eqwidth: determine equivalent width
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. 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 M1(A1+A2+A3+A4+M2(A5)), where the Mx models are multiplicative and the Ax models are additive.
XSPEC12> eqwidth 3
// Calculate the total flux of component M1A2 (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 M1(A1+A3+A4+M2(A5)), using the
// 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 M1A1 is used as the feature, and
// M1(A2+A3+A4+M2(A5)) are used for the continuum. The range
// has been reset to the original value.
XSPEC12> eqwidth 1
// Illegal, as M1 is not an additive component.