There are currently two ways to run WebSpec, through the initial WebSpec page, which provides a simpler means for specifying the desired model and, for the more experienced XSPEC user, or the daring, there is a more advanced interface available via a link from the initial page.
The more advanced page allows you to put together more complex models by allowing you to build your own model expressions. The mission/instrument choice is the same as for the simpler interface. The form to build your own expression is made up of operator and component fields. You can build your expressions by clicking on the desired operator and component to indicate the way in which you wish them to be combined. You are merely building an algebraic expression that represents the model you wish to use in the simulation. As an example, if you want to combine a Gaussian, a Black Body, and a Power Law component with Photoelectric Absorption, you want to build either:
( Gaussian + Black Body + Power Law )* Photoelectric Absorption
or:
Photoelectric Absorption *( Gaussian + Black Body + Power Law )
For the first expression, choose ( in the first operator field, Gaussian in the Component 1 field, + in the second operator field, Black Body in the Component 2 field, + in the third operator field, Power Law in the Component 3 field, )* in the fourth operator field, and Photoelectric Absorption in the Component 4 field. The second expression, and any other expression for that matter, are constructed in a similar fashion.
Once you have specified the mission/instrument and the desired model, click on the "Specify Model Parameters" button to continue the process.
Next, you get the first (and the only blank) entry to make, the exposure. Enter the desired exposure in seconds. Now you get to enter the energy boundaries. There are two types of energy boundaries. The first set is a lower and upper boundary for the purposes of spectral fitting. This specifies the energy range over which the spectra is being fit. The second type is a way to specify energy band(s) for flux calculations. Note, that some mission/instrument combinations give the option to enter more than one band, e.g. RXTE/PCA. The lower and upper energy ranges given here do NOT affect the fitting they merely give an energy range for which a flux should be calculated.
After the energy ranges are entered, you are given one or more sets of model parameters to specify. There is a set for each component chosen on the previous page. For each parameter, the default value is already entered. For each parameter, you get the opportunity to freeze (fix at the specified value) or thaw the parameter for fitting purposes. Also, you can specify whether or not an error for that parameter should be computed. The error calculation is based on finding the parameter values for which the delta fit statistic is 2.706 from the best fit value. This is equivalent to the 90% confidence region for a single interesting parameter. Again, the default settings for each parameter are loaded when the page is first loaded.
After the exposure, energy ranges, and model parameters have been entered, click on the "Show me the spectrum!" button to submit the simulation. It may take a minute or two to finish. Once it is done, the results page will be loaded.
Next comes the flux table. A table listing the fluxes for the energy band(s) specified is displayed. This table shows the lower and upper energies for the flux calculations, the count rate, photon flux, and energy flux. Inspect these resulting fluxes. If they are not what you wanted or expected, you may wish to go back to the parameter entry page and modify the normalization(s) used. In the simplest case, you would merely multiply the entered normalization by the ratio (flux wanted)/(flux returned).
Finally, you are offered the opportunity to download or view the various files associated with the simulation. You can download the spectra (source and background) and the response matrix used, if you would like to do your own spectral work. You may also download the qdp and pco files that were used to create the plot of the simulated spectrum. Last, but perhaps most useful, you can view the XSPEC script file and the resulting log file. These can be supremely useful if something just doesn't quite look right. The log file will show you the output at every step. The script file will show you what XSPEC was told to do.
For feedback or questions on WebSpec, please contact Stephan Fantasia. fantasia@milkyway.gsfc.nasa.gov