Cora is a program package that implements a line fitting
procedure based on the Maximum Likelihood technique and developed
for emission line spectra, as introduced in Ness & Wichmann 2002.
Basically the program seeks to obtain emission line counts (total number of
photons). Energy fluxes or intensities are not calculated during the procedure,
since the response from the specific instrument must be processed. Our aim is
to provide a tool applicable to all kinds of different spectra.
However, we provide some tools to calculate fluxes from effective
areas to be given after the calculation of line counts. In addition,
line positions and line widths can be iterated in order to get most
accurate line fluxes. For line profiles we provide Gauss- and Lorentz profiles.
A Pseudo-Voigt profile is implemented consisting of a weighted sum of
a Gauss- and a Lorentz profile, with common line-widths also calculated from
individual Gauss and Lorentz line-widths. More profiles can be used by giving
an ASCII file containing data points representing the line profile and line width.
The program is especially applicable to spectra with low count numbers like
UV spectra or X-ray spectra. It has been developed in the context of analyzing
CHANDRA LETGS data, and is especially useful for these data, but sufficiently
general to be applicable for other data as well. A supplemental program
cora_rgs is provided, converting the fits files returned by the XMMSAS task
rgsproc into the CORA format, such that XMM-RGS data can easily be
analyzed with CORA.
In Sect. 3, installation instructions are given.
The basic fit routine is delivered with a Graphical
User Interface (GUI) supporting the specification
of the input parameters. The acceptable ranges of the input parameters are
described in Sect. 6.
The output is organized in two ways: In order to obtain a visual impression of
the fit result, a plot can be produced, thus allowing comparison of the
fit result with the original spectrum. This plot can be produced in different
formats, i.e., as postscript file designed for inclusion in publications and
as GIF and PPM file for inclusion in, e.g., power point presentations, or in an
X window. In addition the numerical fit
results, i.e., the parameters with their errors, can be written into an ASCII
file for further processing. This is useful when the program is to be used for
searching a whole
spectrum automatically, listing all lines with wavelengths, fluxes, etc. in
a log-file. This log-file can be converted into a LATEX table for publication
etc. A detailed description can be found in Sect. 8.
We would like to point out that the program is still being developed and improved. Some new features that are provided may not yet be included in the manual.