XSPEC Data Formats

XSPEC is designed to support multiple input data formats. Support for the earlier SF and Einstein FITS formats have been removed. XSPEC reads OGIP standard data as well as a modified data format used by the INTEGRAL/SPI detector.

New data formats can be implemented independently of the existing code, so that they may be loaded during program execution. The “data format” includes the specification not only of the files on disk but how they combine with models.

OGIP Data

The OGIP data format both for single spectrum files (Type I) and multiple spectrum files (Type II) is fully supported. Links to all the standard documents are available at https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/caldb_doc.html and library routines and programs to create and manipulate these files are described in Appendix E.

INTEGRAL/SPI Data

XSPEC also includes an add-in module to read and simulate INTEGRAL/SPI data, which can be loaded by the user on demand. The INTEGRAL/SPI datasets are similar to OGIP Type II, but contain an additional FITS extension that stores information on the multiple files used to construct the responses.

The INTEGRAL Spectrometer (SPI) is a coded-mask telescope, with a 19-element Germanium detector array. The spectral resolution is 500, and the angular resolution is 3$\deg$. Unlike focusing instruments however, the detected photons are not directionally tagged, and a statistical analysis procedure, using for example cross-correlation techniques, must be employed to reconstruct an image. The description of the XSPEC analysis approach which follows assumes that an image reconstruction has already been performed; see the SPIROS utility within the INTEGRAL offline software analysis package (OSA), OR, the positions on the sky of all sources to be analyzed are already known (which is often the case). Those unfamiliar with INTEGRAL data analysis should refer to the OSA documentation. Thus, the INTEGRAL/SPI analysis chain must be run up to the event binning level [if the field of view (FoV) source content is known, e.g. from published catalogs, or from IBIS image analysis], or the image reconstruction level. SPIHIST should be run selecting the “PHA” output option, and selecting detectors 0-18. This will produce an OGIP standard type-II PHA spectral file, which contains multiple, detector count spectra. In addition, the SPIARF procedure should be run once for each source to be analyzed, plus one additional time to produce a special response for analysis of the instrumental background. If this is done correctly, and in the proper sequence, SPIARF will create a table in the PHA-II spectral file, which will associate each spectrum with the appropriate set of response matrices. The response matrices are then automatically loaded into XSPEC upon execution of the data command in a manner very transparent to the user. You will also need to run SPIRMF (unless you have opted to use the default energy bins of the template SPI RMFs). Finally, you will need to run the FTOOL SPIBKG_INIT. Each of these utilities - SPIHIST, SPIARF, SPIRMF and SPIBKG_INIT - are documented elsewhere, either in the INTEGRAL or (for SPIBKG_INIT, the HEAsoft) software documentation.

There are several complications regarding the spectral de-convolution of coded-aperture data. One already mentioned is the source confusion issue; there may be multiple sources in the FoV, which lead to different degrees of shadowing on different detectors. Thus, a separate instrumental response must be applied to a spectral model for each possible source, for each detector. This is further compounded by the fact that INTEGRAL's typical mode of observation is “dithering.” A single observation may consist of 10's of individual exposures at raster points separated by $\sim2\deg$. This further enumerates the number of individual response matrices required for the analysis. If there are multiple sources in the FoV, then additional spectral models can be applied to an additional set of response matrices, enumerated as before over detector and dither pointing. This capability - to model more than one source at a time in a given $\chi^2$ (or alternative) minimization procedure - did not exist in previous versions of XSPEC. For an observation with the INTEGRAL/SPI instrument, where the apparent detector efficiency is sensitive to the position of the source on the sky relative to the axis of the instrument, the statistic is:

  $$\chi^2 = \sum_P\sum_d\sum_I ({{D_{d,p}(I) - \sum_j\sum_E R_{d,p;j} \times M_j(E;x_s) - B_{d,p}(I;x_b)}\over{\sigma_{d,p}(I)}})^2$$ (2.10)

where $p,d$ run over instrument pointings and detectors; $I$ runs over individual detector channels; $j$ enumerates the sources detected in the field at different position ($\theta,\phi$); $E$ indexes the energies in the source model; $x_s$ are the parameters of the source model, which is combined with the response; and $x_b$ are parameters of the background model.

Examination of this equation reveals one more complication; the term $B$ represents the background, which, unlike for chopping, scanning or imaging experiments, must be solved for simultaneously with the desired source content. The proportion of background-to-source counts for a bright source such as the Crab is 1%. Furthermore, the background varies as a function of detector, and time (dither-points), making simple subtraction implausible. Thus, a model of the background is applied to a special response matrix, and included in the de-convolution algorithm.

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Last modified: Friday, 23-Aug-2024 13:20:40 EDT

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