Response functions

The instrument response functions (IRFs) are a critical part of ixpeobssim, and they are used (in identical form) both in the event simulation and in the analysis of the data products from the simulation itself.

All the response functions are stored in FITS files in the OGIP format defined in CAL/GEN/92-002 (and modifications CAL/GEN/92-002a) and are intended to be fully compatible with the spectral analysis tools provided by XSPEC (see this page for more details).

We identify six different types of response functions:

If you are familiar with basic spectral analysis in XSPEC, the .arf and .rmf files have exactly the meaning that you would expect, and can be in fact used in XSPEC; the .mrf files have the exact same format as the .arf files.

ixpeobssim provides facilities for reading, displaying and using IRFs, as illustrated below.

IRF summary

This is a short, top level description of the version v13 of the default IXPE response functions used by ixpeobssim. As explained down in this page, these maps one-to-one with the response files shipped with the official IXPE CALDB released to the public with HEASoft version 6.33.


New in version 31.0.0: As of version 31.0.0, ixpeobssim comes with a default set of response files (dubbed v13) whose validity time is binned in 6-month interval. The beginning of each interval is encoded in the file name, in a way that should be human-readable.

Contrary to the HEASOFT framework, ixpeobssim has no internal mechanism to automatically pick the right set of response files for the analysis of a given observation, and it is a responsibility of the user to make sure that the start of the observation is included within the validity interval of the files themselves. (The latter is properly encoded with each file in the form of the canonical header keywords.)

All the response functions are defined between 1 and 12 keV in steps of 40 eV, and are detector-unit based (the pulse-invariant channel space is defined between 0 and 15 keV in steps of 40 eV). As such, there are differences between the three modules in terms of effective area, modulation factor and point-spread function. Since the eight iteration of the response functions, all response files come in two distinct flavors—un-weighted and weighted. Accordingly, all the analysis tools support weights.

Effective area

Below is the nominal IXPE on-axis effective area, in both the un-weighted and weighted flavors. The latter is roughly 15% smaller at the peak value but, as we shall see in a second, the increase in the modulation factor drives the overall sensitivity slightly up.


On-axis effective area as a function of the energy. The solid lines represent the total effective area for the sum of three telescopes, in the un-weighted and weighted version, while the un-labeled dashed lines represent the curve for each of the three single telescopes.

The effective-area curves for the three telescopes are within a few % from each other, the small differences being due to the slightly different mirror effective areas measured during the MMA calibration, as well as the different asymptotic pressure values for the three GPDs at the focal plane.

The effective-area calculation in ixpeobssim includes all the relevant contributions, namely:

  • the mirror effective area;

  • the transparency of mirror-module-assembly thermal shield;

  • the transparency of the detector-unit UV filter;

  • the transparency of the GPD window;

  • the efficiency of the GPD gaseous active medium;

  • the efficiency of the event weighting (in the weighted flavor).

The plots below show the principal ingredients that go into the calculation.


On-axis effective area as a function of the energy for three Mirror-Module Assemblies (MMA) and for a single module.


Transparency of the UV filter as a function of the photon energy.


Quantum efficiency of the GPD as a function of the energy, broken up in its two main components—the Be window transparency, and the gas cell absorbing efficiency.

The vignetting function shown below comes from a preliminary study by MSFC based upon ray-trace simulations for a perfect mirror module assembly, and is relevant for the simulation of extended sources.


Preliminary estimation of the vignetting of the optics as a function of energy and off-axis angle.

The vignetting, along with the relative orientation of the three IXPE detector units, defines the relative exposure across the field of view of the instrument, as shown in the following two plots. (Note that above 6 keV the drop of the effective area at the edge of the field of view is relatively more important.)


Relative exposure at 3 keV across the field of view for the set of three telescopes clocked in the IXPE configuration.


Relative exposure at 8 keV across the field of view for the set of three telescopes clocked in the IXPE configuration.

Of course, unless you specifically decide to disable this functionality, ixpeobssim handles all of this behind the scene, so you don’t have to worry about it—but keep it in mind when you do back-of-the-envelope calculations.

Energy dispersion

The energy dispersion (a.k.a. the response matrix) comes from a series of line Monte Carlo simulations performed with the IXPE GPD Geant 4 simulation framework. Below is a color representation of the energy dispersion as a function of the energy, which is essentially the content of the binary table in the MATRIX extension of the rmf file.


Representation of the GPD response matrix.

For illustration purposes, here are the corresponding one-dimensional pdfs at a few fixed true energies (i.e., these are just vertical slices of the color plot above).


Energy dispersion (one-dimensional probability density function) at a set of discrete energies. The FWHM energy resolution is indicated for completeness.

Point-spread function

The PSF model is derived from one of the early point-source observations, as described in issue #158. Starting with version 6 of the instrument response function each DU comes with a different PSF scaling factor to account for the differences measured during the mirror calibration. As shown in issue #387, MMA 1 has a significantly better PSF (less than 20 arcsec HPD) than MMAs 2 and 3 (running at more than 25 arcsec HPD).


Encircled energy fraction (EEF) for the PSF of the three IXPE telescopes.

For completeness, the current set of response functions do not include the mirror aberration, which is nonetheless much smaller than the PSF half-power diameter across the entire field of view and is therefore, to first order, negligible.

Modulation factor

Our parametrization of the modulation factor comes from a series of line-type Monte Carlo simulations, informed by the ground calibrations of the three detector units.


Modulation factor as a function of the photon energy for the IXPE detectors. The solid line represents the average for the three GPD, in the un-weighted and weighted version, while the un-labeled dashed lines (admittedly, barely visible) represent the curve for each of the three detectors.


The noticeable edge around 9 keV is due to the K-edge of the copper, above which the extraction of photoelectrons from X-rays absorbed in the GEM becomes significantly more likely. This causes an increase of effective area, accompanied by a dilution of the modulation. While we provide a tabulation of all the IRFs in the standard grid between 1 and 12 keV, significant more work is needed to validate the response of the detector above the Cu K-edge, and simulations outside the 2–8 keV standard range should be interpreted with caution.

Below is an alternative representation of the overall IXPE spectro-polarimetric response, combining the elements described above.


Overall IXPE spectro-polarimetric response (all the elements are described in the first part of this section).

Minimum detectable polarization

The effective area curve (for the sum of three mirror modules) and the modulation factor are enough for a crude estimation of the minimum detectable polarization for a point source, and for reference we produce the basic performance plot below for each iteration of the response functions using xppimms for definite sets of spectral indices and rescaling for the source flux and the observing time.


IXPE Minimum Detectable Polarization (MDP) as a function of the source flux for several different exposure times and spectral indices.

Reading and visualizing IRFs

In a nutshell, the recommended way to load the default set of response functions (whatever that means at any point in time) is

from ixpeobssim.irf import load_irf_set

# Load all the default response functions.
irf_set = load_irf_set(du_id=1)

# Access the actual response functions.
aeff = irf_set.aeff
vign = aeff.vignetting
edisp = irf_set.edisp
psf = irf_set.psf
modf = irf.modf

# Print the effective area and modulation factor at 5 keV.

The reader is referred to the documentation and the source code of the relevant classes for a full description of the interfaces that ixpeobssim provides.

For completeness, ixpeobssim makes available as a single visualization interface to all the response file. Just type path/to/the/response/file.fits

and you should get back some sensible visualization of the thing.


For convenience, at this point in time, ixpeobssim is effectively implementing its own, self-contained CALDB—that we sometimes refer to as the ixpeobssim pseudo-CALDB. The plans for interfacing ixpeobssim with the actual IXPE CALDB are not yet defined as there are definitely peculiarities on both sides (simulation and real data) that make having a drop-in replacement structure less than trivial.

The actual FITS files with the response data live (provisionally) in the ixpeobssim/caldb folder and the basic logic determining the naming and the file location is defined here.


New in version 21.0.0.

Starting from version 21.0.0 the structure of the pseudo-CALDB has been drastically changed to match as closely as possible that of the actual CALDB submitted to HEASARC.

The ixpeobssim internal rules for the IRF-name designation have also been modified changing the delimiter, in order to have a better match with the CALDB file names and provide support for weights in a more straightforward fashion.

ixpeobssim will, at least provisionally, maintain a separate version numbering with respect to the official CALDB.

The convention we use to name response file is [base][unit][calibtype][intent][ver], where:

  • [base] is the base name for the set of response functions, e.g., ixpe_;

  • [unit] indicates the telescope unit (du1, du2, du3).

  • [calibtype] provides an identifier for the calibration data (e.g., vign or psf), with the exception of the arf, rmf and mrf files, where the data type is indicated by the file extension;

  • [intent] is the intent of a particular set of response functions, e.g., _obssim_;

  • [version] is the CALDB version number for any given file.

Additionally, each coherent set of response functions is identified within ixpeobssim by a name, in the form [base]:[intent]:[ver]. ixpeobssim is able to parse a string formed according to this rule and resolve all the relevant paths to the actual response files.


The pseudo-CALDB contains a number of response files that are not shipped with the real CALDB, including pre-launch estimates that we retain for bookkeeping purposes, but should not be used to analyze flight data.

In a nutshell: all the set of response files named as *_legacy_* should be considered of solely historical interest and should never be used in conjunction with flight data samples.

Response file versioning

This is a short description of the main features of different sets of response files that are useful for simulation and science analysis:

  • ixpe:obssim*:v13: compared to the previous iteration (ixpe:obssim:v12), this collection of response files includes an updated parametrization of the MMA effective area at high energy (that should improve the spectral residual and the spectral fitting in general), as well as a better estimate of the pressure inside the gas pixel detectors.

  • ixpe:obssim:v12: compared to the previous iteration (ixpe:obssim:v11), this version includes an updated parametrization of the effective area for MMA 3, accounting for its thicker thermal shield (the effect is of a few % at 2 keV, and negligible above 4 keV, but has been shown to improve the spectral residuals at low energy for bright sources), as well as a new set of weighted response files with the SIMPLE prescription, that are necessary for a proper weighted model-independent analysis.

  • ixpe:obssim:v11: identical to ixpe:obssim:v10, except that the PSF parametrization (used on the simulation side of things) has been improved to match the on-orbit radial dependence measured with point sources; all the other response files are unchenged.

  • ixpe:obssim:v10: this is the first iteration of the response files matching the structure of the actual CALDB, and the first that can be used with real data.

Mapping to the real CALDB

Although the pseudo-CALDB and the real-CALDB are fundamentally different in some key aspects, most of the relevant files (e.g., those containing the effective area, the response matrix and the modulation response function) have a definite, one-to-one correspondence between the two databases—meaning that they are identical, modulo a few header keywords.

  • ixpe:obssim*:v13 maps to equivalent files in the CALDB with the same starting date.

  • ixpe:obssim:v12 maps to 20170101_02

  • ixpe:obssim:v11 maps to 20170101_01

  • ixpe:obssim:v10 maps to 20170101_01

The structure of the ixpeobssim pseudo-CALDB is designed to match as closely as possible that of the real IXPE CALDB.

    |---- cpf

The naming conventions for the FITS files have been aligned to the IXPE CALDB starting from version 10, and the file names for the previous iterations have been changed accordingly.

There are two subtle but noticeable differences between the IXPE CALDB and the ixpeobssim pseudo-CALDB, namely:

  • the pseudo-CALDB has no concept of validity date, and that is reflected both in the file names and in the corresponding header keywords;

  • the versioning scheme has a different meaning in the two contexts, and version numbers are physically assigned by different people at different times (more specifically, ixpeobssim uses a unique, sequential identifier that is tied to the basic ingredients going into the response functions, while the actual IXPE CALDB can have the same identifiers for files with a different validity epoch).

The latter difference is also reflected in the format string for the version identifier in the file name, which is '%02d' for the IXPE CALDB and 'v%03d' in the pseudo-CALDB.

If you are really careful you will also notice that, for weighted response files, the position of the weight identifier in the file name for the modulation factor is different in the pseudo-CALDB, compared with the real one, i.e., the mapping is in this case


This is just an historical accident that, at this point, is not worth correcting. (And you probably will never need the modulation factor, anyway.)

Historical notes


The links provided below are ultimately pointing to IXPE private repositories to which only collaboration members have access.

The release process and the differences with respect to the previous iterations are summarized on our issue tracker at:

Version 8 of the response files is the first supporting XSPEC spectro-polarimetric analysis with weights. Version 9 is fairly similar, with a refined parametrization of the MMA effective area. Version 10 features a few new header keywords, and is the one on which the first version of the CALDB submitted to HEASARC is based.

Version 7 of the response files features the first non-diagonal response matrix.

Version 6 of the response function is the first iteration taking full advantage of the flight DU calibration and the telescope end-to-end calibration. This is also the last iteration of the response function using the now infamous 80% cut—the next ones will hopefully support ensemble-weighted analyses.

Starting with version 6 of the instrument response function the PI runs from 0 to 374 (included), corresponding to a physical-space binning spanning the 0–15 keV energy range in steps of 40 eV. (In previous iterations the PI spanned the very same energy interval used to define the response functions, i.e., 1–12 keV.)

All the non-standard versions of the response files (e.g., without the standard cuts or with the MMA alone) have been dropped altogether starting from version 4.

In iterations of the response functions up to v3, ixpeobssim used to ship combined versions of the effective area and modulation factor, that were useful for back-of-the envelope sensitivity calculations. From version 4 onward this is no more the case, and all the relevant applications have been modified to make the appropriate loop over the three detector units where the combined response functions were used before.