Underground muon energy spectrum and its relevance for muon astronomy

A striking result from the NUSEX (1) and SOUDAN (2) experiments is the discovery of a high flux of muons from the direction of Cygnus X-3 displaying the typical 4.8 hour time modulation of this source. The differential energy spectrum of these muons is much flatter (γ∼2) than that for ordinary atmospheric muons (γ∼3.71). This implies a very different shape for the local deep underground muon energy spectrum. We show that a measurement of the content of high-energy muons (> 500 GeV) in underground experiments is a strong signature of their origin.