WODEN sky model format

The WODEN source catalogue is a modified version of the RTS srclist. In the current version of WODEN, you create one single SOURCE which can include as many COMPONENTS as desired, each of type POINT, GAUSSIAN or SHAPELET. A POINT is a dirac delta point source model, a GAUSSIAN is a 2D Gaussian model (with a major, minor, and position angle), and a SHAPELET model uses multiple ‘shapelet’ basis functions to build a model. For details on the model types, see Line et al. 2020. If you want to build a shapelet model, you can use the software SHAMFI, which you can read about on the SHAMFI readthedocs.

Currently, every source is given a simple power-law frequency behaviour as:

\[S = S_0 \left( \frac{\nu_0}{\nu} \right)^\alpha\]

where \(S\) is the flux density at frequency \(\nu\), with a reference flux density \(S_0\), reference frequency \(\nu_0\), and spectral index \(\alpha\).

Point sources

An example of a single SOURCE with a single point source COMPONENT is:

SOURCE source_name P 1 G 0 S 0 0
COMPONENT POINT 4.0 -27.0
LINEAR 1.8e+08 10.0 0 0 0 -0.8
ENDCOMPONENT
ENDSOURCE

An explanation of each line and value follows.

SOURCE source_name P 1 G 0 S 0 0

Initialises the SOURCE, giving it the name source_name, and specifying the number and type of components (P = point, G = gaussian, S = shapelet). For shapelet, the two numbers are total number of coefficients and total number of components. Read on further for more explanation of shapelets.

COMPONENT POINT 4.0 -27.0

Initialises a component, specifying the type (either POINT, GAUSSIAN, SHAPELET) and the RA and DEC (hours, deg). So this line means a point source at RA,DEC = 4h, -27deg.

LINEAR 1.8e+08 10.0 0 0 0 -0.8

Specifies a reference Stokes flux density as LINEAR Freq I Q U V SI, where the Freq is in Hz, Stokes params I,Q,U,V are all in units of Jy, and SI is the spectral index. It’s labelled LINEAR as a power-law is linear in log-log space. This example line specifies we have a source that has a flux density of purely Stokes I of 10 Jy at 180 MHz, with a spectral index if -0.8.

ENDCOMPONENT

This line ends the component.

ENDSOURCE

This line ends the source.

To add multiple point sources, simply repeat the COMPONENT / ENDCOMPONENT sections with new details, i.e.

SOURCE multi_point P 3 G 0 S 0 0
COMPONENT POINT 4.0 -27.0
LINEAR 1.8e+08 10.0 0 0 0 -0.4
ENDCOMPONENT
COMPONENT POINT 3.0 -37.0
LINEAR 1.3e+08 1.0.0 0 0 0 -0.786
ENDCOMPONENT
COMPONENT POINT 5.0 -47.0
LINEAR 3.9e+08 0.04 0 0 0 .02
ENDCOMPONENT
ENDSOURCE

noting that at the very top line, I have updated P 3 to reflect there are now three point sources. These numbers are used to quickly allocate memory, that’s why they re included.

Note

WODEN crops everything below the horizon out of the sky model. It can do this one of two ways - either by COMPONENT or by SOURCE. In the example above, we have three COMPONENT in one SOURCE. If you ask WODEN to crop by SOURCE, if just one of the COMPONENTS is below the horizon, it’ll crop the entire source.

Gaussian sources

An example srclist containing a single gaussian:

SOURCE gaussian_source P 0 G 1 S 0 0
COMPONENT GAUSSIAN 3.378 -37.2
LINEAR 1.8e+08 10.0 0 0 0 -0.8
GPARAMS 45.0000000000 6.0 3.0
ENDCOMPONENT
ENDSOURCE

where all lines have the same meaning as the point source, and the meaning of the extra line:

GPARAMS 45.0000000000 6.0 3.0

which specifies the Gaussian parameters as GPARAMS pa(deg) major_axis(arcmin) minor_axis(arcmin). The major and minor axes are specified as FWHM. Note this line needs to sit in between the lines starting with `COMPONENT GAUSSIAN and `ENDCOMPONENT.

Shapelet sources

To generate shapelet models compatible with WODEN, simply use SHAMFI to fit an image with the --woden_srclist option (again see SHAMFI readthedocs. for more detail). This will ensure all normalisations are correct. An example sky model (made by hand so the normalisations won’t be correct) is:

SOURCE shapelet_source P 0 G 0 S 1 3
COMPONENT SHAPELET 3.378 -37.2
FREQ 1.8e+08 10.0 0 0 0
SPARAMS 45.0000000000 6.0 3.0
SCOEFF 0 0 0.92342
SCOEFF 1 10 0.0002354
SCOEFF 4 5 0.004567
ENDCOMPONENT
ENDSOURCE

which generates a single shapelet component, including 3 shapelet basis functions, hence S 1 3 in the first line. The SPARAMS line is similar to the GAUSSIAN line with SPARAMS pa(deg) major_axis(arcmin) minor_axis(arcmin). The extra lines like:

SCOEFF 0 0 0.92342

encode the order of the shapelet basis function (see Line et al. 2020 for details) and fitted coefficient as SCOEFF p1 p2 coeff_value. You can add as many SCOEFF lines as necessary, with a maximum order < 100. If you use SHAMFI, the coefficients will be scaled such that the Stokes I flux density of the full source will be 10 Jy at 180 MHz for this example. You may have noticed the SED information here is different:

FREQ 1.8e+08 10.0 0 0 0

This line will still assume a power-law frequency behaviour, with a reference flux of 10 Jy at 180 MHz, but use a default SI = -0.8.

Putting it all together

An example skymodel with four sources, the first with all component types, the next three with a single component of each type, would look something like this:

SOURCE multi_sources P 3 G 1 S 2 7
COMPONENT SHAPELET 3.378 -37.2
FREQ 1.8e+08 10.0 0 0 0
SPARAMS 45.0000000000 6.0 3.0
SCOEFF 0 0 0.92342
SCOEFF 1 10 0.0002354
SCOEFF 4 5 0.004567
ENDCOMPONENT
COMPONENT SHAPELET 3.12 -32.2
FREQ 1.8e+08 3.1 0 0 0
SPARAMS 56.0000000000 9.0 3.0
SCOEFF 0 0 0.02345
SCOEFF 3 0 -0.234234
SCOEFF 21 34 0.82342
SCOEFF 31 5 -0.00876234
ENDCOMPONENT
COMPONENT GAUSSIAN 3.378 -37.2
LINEAR 1.8e+08 10.0 0 0 0 -0.8
GPARAMS 45.0000000000 6.0 3.0
ENDCOMPONENT
COMPONENT POINT 4.0 -27.0
LINEAR 1.8e+08 10.0 0 0 0 -0.8
ENDCOMPONENT
COMPONENT POINT 3.0 -37.0
LINEAR 1.8e+08 0.6 0 0.2 0 -0.8
ENDCOMPONENT
COMPONENT POINT 5.0 -47.0
LINEAR 70E+6 87.0 0 0 0 -0.8
ENDCOMPONENT
ENDSOURCE
SOURCE source_name P 1 G 0 S 0 0
COMPONENT POINT 4.0 -27.0
LINEAR 1.8e+08 10.0 0 0 0 -0.8
ENDCOMPONENT
ENDSOURCE
SOURCE gaussian_source P 0 G 1 S 0 0
COMPONENT GAUSSIAN 3.378 -37.2
LINEAR 1.8e+08 10.0 0 0 0 -0.8
GPARAMS 45.0000000000 6.0 3.0
ENDCOMPONENT
ENDSOURCE
SOURCE shapelet_source P 0 G 0 S 1 3
COMPONENT SHAPELET 3.378 -37.2
LINEAR 1.1e+08 10.0 2.0 0 0.8 -0.7
SPARAMS 45.0000000000 6.0 3.0
SCOEFF 0 0 0.92342
SCOEFF 1 10 0.0002354
SCOEFF 4 5 0.004567
ENDCOMPONENT
ENDSOURCE