Estimating the broadband polarization degree and angle, or Stokes parameters, from a (real or simulated) observation is a fairly common task—possibly the most common one. This section is devoted to a succinct discussion of the various means ixpeobssim is offering for the job and the associated merits and pitfalls.
The section is largely focused on point sources, at the moment. Updates for extended sources are on their way.
(For completeness, by the term broadband, in this context, we mean averaged over an energy range much larger than the intrinsic binning of the underlying response functions.)
This is a slippery business, and mileage might vary considerably depending on your precise analysis setup, so take the whole discussion in this section cum grano salis and make an effort to gauge its validity to the problem at hand.
To make a long story short, going through the effort of a full spectro-polarimetric fit with a proper model is likely to give you the right answer, but there are simpler and faster ways around that might bring you close enough in specific circumstances.
The ixpeobssim toolbox¶
ixpeobssim offers several different tools designed to estimate the broadband polarization, given a simulated photon list. We briefly recap the laundry list, here; more details can be found in the chapters about Binned data products and XSPEC support.
Fitting with XSPEC¶
Whenever practically possible, one should fit the I, Q and U Stokes spectra with a proper spectro-polarimetric model (e.g., in XSPEC). This is properly taking into account both the effective area and the energy dispersion, and is supposed to provide the ultimate statistical precision—at least for point sources.
Fitting with a constant polarimetric multiplicative model (i.e.,
in the ixpeobssim language) will often yield sensible results in terms of the
average broadband polarization degree, but it is not guaranteed to be unbiased.
If a suitable spectral model, for any reason, is not available, fitting the normalized Stokes parameters Q/I and U/I with an additive polarimetric model is a possible alternative, although it is not guaranteed to be unbiased, either—the reason being that, strictly speaking, the detector response matrix is not directly applicable to the normalized Stokes spectra.
Polarization cubes are a legitimate, model-independent, alternative to a fully fledged forward-folding fit. The automatically incorporate the detector acceptance correction, but, by their very nature, cannot handle the energy dispersion. Whether this is an important effect or not depends very much on the setup under study, but the good news is that one can gauge the magnitude of any possible bias by comparing polarization cubes binned in measured and true energy. Polarization cubes binned in true energy are supposed to provide the right answer.
(It goes without saying that the true energy is not available in real life, but still this is a perfectly valid thing to do for sensitivity studies and/or debugging purposes.)
In a nutshell: do not use them. Modulation cubes perform the relevant sums and/or averages over the count spectrum (as opposed to a proper proxy of the input source spectrum), and do not handle neither the effective area nor the energy dispersion of the detector. Binning in true energy will save you from the latter, but not from the former, which is typically a bigger effect.
Modulation cubes are retained in ixpeobssim, mainly for diagnostic purposes, but their use for science analysis is deprecated.
A toy case study¶
In the spirit of substantiate the somewhat generic statements in the previous section, the following plot summarizes the performance of the various methods for retrieving the broadband polarization (between 2 and 8 keV) for a toy setup in which the polarization degree increases linearly with energy and the polarization angle is constant. (For completeness, the spectrum was a simple power law.)
We generated 1000 independent, high-statistics (but unrealistic) realizations of the very same model and, for each single one we applied all the methods described in the previous section.
A few comments are in order:
The full XSPEC fit (with a proper model) to the I, Q and U spectra provides the right answer; fitting the normalized Q/I and U/I Stokes parameters to the polarimetric part of the model features a measurable negative bias, as does a fit with a constant polarization model;
a polarization cube with a single energy bin between 2 and 8 keV features a small, but measurable, negative bias—which is completely recovered when binning in true energy;
the corresponds single-bin modulation cube is way off in both flavors.