# Converting between photons/s/cm^2 to spectral flux (e.g. Jy) - Fermi LAT Sensitivity?

1. Jul 13, 2011

### zhermes

I'm trying to figure out the sensitivity of the Fermi LAT, in units of spectral flux---e.g. Jansky, or a limiting magnitude which could easily be converted. I've only found sensitivity in traditional high-energy terms, e.g:
$$4\times10^{-6} \textrm{ photons/s/cm}^2$$
for photons above 100 MeV, with a photon spectral index of -2.1
from http://arxiv.org/abs/1003.1436" [Broken].

The published spectral range of the LAT is 20 MeV to 300 GeV, thus for a spectral index of -2.1, I find that the average photon energy (above 100 MeV) should be about 606 MeV.
This gives a sensitivity of about $$3.9\times10^{-9} \textrm{ ergs/s/cm}^2$$ That seems fair.

Now, to get spectral flux, one needs to divide by the bandpass to yield units of ergs/s/cm^2/Hz, and then 1 Jy = 10^-23 erg/s/cm^2/Hz (wikipedia).

Dividing by a bandpass of 300 GeV (~7e25 Hz) gives a sensitivity of ~$$5\times10^{-12} \textrm{ Jy}$$!!! That can't be right! This would correspond to a limiting magnitude of 37!

What am I doing wrong?
Thanks!
Z

EDIT: This value is for an integration of 100s --- not very long.

Last edited by a moderator: May 5, 2017