V11.3 is an incremental release over V11.2.

The following list summarizes the major changes to XSPEC since the previous edition of the user's manual. Bug fixes are listed in Appendix E.

- Commands
- Added new option
`forcecalc on/off`to`xset`to force calculation of the model for all datasets. - Added the following new options to
`tclout`: ftest, query, response, arf, backgrnd, xflt, areascal, backscal, expos - If
`steppar`is run with a single parameter then followed by`plot contour`a plot of statistic vs. parameter value will be produced. - The model description written as part of the standard output at the
end of a fit can now be cut-and-pasted directly into the
`editmod`command. - The commands
`datafile{n}`and`arf{n}`can now mean n-1th extension following the first spectral extension. This only works for type I spectral extensions. If the first spectral extension is type II then the commands have their usual interpretation as the nth spectrum in the extension. - When using the C-statistic the command
`plot chisq`now gives contributions to C shown either +ve or -ve depending on the sign of data-model. - There is a new option to the command
`log filename`to add to the filename a string giving a date and time stamp.

- New Models
`acisabs`- the absorption due to the contamination on ACIS. Should only be used with response matrices that do not have this effect included !`bkn2pow`- a three-segment broken power-law.`expdec`- simple exponential decay model.`nsa`- neutron star H atmosphere model with different values of magnetic field.`pwab`- absorption with a power-law distribution of covering fraction.`smaug`- deprojection model for an extended an optically-thin source (eg a cluster).`xmmpsf`- model to mix spectra between regions to take into account XMM-Newton PSF.

- Model changes
- The command
`xset APECROOT`can now take an argument giving just the APEC version number. The default has been updated to 1.3.1. - The
`projct`model works correctly for multiple datasets. - The mekal cooling flow models now come in two versions. The default
version has a variable number of emission measure steps that can be set
using the command
`xset cflow_ntemps`followed by a number. The old version of the models, which included a bug, are available by setting`xset cflow_version 1`.

- General
- The
`ignore`command runs much faster on spectra with big response matrices. - The screen output from
`error`now also includes the parameter range in terms of plus and minus difference from the best fit. - The abundance tables have been extended to include all elements up
to Zn.
- A new
`lodd`abundance tables has been added for the solar photospheric abundances from Lodders 2003. - The abundance table and cross-section in use have been added to
`(auto)save`. - The default cosmology has been set to the ``WMAP standard'' of
*H*_{0}=70, in a flat Universe.

- Markov-Chain Monte Carlo
- Preliminary code for Markov Chain Monte Carlo is included. The
`chain`command runs MCMC chains and the results can be displayed using`plot mcmc`. This is still in the experimental stage. At the moment it seems to work reasonably well if a fit is done then chains run to generate the posterior probability distribution. The aim is to improve this so that it is not necessary to first find the minimum.