8.2.3. Sectors and regions

8.2.3.1. Introduction

In many cases an observer analyses the observed spectrum of a single X-ray source with a single instrument. There are however situations with more complex geometries or multiple instruments involved that require a more complicated setup of model components and data.

Example 1: An extended source, where the spectrum may be extracted from different regions of the detector, but where these spectra need to be analysed simultaneously due to the overlap in point-spread function from one region to the other. This situation is e.g. encountered in the analysis of cluster data with ASCA or BeppoSAX.

Example 2: For the RGS detector of XMM-Newton, the actual data-space in the dispersion direction is actually two-dimensional: the position z where a photon lands on the detector and its energy or pulse height E as measured with the CCD detector. X-ray sources that are extended in the direction of the dispersion axis \phi are characterised by spectra that are a function of both the energy E and off-axis angle \phi. The sky photon distribution as a function of (\phi,E) is then mapped onto the (z,E)-plane. By defining appropriate regions in both planes and evaluating the correct (overlapping) responses, one may analyse extended sources.

Example 3: One may also fit simultaneously several time-dependent spectra using the same response, e.g. data obtained during a stellar flare.

Example 4: When one wants to fit the instrumental and/or particle background simultaneously with the source spectrum, the particle background needs to be folded with a different response function.

These examples can be modeled and fitted using the SPEX concept of sectors and regions.

8.2.3.2. Sectors

Sectors are a way in SPEX to organize or group model components that are related to each other. For example, we may put components related to the source spectrum and components related to the (particle) background into separate sectors.

A sector may also contain the components for different sources on the sky. For example if there is a bright point source superimposed upon the diffuse emission of the cluster, we can define two sectors: an sector for the cluster emission, and sector for the point source. Both sectors might even overlap, as this example shows!

Another example: the two nearby components of the close binary \alpha Centauri observed with the XMM-Newton instruments, with overlapping point-spread-functions of both components. In that case we would have two sectors that each describe one of the double star’s components.

The model spectrum for each sector may and will be different in general. For example, in the case of an AGN superimposed upon a cluster of galaxies, one might model the spectrum of the point-like AGN sector using a power law, and the spectrum from the surrounding cluster emission using a thermal plasma model.

Sectors can be defined using the command Sector: creating, copying and deleting of a sector in combination with the Comp: create, delete and relate spectral components command.

8.2.3.3. Regions and instruments

The observed count rate spectra are extracted in practice in different regions of the detector. It is necessary here to distinguish clearly the model sectors and detector regions. A detector region for the XMM EPIC camera would be for example a rectangular box, spanning a certain number of pixels in the x- and y-directions. It may also be a circular or annular extraction region centered around a particular pixel of the detector, or whatever spatial filter is desired. For the XMM RGS it could be a specific “banana” part of the detector-coordinate CCD pulse-height plane (z,E).

Note that the detector regions need not to coincide with the sectors, neither should their number to be equal! A good example of this is again the example of an AGN superimposed upon a cluster of galaxies. The sky sector corresponding to the AGN is simply a point, while, for a finite instrumental psf, its extraction region at the detector is for example a circular region centered around the pixel corresponding to the sky position of the source.

Also, one could observe the same source with a number of different instruments and analyse the data simultaneously. In this case one would have only one sky sector but more detector regions, namely one for each participating instrument.

Regions and instruments have a very similar role in SPEX. The subtle difference is that regions usually have the same or similar response properties, while different instruments have different responses. Although this difference exists, in practice instruments and regions are often mixed.

8.2.3.4. Including sectors and regions in trafo

If certain model components need to be applied to particular spectra, the trafo program (Trafo) can be used to predefine the sector and region numbers in the .res and .spo files. This can be helpful in the case of modelling particle background simultaneously with the source spectrum. In trafo, two sectors can be defined to contain the source and background components, to make sure that the background components are folded with a different response than the source components. In addition, model spectra from both sectors can be added and applied to one detector region. This example is explained in more detail in Modeling particle background.