Spectral fitting


At this website it is possible to model X-ray spectra of a large subset of XMM-Newton detections for which SAS based pipeline has extracted necessary data products. For the 4XMM-DR11 catalog, out of 895 415 detections 319 565 (or 35.7%) have spectra. Note that they were extracted in an automated regime so in complex cases such as highly variable / diffuse background, aperture centering and background extraction algorithms work imperfectly. In other words, these spectra and present spectral analysis tool are provided for quick-look purposes only.

If you want to try the spectral fitting but are not sure which source to take, just type has_spectrum = true in the search field at the top and pick any item from the list of results. Then click any of the Fit spectrum links in the table at the bottom of the page.


The XMM-Newton pipeline produces the following spectral data products for sources which are used at the core of the spectral fitting functionality:

The fitting application uses most recent version of canned response matrices.

More details on these and other science ready data products from the XMM-Newton are available in the Specifications for individual SSC data products document.

Please note that SRSPEC, BGSPEC, SRCARF data products are available for direct download from this website for further analysis in any OGIP-compatible software package such as XSPEC, Sherpa or ISIS. To find direct download links one needs to fetch exported detections table in FITS, VOTable or CSV formats from any query results and check columns url_data_spectrum_PN, url_data_bckgrnd_PN, url_data_arf_PN (replace 'PN' to 'M1' or 'M2' in column names if you are interested in MOS data).


Under the hood the spectral fitting engine is essentially a wrapper over Sherpa modelling and fitting application and at the time of writing we use its version 4.5. The flexible interface between the web application and the Sherpa engine allows rapid development of new features, so if you miss any functionality (e.g. specific model from this list) that exists in the Sherpa, let us know and there is a fair chance we implement it here.

Spectral fitting interface

Model parameters options

If the fit is successful, the X-ray spectrum plot is updated with fitted model and its residuals plotted. The text output of the Sherpa fit() function is shown with reduced χ2 value and the total model flux within the specified energy range.