Neutron astronomy

Abstract

Neutrons travel along straight lines in free space, but only survive for a distance which depends on their energy. Thus, detecting neutrons in space in principle provides directional and distance information. Apart from secondary neutrons produced by cosmic-ray interactions in the Earth atmosphere, which are the dominant background, direct neutron emission is caused by solar flares, with clear time correlation with X-rays, which can be measured from few tens MeV up to few GeV. There is no detectable astrophysical source up to the PeV scale, when neutrons coming from supernova remnants may reach the Earth before decaying. In addition, ultra high energy neutrons are the most plausible explanation for the measured anisotropy of cosmic-ray showers produced in the atmosphere above 10 18 eV. From the GeV to the PeV scale, the expected neutron flux is very low and not too different from the antiproton flux, as the same cosmic-ray collisions with the interstellar medium which can produce antiprotons can also produce neutrons and antineutrons. This background flux of cosmic-ray neutrons is very low and has not yet been detected. Measuring the neutron energy spectrum in space is a very effective way of searching for decays of exotic particles. For example, dark matter could consist of Weakly Interacting Massive Particles (WIMPs), which may annihilate into final states with particle and antiparticle pairs. Consequently, a number of indirect WIMP searches are being carried on, focusing on positron and antiproton spectra. However, no experiment is presently foreseen to look for "bumps" in the neutron energy spectrum, which is virtually background free. Here we consider the implications of a measurement of the neutron energy spectrum in astronomy and astrophysics and list the interesting energy regions in the search for WIMPs.