Most astrophysical sources are modelled as power laws. These are simply described by a reference Stokes \( \text{I} \), \( \text{Q} \), \( \text{U} \) and \( \text{V} \) flux density at a frequency \( \nu \) alongside a spectral index \( \alpha \).

Curved power laws are formalised in Section 4.1 of Callingham et al. 2017. These are the same as power laws but with an additional "spectral curvature" parameter \( q \).

Both kinds of power law flux-density representations are preferred in hyperdrive.

The list type is simply many instances of a Stokes \( \text{I} \), \( \text{Q} \), \( \text{U} \) and \( \text{V} \) value at a frequency. Example: this source (in the RTS style) has 3 defined frequencies for flux densities:

SOURCE J161720+151943 16.2889374 15.32883
FREQ 80.0e+6 1.45351 0 0 0
FREQ 100.0e+6 1.23465 0 0 0
FREQ 120.0e+6 1.07389 0 0 0
ENDSOURCE

In this case, Stokes \( \text{Q} \), \( \text{U} \) and \( \text{V} \) are all 0 (this is typical), but Stokes \( \text{I} \) is 1.45351 Jy at 80 MHz, 1.23465 Jy at 100 MHz and 1.07389 Jy at 120 MHz. This information can be used to estimate flux densities within the defined frequencies (\( 80 <= \nu_{\text{MHz}} <= 120 \); interpolation) or outside the range (\( \nu_{\text{MHz}} < 80 \) or \( \nu_{\text{MHz}} > 120 \); extrapolation).