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

  1. Jul 13, 2011 #1
    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:
    [tex]4\times10^{-6} \textrm{ photons/s/cm}^2[/tex]
    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 [tex]3.9\times10^{-9} \textrm{ ergs/s/cm}^2[/tex] 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 ~[tex]5\times10^{-12} \textrm{ Jy}[/tex]!!! That can't be right! This would correspond to a limiting magnitude of 37!

    What am I doing wrong?

    EDIT: This value is for an integration of 100s --- not very long.
    Last edited by a moderator: May 5, 2017
  2. jcsd
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