National Aeronautics and Space Administration

Source Confusion: The most important addition to the science capabilities added in XSPEC12 is the ability to fit multiple sources simultaneously with models of different functional form. For coded mask apertures, a different effective response arises from different parts of the field of view. XSPEC12, unlike XSPEC11, can assign separate model components to these different responses.

This has use where source confusion is present and also when the data are dominated by background flux. This capability was largely motivated by support for the SPI instrument on the INTEGRAL spacecraft, but is likely to have wider applicability to gamma ray astronomy conducted through coded mask instruments if the field of view is wide and/or the instrumental background is high.

In XSPEC12 the user can enter distinct named models, derive fits to each simultaneously, and plot both the joint fit and the separate components. XSPEC11 and prior, in contrast, store a single model that is convolved with a single response per spectrum. The capability of working with multiple model objects also provides the facility for the user to work simultaneously with models of different functional form (define multiple models and interchange them at will, rather than having to re-enter and recompute).

New Response formats: The capability to work with detector responses that are linear combinations of a basis set as required by the INTEGRAL/SPI instrument.

Efficiency With a new algorithm for computing numerical derivatives, XSPEC12 halves the number of convolution operations during iteration. This promises equal or better performance where the program's runtime is dominated by convolution (relatively simple models and large arrays). This gain is partly offset, however, by using double precision computations.

Indefinite nesting depth in model definitions. It also allows polynomial relationships defining links between multiple parameters. By contrast, XSPEC11 offers only linear relationships between one parameter and another.

Better support for OGIP Type II files by upgrading the syntax understood by the data command. Type II Background and Auxiliary Response (Efficiency) files are also supported.

Development  tool for user models. The new version allows users to develop local models within XSPEC12 in fortran, C, or C++. Models may be plotted without entering any data, so that their correctness and accuracy can be checked over any desired energy range. Local models can be recompiled, re-initialized, and then plotted from the XSPEC prompt so that they can be modified iteratively during a session.