AHELP for CIAO 4.13 Sherpa v1

# emissiongaussian

Context: models

## Synopsis

Gaussian function for modeling emission.

## Syntax

`emissiongaussian`

## Description

It is for use when the independent axis is in Angstroms.

## Example

```>>> create_model_component("emissiongaussian", "mdl")
>>> print(mdl)```

Create a component of the emissiongaussian model and display its default parameters. The output is:

```mdl
Param        Type          Value          Min          Max      Units
-----        ----          -----          ---          ---      -----
mdl.fwhm     thawed          100  1.17549e-38  3.40282e+38       km/s
mdl.pos      frozen         5000  1.17549e-38  3.40282e+38  angstroms
mdl.flux     thawed            1 -3.40282e+38  3.40282e+38
mdl.skew     frozen            1  1.17549e-38  3.40282e+38           ```

### ATTRIBUTES

The attributes for this object are:

Attribute Definition
fwhm The full-width half-maximum of the model in km/s.
pos The center of the gaussian, in Angstroms.
flux The normalisation of the gaussian.
skew The skew of the gaussian.
limit The model is only evaluated for points that lie within limit sigma of pos. It is a hidden parameter, with a default value of 4.

The functional form of the model for points is:

```f(x) = flux * exp(-0.5 * d(x)^2) / s2   if skew = 1

= 2 * flux * exp(-0.5 * d2(x)^2) / (s2 * (1 + skew))

otherwise

d(x) = (x - pos) / s

d2(x) = d(x)                  if x <= pos

= d(x) / skew           otherwise

s2 = 2.50662828 * s

s = pos * fwhm / (2.9979e5 * 2.354820044)```

and for integrated data sets the low-edge of the grid is used.

The calculation is only done for those points that are in the range:

`|x - pos| < limit * sigma`

Outside this range the model is set to 0.

## Bugs

See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.