussions with the mute button on were much franker than those with it live.