GJy

GJy, short for gigajansky, is a non-SI unit of spectral flux density used in radio astronomy. It denotes 1 × 10^9 jansky, where the jansky (Jy) is defined as 10^-26 W m^-2 Hz^-1. Therefore, 1 GJy equals 1 × 10^9 Jy or 1 × 10^-17 W m^-2 Hz^-1.

The gigajansky is a very large value by astronomical standards and is rarely used in practice. Most observed radio sources have flux densities from millijansky (mJy) to tens or hundreds of jansky. For reference, the Sun’s radio flux at centimeter wavelengths is typically on the order of 10^4 to 10^5 Jy, so gigajansky would exceed conventional solar or galactic brightness.

Because of this, GJy is mainly encountered in theoretical discussions, simulations, or as an upper-limit or calibration figure rather than in routine measurements. It should not be confused with gigajoule (GJ), which is a unit of energy and has a different dimensional meaning.

Notes on usage: the prefix giga- multiplies the Jansky by one billion, and is applied to spectral flux density, not to standalone energy quantities unless the surrounding context specifies a different definition.

See also: Jansky, radio astronomy, spectral flux density, units of measurement prefixes.