# On "cosmic flux" units

I've trouble understanding the following graph, taken from Wikipedia: It's supposed to show the cosmic ray flux vs particle energy. I've never seen a "flux" written in these units... Why $${GeV}^{-1}$$?

• Why not? This is a distribution over particle energy. That factor is just saying "per unit of energy". Feb 27 at 11:58
• Spectra are dN/dE which is per unit energy. But be aware that in astroparticle research, often that same information is shown multiplied by some E^(-2.7) or some such power law. That is because this spectrum is so extremely steep. So always check the vertical axis units first with those cosmic ray spectrum plots ...
– rfl
Feb 27 at 14:21

It is a differential flux. If you wanted to know the total particle rate per unit area, per unit solid angle, per unit time, with units $$\rm m^{-2}\,sr^{-1}\,s^{-1}$$, you would have to choose an energy interval that you care about and integrate the curve in the figure.
Some authors would write a monstrous differential symbol rather than $$F$$, like
$$F = \frac{\mathrm dN}{\mathrm dA\ \mathrm d\Omega\ \mathrm dt\ \mathrm dE}$$
to make explicit that the number of particles observed $$N$$ depends on your detector’s area $$A$$, its solid angle acceptance $$\Omega$$, your experiment’s running time $$t$$, and your choice of energy window.
The horizontal lines on the diagram are probably integrated over $$4\pi$$ steradians for all particle energies higher than the intersection of the horizontal line and the blue differential flux curve.