Monochromatic AB (Oke 1964) magnitudes are defined by

magAB = -2.5 x log(fν) - 48.60 

The constant is set so that AB is equal to the V magnitude for a source with a flat spectral energy distribution. We adopt the Vega flux densities recommended by Bohlin & Gilliland (2004; their alpha_lyr_stis_002.fits file). The isophotal wavelength at V is 5546Å, and the isophotal flux density is 3.63 x 10^-20 ergs s^-1 cm^-2 Hz^-1. A V (Landolt V filter) magnitude of 0.026 from Bohlin & Gilliland (2004) is assumed. Then

magAB = -2.5 x log(fν) - 48.574

This means that a reference spectrum of constant flux density per unit frequency:

fAB,ν0 = 3.718 x 10-20 erg s-1 cm-2 Hz-1 = 3718 Jy

will have AB magnitudes m_AB,ν⁰ = 0 at all frequencies ν
Å

Reference: Tokunaga & Vacca, Publication of the Astronomical Society of the Pacific, 117:421-426, 2005 April.

-- JeanMalapert - 17 Aug 2006