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More on electron/positron excesses…

By Hamish Johnston

Yesterday we reported that the excess number of cosmic positrons seen by PAMELA and other detectors could be coming from a nearby pulsar called Geminga — rather than from annihilating dark matter.

While the pulsar explanation is significant, the direct detection of dark matter via cosmic rays could bag someone a Nobel Prize. But alas, it is looking less likely that the current generation of cosmic ray detectors are up to the prize-winning task (assuming dark matter exists, and annihilates to create cosmic rays!).

One of the physicists arguing the pulsar explanation is Todor Stanev of the University of Delaware.

Stanev has also teamed up with researchers in Germany, Sweden and the US to show that electron and positron excesses seen by PAMELA and other experiments can also be attributed to the violent acceleration of matter that is believed to occur in the polar caps of certain supernovae.

According to the team, the energy spectra of electrons and positrons emitted in such a polar cap match what has been seen by several detectors. You can read about it in Physical Review Letters .

However, unlike the pulsar result, the researchers don’t seem to link recent electron- and positron-excess measurements to a specific astronomical event.

Instead, they seem to be suggesting that these positrons and electrons are present in the general background of cosmic rays reaching Earth.

So, does this mean that it will be even more difficult to winnow a dark-matter signal?

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