Cora is a program package that implements a line fitting
procedure based on the Maximum Likelihood technique proposed by Cash
1979 (ApJ 228, 939) and has been developed
for emission line spectra, as introduced in Ness & Wichmann 2002
(Astronomische Nachrichten 323, 129).
The program measures the number of counts contained in emission lines.
Energy fluxes or intensities are not calculated during the procedure,
since the response from the specific instrument would have to be processed.
This would go beyond the general purpose of this tool.
However, the line counts can be converted to photon fluxes by use of the
effectives areas that can be delivered with the spectrum. The point-spread
function is not obtained from the response but is approximated analytically
by use of parameterized Gaussian/Lorentzian profile functions. A Pseudo-Voigt
profile is implemented consisting of a weighted sum of a Gaussian- and a
Lorentzian profile, with common line widths, also calculated from
individual Gaussian and Lorentzian line widths. More profiles can be
provided as ASCII files containing a non-analytical line template that
represents any type of line shape and line width.
We provide some extra tools to calculate fluxes from given effective areas.
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. Supplemental IDL scripts are provided to convert the fits output produced by the Chandra CIAO software and the XMMSAS software into the CORA format.
Most flexible application is provided by the IDL version, but the C-Version is fully functional and the two versions require and produce compatible input/output. This manual focuses on the C-Version and some short description about the IDL usage is given in Sect. 9.
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. An interactive graphic is available to choose lines
to be fitted directly from the spectrum. The acceptable ranges of the input
parameters are described in Sect. 6.
The output is organized in two ways: a plot showing spectrum and fit and a
log file recording the fit parameters. The plot can be produced in different
formats, i.e., as postscript file designed for inclusion in publications and
as GIF or PPM files for inclusion in, e.g., power point presentations or html.
In addition the resulting fit parameters with their errors can be written into
an ASCII file for further processing.
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.