Although for relatively bright point sources background is typically not an issue for IXPE, it might become important for dimmer point sources and/or extended sources, especially at high energy.
We distinguish three different types of backgrounds: galactic, extra-galactic, and instrumental—as it turns out, the latter is typically the most important. ixpeobssim provides ways to simulate background components and handle backgrounds in high-level analysis. In the remaining of this section we shall briefly cover the simulation part.
All the background components are assumed to be completely unpolarized, which is consistent with our best understanding of the detector response.
ixpeobssim.srcmodel.bkg module contains all the background-related
This is, admittedly, the part that is less extensively covered in ixpeobssim.
We do have a
ixpeobssim.srcmodel.bkg.xGalacticBkg class that is designed
to interface to the the
HEASARC background tool
and, particularly, use the ROSAT count rate in the 1–2 keV energy band (R7).
The model is overly simple (uniform in sky coordinates over the field of view
and with a fixed spectral index), with the overall intensity being the only
parameter that can be changed by the user.
Tuning the galactic X-ray background to a specific observation is still tricky in the current ixpeobssim setup. Any help in making this more user-friendly (and, possibly, developing an iterface to the e-ROSITA data) is more than welcome.
The extra-galactic background, although not really the relevant contribution in
any practical situation, is relatively straightforward to handle. We provide a
ixpeobssim.srcmodel.bkg.xExtragalacticBkg, with the basic parametrization
taken from Gruber et al., 1999.
This is most often the primary source of background, its main characteristics being that the instrumental background is modeled in instrument coordinates and not convolved with the instrument response functions—not with the effective area, nor with the vignetting, and typically, not with the energy dispersion.
In terms of simulation facilities, the two most important data structures are
ixpeobssim.srcmodel.bkg.xPowerLawInstrumentalBkg: a background component with a power-law energy spectrum with adjustable normalization and spectral index;
ixpeobssim.srcmodel.bkg.xTemplateInstrumentalBkg: a background component with an energy spectrum derived from an actual observation—typically by using a dim point source and removing the source counts at the center of the field of view, and then properly rescaling the the relevant area.
By deafault the instrumental background is generated uniformly in instrument coordinates, although, based on actual observations, we do support a generic linear radial dependence, as briefly summarized in the next section.
IXPE celestial observations show that the distribution of the instrumental background in detector coordinates is not rigorously flat—in fact it is typically higher close to the walls of the detector—and can be reasonably modeled with a linear function of the radial distance from the center.
The radial slope alpha represents the fractional half-excursion of the variation across the size h of the fiducial rectangle. For alpha = 0 the detector position are distributed uniformly over the fiducial rectangle. For alpha = 2 the radial dependence is maximal, and the density of events is zero at the center of the detector.
The values of alpha found experimentally vary from observation to observation (at a level which is consistent with expectations based on the typical variations of the radiation environment in low-Earth orbit), and are typically between 0 and 0.2.
Creating a template¶
To be filled in.