WebSpec Help
WebSpec provides a facility for simulating spectra for a variety of mission/ instrument combinations and several different models. It utilizes the X-ray spectral fitting package, XSPEC.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.
Mission and Model Choice Page
On the first page, you choose a mission/instrument and the desired model to be used in the simulation. For the simpler interface, the choice of model involves two decisions. First decide whether you want to apply Photoelectric Absorption or not. This choice is made by clicking on the box to apply the absorption; leave the box unclicked for no absorption. Next choose a model from the list of available models. Note, this is a scrolling list; more choices are available than are shown.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 "I'm ready to set model parameters" button to continue the process.
Model Parameter Entry Page
The next page is a dynamic page. Its contents are determined by your choices on the previous page. The first thing on this page is a double check for you to see what WebSpec thinks you have asked it to do. Seen here are an English description of the model chosen and the mission/instrument combination chosen. Also shown is the exact expression that XSPEC is going to use as the model. If these aren't what you wanted, you should go back to the previous page and try making a different selection.The information that can be entered is as follows.
Exposure
The desired exposure in seconds. This is the only entry you must make.Fit Energies
The energy range over which the fit is performed. This is set to a sensible default for the requested instrument.Flux Energies
The energy range over which fluxes will be calculated. These energies do not affect the fitting.Model Parameters
Each model component selected on the previous page is listed with an entry for each parameter. You can set the value of the parameter and use the check boxes to freeze (or thaw) the parameter and request the estimation of its 90% confidence range. Note that you cannot freeze all parameters.Background File
If a background file is provided for this instrument it will be listed with a check box already filled. You can click on this to turn off including the background but this is not recommended. If there is a systematic uncertainty associated with the background normalization this is also specified with a default set appropriately for the instrument. You should not change this unless you know what you are doing.Fit Statistic
The fit statistic is set by default to a sensible option for this instrument. You can change this but should not do so unless you know what you are doing.Plot Rebinning
The plot that will be generated from the simulation will use the xspec setplot rebin to bin up channels either to the specified minimum significance or maximum number of channels, whichever comes first.The response file information is also written out but cannot be changed. When you are happy with the page click on the "Do the simulation" button. The simulation may take a few minutes to run, particularly if you choose to determine errors on many parameters. Once it is done, the results page will be loaded.
Results Page
If you have gotten to this page, you probably have entered things properly. The first thing offered is an image of the resulting spectrum. You can click on the image to get a larger version for closer examination. At this point, you can change the plotting specifications, (x and y scales, labels, etc.) and replot the simulated data and model. You are also offered the opportunity to download a postscript version of the plot. After the image, the model expression is echoed again, as well as the mission/instrument combination and exposure. The resulting model parameters, along with their errors where requested, are listed next. The reduced chi^2 for the model and the count rate over the fitted energy range is given next.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 fitting. 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.