XSPEC User's Guide

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groups. When files are read in they can be labeled as belonging to a particular data group.
When a model is defined a set of model parameters is allocated for each data group.
These parameters can all vary freely or they can be linked together across data groups as
required.
To set up data groups, the data command should be given as in the following
example :
XSPEC12> data 1:1 file1 1:2 file2 2:3 file3
which sets up two data groups. The first data group comprises data sets from file1
and file2, and the second data group takes the data set from file3. Now when a model is
defined, XSPEC will give two sets of model parameters, one for the first datagroup and
one for the second.
3.8 Model Commands
XSPEC allows users to fit data with models constructed from individual
components. These components may be either additive, multiplicative, mixing, or
convolution. Multiplicative components simply multiply the model by an energydependent
factor. Convolutions apply a transformation to the model component they
operated on whereby the output can be affected by a range of input energies, such as in
smoothing operations. Mixing components are two dimensional and designed to
transform fluxes between different spatial regions (such as in projection). Multiplicative,
and convolution components can act on individual components, on groups of
components, or on the entire model, whereas mixing transformations apply to the whole
model.
The model command defines the model to be used and prompts for the starting
values of its parameters. The user also can set the allowed ranges of the parameter.
Parameters can be linked to an algebraic function of the other parameters, and unlinked
using the untie command. The value of an individual parameter can be changed with the
command newpar (and the current setting queried with newpar 0). Parameters can be
fixed at their current value with the freeze command and allowed to vary freely with the
thaw command. Individual components can be added or subtracted from the model using
addcomp, delcomp, and editmod. The plasma emission and photoelectric absorption
models require an assumption about relative elemental abundances.
The flux command calculates the flux from the current model in the given energy
range. This energy range must be within that defined by the current response matrix. If a
larger energy range is required, then the energies command can be given to compute the
model over the desired range. The lumin command calculates the luminosity for the
source redshift given. The eqwidth command determines the equivalent width of a model
component, usually a line. The user of either of these last two commands should read the
help descriptions carefully. The Tcl script addline can be used to automatically add lines
to a model. These can be identified using identify and modid.
New model components which can be described by a simple algebraic formula
can be set up using mdefine and used in the same way as the standard models except they
will run slower being interpreted rather than compiled.