Determine the equivalent width of a model component.

**Syntax: ****eqwidth** [[RANGE <frac range>]
<[model
name:]model
component number>] [err <number> <level> | noerr]

The command calculates the integrated photon flux produced by an additive model component (combined with its multiplicative and/or convolution pre-factors) (FLUX), the location of the peak of the photon spectrum (E), and the flux (photons per keV) at that energy of the continuum (CONTIN). The equivalent width is then defined as {EW = FLUX / CONTIN} in units of keV. New for version 12: the continuum is defined to be the contribution from all other components of the model.

There are certain models with a lot of structure where, were they the continuum, it might be inappropriate to estimate the continuum flux at a single energy. The continuum model is integrated (from E(1—<frac range)> to E(1+<frac range)>. The initial value of <frac range> is 0.05 and it can changed using the RANGE keyword.

The err/noerr
switch sets whether errors will be estimated on the equivalent width. The error
algorithm is to draw parameter values from the distribution and calculate an
equivalent width. <number>
of sets of parameter values will be drawn. The resulting equivalent widths are ordered
and the central
<level> percent selected to give the error range. You can get the
full array of simulated equivalent width values by calling ‘tclout eqwidth’
with the ‘errsims’ option (see **tclout** command).

When Monte Carlo Markov Chains are loaded (see **chain**
command), they will provide the distribution of parameter values for the error
estimate. Otherwise the parameter values distribution is assumed to be a
multivariate Gaussian centered on the best-fit parameters with sigmas from the
covariance matrix. This is only an approximation in the case that fit statistic
space is not quadratic.

Examples:

The current model is assumed to be *M _{1}(A_{1}+A_{2}+A_{3}+A_{4}+M_{2}(A_{5})),*
where the

XSPEC12> eqwidth 3

// Calculate the total flux of
component *M _{1}A_{2}* (the third

// component of the model with its multiplicative pre-factor)

// and find its peak energy (E). The continuum flux is

// found by the integral flux of *M _{1}(A_{1}*+

// range of 0.95E to 1.05E to estimate the flux.

XSPEC12> eqwidth range .1 3

// As before, but now the continuum is estimated from

// its behavior over the range 0.9E to 1.1E.

XSPEC12> eqwidth range 0 3

// Now the continuum at the single energy range (E)

// will be used.

XSPEC12> eqwidth range .05 2

// Now the component *M _{1}A_{1}*
is used as the feature, and

// *M1(A _{2}+A_{3}+A_{4}+M_{2}(A_{5}))*
are used for the continuum. The range

// has been reset to the original value.

XSPEC12> eqwidth 1

// Illegal, as *M _{1}*
is not an additive component.