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XMM-EPIC status of calibration and data analysis XMM-Newton Science Operations Centre XMM-SOC-CAL-TN-0018 Page: - 1 ­ Issue: 2.4 M. Kirsch Date: 11.02.05

EPIC status of calibration and data analysis
Marcus Kirsch with the inputs from the whole EPIC Consortium please send all comments to mkirsch@sciops.esa.int This document reflects the status of the calibration of the EPIC camera as implemented in SAS 6.1 with all available CCFs at 31/12/2004. Furthermore the outlook is considered for improvements of calibration, which at the moment can be expected for the next SAS release.

Contents:
1 Calibration Overview __________________________________________________________ 2 1.0 Summary _______________________________________________________________ 2 1.1 Imaging ________________________________________________________________ 6 1.1.1 Astrometry __________________________________________________________ 6 1.1.2 Point Spread Function and Encircled Energy ________________________________ 7 1.2 Eff 1.2.1 1.2.2 1.2.3 1.2.4 ective Area __________________________________________________________ Mirror collecting area _________________________________________________ Filter transmission ____________________________________________________ CCD Quantum efficiency ______________________________________________ Vignetting __________________________________________________________ 11 11 11 12 13

1.3 Energy Redistribution ___________________________________________________ 15 1.3.1 MOS ______________________________________________________________ 15 1.3.2 PN ________________________________________________________________ 15 1.4 1.5 1.6 CTI/Gain ______________________________________________________________ 16 Background ____________________________________________________________ 18 Timing ________________________________________________________________ 20

Examples of EPIC spectra ________________________________________________ 21 1.7 1.7.1 PKS 0558-508 _______________________________________________________ 21 1.7.2 PKS 2155-304 _______________________________________________________ 22 1.8 2 Cross Calibration with RGS and other Satellites _____________________________ 23 features in SAS _____________________________________________________ SAS 5.3.0 __________________________________________________________ SAS 5.3.3 __________________________________________________________ SAS 5.4.1 __________________________________________________________ SAS 6.0.0 __________________________________________________________ SAS 6.1: ___________________________________________________________ 23 23 23 23 24 24 Data Analysis _______________________________________________________________ 23 2.1 New 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5

2.2 Data Analysis ___________________________________________________________ 24 2.2.1 MOS ______________________________________________________________ 25 2.2.2 PN ________________________________________________________________ 25


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1

Calibration Overview

This section gives a short overview of the status of the calibration of the EPIC instruments MOS1, MOS2 and PN, operating on-board the XMM-Newton observatory. It summarises the quality of the calibration to the extent that this may influence the scientific interpretation of the results. The instrument calibration is based on a physical model of the various components including mirror response, filter transmission and detector response (energy redistribution, gain, Charge Transfer Efficiency). During ground calibration various components were calibrated and the physical model for each component was optimised. These models were verified in flight and are, where relevant, continuously monitored (e.g. contamination of the detector, changes in gain and CTI due to radiation damage). Corrections, which are needed for these time-variable changes, will be applied to the Current Calibration Files (CCFs) and/or the processing software. For more detailed information see the release notes of the CCFs at: http://xmm.vilspa.esa.es/ccf/releasenotes/ Blue coloured text gives distilled information or html links. Red coloured text marks current problems.

1.0

Summary

We give in the next table a summary of the status of the calibration:

Effect Absolute Astrometry Relative Astr. within 1 camera Relative Astr. between 2 cameras Point Spread Function (PSF) Relative Effective Area Absolute Effective Area Absolute Energy scale Relative Timing Absolute Timing

Max. Error 1''(r.m.s.) 1.5''(r.m.s) 1.5''(r.m.s) 2% ±5% ± 10 % ± 10 eV P/P<1E-8 250-500 µs

Energy dependent NO NO NO YES YES YES YES NO NO

Off axis angle dependent YES YES YES YES YES YES YES NO NO

Summary of improvements since the 2.3 issue of the document: · PN telescope effective area: XMM-Newton EPIC-pn observations with very high statistical precision show residuals at the silicon edge around 1.8 keV and gold edge around 2.2 keV. The effective area around the gold edge has been changed in order to improve the residuals around that edge in the pn-spectra (see: XMM-CCFREL-174). PN Small Window mode CTI: EPN_CTI_0013.CCF reduces the effect of under-correction of the CTI of 2-3 % for the pn Small Window mode at energies between 550 and 700 eV where O-lines are found in supernova remnants (SNRs). In addition, in this CCF the Large Window mode CTI was adjusted to the Small Window mode CTI for energies below 500 eV. This improves the former residuals in the fitted spectra (see XMM-CCFREL-172). PN extended Full Frame mode CTI: The internal calibration source showed an over-correction of up to 15 eV at Mn-K line position (5896 eV) in pn extended Full Frame (eFF) mode, that was related to imperfect Gain/CTI

·

·


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correction. Special calibration observations have been performed on the SNR CAS-A leading to a correction function for the eFF mode implemented in SAS 6.1. Line positions are now accurate to 0.1 % compared to the Full Frame mode. epevents-6.41 [SAS-6.1] has default setting "withgaineff=N" while in the DT it is already "withgaineff=Y" (correspondingly for epchain-8.53 in SAS-6.1]. After the deadline for SAS-6.1 the tests for the setting "withgaineff=Y" were finished and successful. It is highly recommended to use the setting "withgaineff=Y". · PN long-term CTI correction: The long term CTI behaviour of all modes is now modelled with an additional quadratic term in order to tail off the time dependence (see XMM-CCF-REL-172). This affects all PN modes and thus all event files should be reprocessed. PN redistribution: For SAS 6.0 the re-distribution was strongly increased for energies below the O-edge (0.53 keV) in order to reduce excesses seen at low energies. In particular for the isolated neutron star RX J1856.53754 a soft excess in the EPIC-pn spectrum resulted in an absorption column density of almost zero in comparison to NH = 91019 cm-2 obtained from a 500 ksec Chandra LETGS high resolution spectrum. At that time the re-distribution parameters were adjusted to match the column density value obtained with LETGS.

·

Zeta puppis with SAS6.0 (upper) and with SAS6.1 (lower). Both plots show spectra and ratio plots for data versus model. The second plot shows an improvement in the agreement between data and model compared to the first one.


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Subsequent analysis of EPIC-pn spectra of Zeta Puppis (with SAS 6.0) however revealed large residuals around 430 eV (N lines) which clearly showed that the redistribution was modelled wrongly. The original redistribution parameters model the N lines in the Zeta Puppis spectrum better. Therefore, values near the old (from ground calibration) ones were adopted for SAS6.1. As a result the column density derived from the EPICpn spectrum of RX J1856.5-3754 is again lower (by 71019 cm-2) than the LETGS value (see also same topic in "Summary of important ongoing calibration topics"). Summary of important ongoing calibration topics: · · EPIC-MOS cameras show, for energies above 3keV, an excess of up to 15 % with respect to EPIC-pn that might be related to various system components and is under investigation. PSF core: The improved King function of the PSF is a good but not perfect fit to the PSF of the telescopes, as the core of the PSF is very slightly underestimated. This affects the MOS more than the pn (as the MOS detector pixels are much smaller than the pn pixels). This can produce an error in the enclosed energy of at most ~2 %, depending on instrument, energy and extraction radius. Work is currently underway to model the PSF as a combination of a King function plus a Gaussian function (the latter to model the slight excess at the core). Astrometry: A possible residual in the position angle rotation (Euler angle) of the order of 0.1 deg is under investigation. This could lead to an uncertainty of up to 1.5'' at the edge of the XMM-Newton field of view. MOS columns with corrupted energy: In addition to those columns already declared as dead in the CCF, other columns of the MOS cameras provide currently (after cooling at revolution 533) wrong energy information, because they suffer from higher CTI losses than nominal columns. This can lead to errors in the energy determination of up to 200 eV if spectra are only reduced from that very column. The feature will be addressed in the future by a column dependent CTI correction also for the MOS cameras. For the time being we recommend to exclude those columns for the analysis of line features etc. The table below lists the effected columns in CCD1 of both MOS cameras: MOS1 ADU offset 27.3 26.4 37.8 19.5 23.5 41.9 31.5 28.3 MOS2 ADU offset 20.2 16.7 19.2 53.6 15.1

· ·

Column 111 259 287 320 342 360 437 537

eV offset 92.4 89.6 126.9 67 80.2 140.2 106.3 95.8

Column 18 112 231 291 304

eV offset 63.7 51.8 60.2 176.3 46.5

·

Epoch and spatial dependent aspects of the MOS calibration: The response of the MOS detectors is epoch dependent. Ageing of the CCDs due to particle-induced traps has decreased the average charge transfer efficiency and subsequent spectral resolution with time since launch. From orbit 534 onwards the detector plane was cooled by 20 degrees centigrade down to its current operating temperate of -120 degrees centigrade. This improved the CTI and energy resolution immediately and also reduced the "rate" of degradation. These changes are regularly monitored and the relevant current calibration files (CCFs) in the SAS adjusted accordingly. An additional epoch dependent effect is the change of the energies. Towards lower energies the RMF deviates from a effects, thought to occur at the surface of the CCD, become be seen, for example, in sources with high column densi energies well below the cut-off due to the absorbing column. shape of the redistribution function (RMF) at low typical Gaussian response as additional charge loss increasingly important. The result of this effect can ties, which still show detected source photons at


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Analysis of various targets has shown that the scale of this surface loss effect is increasing with time. This is partially accounted for in the SAS because rmfgen produces epoch dependent MOS RMFs. The calibration of this effect has employed a number of targets with reasonably well-constrained astrophysical parameters such as isolated neutron stars, BL Lac objects with well-established column densities and line dominated galactic objects. It has become apparent, however, that the RMF change has also developed a spatially dependent component in that the response for photons detected near the telescope boresights shows an enhanced surface charge loss with respect to photons detected away from the boresight. The scale and magnitude of this effect is in the process of being calibrated. The affected portion of the spectrum is primarily below around 650 eV. The systematic error in the redistribution function means that the error in spectral fitting in a given portion of the spectrum depends primarily on the fraction of observed redistributed photons at that energy. The current RMF produced by rmfgen has been tuned at low energies using observations of the isolated neutron star RXJ1856.6-3754 and as a consequence is most accurate for on-axis point sources, which have a strong soft component. For on-axis sources with harder spectra the current RMF is more accurate for photons detected in the WINGS of the psf than the CORE and it is recommended that a region of about 15 arcseconds (radius around the core) be excised fro m the analysis. If a target is highly cut-off and has little direct flux below 650 eV there is no need to excise the core if the source is not piled-up. · PN-redistribution: Observations of the white dwarf GD153 with various EPIC-pn read-out modes and filters yielded large inconsistencies between the spectra below 0.5 keV. A strong correlation is seen between apparent count rates and read-out mode (slower read-out results in higher count-rates and harder spectrum) and filters (medium filter reduces the count rate more than expected from the thin/medium ratio). The most-likely explanation for this effect is pile-up. In principle three kinds of pile-up are possible: pile-up of two source photons, a source photon with electronic noise and a source photon with optical photons from the source. The white dwarf spectrum (kT=25 eV, Black-body) has its maximum at 75 eV, which is below the low-energy event threshold of the instrument (20 adu (not CTI corrected) which effectively corresponds to about 115 eV at the focus position). I.e. the bulk of the photons does not produce events above the threshold. However pile-up can bring the energy of a sub-threshold event above threshold. The increasing effective area of the instrument belo w 100 eV supports photon pile-up from very soft sources. A very strong source of pile-up is the steeply increasing number of noise events, from which only a small tail is visible above the threshold. Source photons with energies below the threshold (which would nominally not be detected) have a high probability to "gain energy" by fortuitously adding to noise. For weak sources (and/or fast readout) this is most likely the dominant pile-up effect. PN spectroscopy with double events: Analysis of the pattern distributions in EPIC pn has shown that charge is more often observed to be split along readout direction than perpendicular to it, even after the effect of reemission has been taken into account. These additional `false doubles' show peculiarities in their pulse height distribution, which may introduce a systematic error in the energy. Thus, in particular for sources with bright emission lines, it is recommended to restrict the spectral analysis to single events.

·


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1.1

Imaging
be assigned to source images in the EPIC relative to optical coordinates), Relative systematic offsets due to spacecraft misrelative to another one) The XMM-Newton absolute astrometry accuracy is limited by the precision of the Attitude Measurement System. Fig. 1-1 shows that the shift from the XMMNewton-EPIC to the optical frame is on average 0'' with a standard deviation of less than 0.8'' per axis. Hence the Absolute Pointing Accuracy is considered to be better than 1" (r.m.s.). The relative astrometry within each camera is accurate to 1.5" for all cameras and over the full field of view. The MOS metrology has been revised with SAS6.0, searching for systematics in the offsets of MOS peripherical CCDs with respect to the central CCD by using observations on rich stellar fields. CCD offsets of up to 2.7" have been corrected in the MOS LINCOORD CCF issue 17. With this new CCF the MOS relative astrometry accuracy has been assessed to be 1.5" (r.m.s.) while it is as good as ~1.0" (r.m.s.) for EPIC-pn. Among all three EPIC cameras the relative astrometry is also estimated to be better than 1.5" across the whole field of view. Note that for faint MOS sources near the detection limit the statistical accuracy of the measurement limits the 90 % confidence contours to 24". A possible residual in the position angle rotation (Euler angle) of the order of 0.1 deg is under investigation. This could lead to an uncertainty of up to 1.5'' at the XMM-Newton field of view.

1.1.1 Astrometry Astrometry means: The precision with which astronomical coordinates can focal plane. We distinguish between: Absolute Astrometry (precision Astrometry per camera (precision within one camera after correction for pointing) and Relative Astrometry between cameras (positions in one camera

Figure 1-1: Histogram of the distribution of offsets for each EPIC camera with respect to the 2MASS reference frame projected on to the two spacecraft axis. Top: MOS1, Middle: MOS2, Lower: pn (Details at XMM-SOC-CAL-SRN-168)


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1 .1 .2 Point Spread Function and Encircled Energy Point Spread Function: The spatial distribution of light in the focal plane in response to an observed (monochromatic) point source. The PSF integrates to 1 over the infinite focal plane. Encircled Energy: The fraction of the energy of a point source collected within a certain radius. Though the shape of the PSF is quite complex, the radially averaged profile can be adequately represented by an analytic function - a King function - whose parameters, core radius r0 and index , are themselves functions of energy and offaxis angle:

PSF

=

È Ôr A É1 + Ã Ãr É Õ0 Ê

Æ Â Â Ü

2

Û Ç Ç Ø

-

It is worth noting that both this function and its integral are analytic. Earlier work (EPIC-MCT-TN-011, EPIC-MCT-TN-012, XMM-CCF-REL-116) used many and off axis to determine the energy dependent PSF. This resulted in a linear dependency off-axis angle. It is shown in XMM-SOC-CAL-SRN-0167 that this linear dependency is no r0 and are seen to be flatter (almost constant) with energy (at least out to ~ 8-10 keV). rapidly turn steeper. bright point sources both on of r0 and with energy and t valid - the dependencies of Thereafter the dependencies

Figure 1-2: Surface brightness radial profiles (crosses) plus fitted King profiles (lines) for two examples: (left) MCG-06-30-15 Rev. 303 pn at 6 keV (right) MCG-06-30-15 Rev. 302 MOS2 at 0.475 keV.

Two threads of analysis using data from very long and clean Small Window mode observations of very bright non piledup sources were followed: One involved the forming of narrow-energy-band images, and fitting the surface brightness radial profiles with a King function to obtain r0 and as a function of energy. A second analysis thread involved the extraction of spectra from narrow annuli around point sources, and once Ancillary Response Files (ARF) had been


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generated (this involving the actual form of the PSF), the spectra were fitted with standard spectral models, to see how (if at all) the spectral parameters obtained varied with extraction radius. This whole process was repeated for several sets of PSF parameters (including those obtained from the surface brightness radial profile fitting described above). Two examples of surface brightness radial profiles plus fitted King profiles are shown in Fig.1-2. The resultant dependencies of r0 and are seen to be flatter (almost constant) with energy, at least up to ~ 8-10 keV (where the r0 and relationships turn over) than in the previous parameterisation of the PSFs as shown in Fig. 1-3. The new PSFs were used in the analysis of spectra extracted from narrow annuli around a number of bright point sources, as described above. Fig. 1-4 shows how the fitted normalization and power-law index vary as a function of the radial distance of the extraction region (0-5", 5-10" etc.) for the old CCF PSFs and the new CCF PSFs described here, for the MCG-06-30-15 Rev.302 data. A point source should show no variation in fitted spectral parameter whether the spectrum is extracted from the very centre of the distribution or from the wings. Usage of previous PSFs results, however, in a very wide range of spectral parameters for different radii. Usage of the new PSFs gives rise to a significantly improved situation, with the fitted normalization and power-law index remaining constant and 'flat' with radius. Figure 1-3: r0 ­ Energy (top) and ­ Energy (bottom) dependencies for the MOS1, MOS2 and pn on-axis PSFs.

Figure 1-4: Plots showing how the fitted normalization (top) and power-law index (bottom) vary as a function of the radial distance of the extraction region (left to right: 0-5" (circle), 5-10" (annulus) , 10-15'' (annulus, etc.), using the current CCF PSFs (left) and the new CCF PSFs (right) for the MCG-06-30-15 Rev. 302 data. The different symbols represent different cameras.


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A major problem with the previous parameterisation was its inability to produce consistent spectral fits for annular extraction regions such as those for instance used for the analysis of piled-up sources. Hence MCG-6-30-15 (from Rev 302) has been extracted from annuli of 5-40", 10-50" and 15-60" and fits have been compared to those obtained for a circular extraction (0-30"). This has been performed using the old and the new PSFs, and the results are presented in Fig. 1-5. Whereas usage of the old PSFs results in a per instrument normalization variation of up to 40%, and changes in the fitted spectral slope of 0.2, the new PSFs give rise to normalization variations of ~ 5% and a spectral slope change of at most 0.03.

Figure 1-5: Plots showing how the fitted normalization (top) and power-law index (bottom) vary as a function of extraction region (left to right: 0-30" circle, 5-40" annulus, 10-50" annulus, 15-60" annulus), using the old CCF PSFs (left) and the new CCF PSFs (right panel) for the MCG-06-30-15 Rev.302 data. Note that the King function is a good but not perfect fit to the PSF of the telescopes, as the core of the PSF is very slightly underestimated. This affects the MOS more than the pn (as the MOS detector pixels are much smaller than the pn pixels), and in Fig. 1-2, the MOS PSF (right) fit can be seen lying underneath the data points at the very centre. This can produce an error in the enclosed energy of at most ~2 %, depending on instrument, energy and extraction radius. Work is


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currently underway to model the PSF as a combination of a King function plus a Gaussian function (the latter to model the slight excess at the core). Off-axis PSF As yet, no sources bright enough for this new type of analysis to be performed off-axis have been observed. Therefore, the general off-axis results of previous work (EPIC-MCT-TN-011, EPIC-MCT-TN-012, XMM-CCF-REL-116) have been used to transform the new on-axis parameters presented here to projected off-axis values. How the PSF varies off-axis is only known reasonably well for small off-axis values. The following two plots indicate the reliability of the PSF at several off-axis locations:

Figure 1-6: Reliability for PSF In the green region: For low energies and nearly on-axis positions a large quantity of data is available and in general statistics for these measurements are good. Error bars are in general small (/r0 (or ) <~ 1 %) and the r0 and evaluations for these curves are not very "far" from the final fit (r/ r < 5% is the worst case). In the yellow regions: for some off-axis angles few measurements are available (< 2-3) at the different energies. In general these measurements have large uncertainties (/rc0 ~ 10%) and the r0 and parameters for these sources sometimes are "far" from the final best-fit values (r /r can be as large as 10-20%). In the red region: no calibration data are available, and best-fit values coming from the model are an extrapolation, not a real calibration. Addendum: Off-axis PSF MPO speciality While the EPIC-pn PSF is azimuthally symmetric, the placing of the CCDs in the MOS cameras to follow the focal plane results in a chip-to-chip variation in the MOS PSF (Saxton, R.D., Denby, M., Griffiths, R.G., Neumann, D.M., 2003, Astron.Nach., 324, 138.). This is not currently modelled in the SAS calibration but will result in an azimuthal variation in the encircled energy fraction which is dependent on the extraction radius and the off-axis angle. This variation is not yet quantified but is estimated to be ± 4% for a source circle of radius 25" at an off-axis angle of 7'.


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1.2

Effective Area

Effective Area means: the collecting area of the optical elements folded with the energy-dependent sensitivity of the detector systems of the EPIC cameras. For on-axis sources an internal accuracy of better than 5% in the determination of the total effective area is reached over the spectral range from 0.4 - 12 keV for each instrument separately. The cross calibration between MOS1 and MOS2 agrees within 5 %. The MOS/PN global flux normalization agrees to 10 % from 0.4 keV to 12 keV. In the range from 0.3-1.0 keV the PN camera shows an up to 10 % higher flux than the MOS, while for energies above 1.5 keV the MOS flux is up to 15 % higher than the PN (see 1.7.2). This is currently under intense investigation and could be due to uncertainties in the vignetting and CCD-Quantum-Efficiency. 1.2.1 Mirror collecting area Mirror collecting area means: the face-on area of the mirror system that reflects X-rays to the focal region. The mirror collecting area has been measured on ground and verified in observations with very high statistics still showed residuals at the Silicon edge keV. The gold edge feature could be explained by an imperfect calibration of area around the gold edge has been changed in order to improve the residual XMM-CCF-REL-174). orbit. However, XMM-Newton EPIC-pn around 1.8 keV and gold edge around 2.2 the telescope effective area. The effective s around that edge in the pn-spectra (see:

1.2.2 Filter transmission Filter transmission means: the fraction of incident X-ray photons that pass through the filter The filter transmission has been measured on ground. The following plot shows filter transmission curves for all the cameras. For the thick filter the green curve shows the single transmission function currently used in the CCFs for MOS1, MOS2 and PN.

Figure 1-7: Filter transmissions in CCF and ground calibration filter measurement data points. green - thick, red - medium, blue- thin (Thin1 & Thin2 have the same CCF) ground: squares-PN, star-M1, diamond M2 , CCF: dashed lines


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1 .2 .3 CCD Quantum efficiency Quantum Efficiency means: the fraction of incident photons on the detector that generate an event in the CCD. Ground calibration measurements have shown that the quantum efficiency of MOS CCDs is spatially uniform above 400 eV. Below this energy spatial variations within a CCD are seen as patches in the outer parts of the CCDs where the response is degraded. This inhomogeneity is currently not taken into account by the SAS.

Figure 1-8:QE ground measurements and CCF (upper MOS1 CCD1, lower PN)


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Figure 1-9: QE spatial inhomogeneities within MOS1 appearing only at very low energies (left 150 eV, right 400 eV).

1.2.4 Vignetting Vignetting means: reduction in the effective area with radial distance from the telescope's axis. The telescope vignetting is well determined for off-axis angles of up to more than 10'. One of the most important outstanding problems of the calibration was an offset of around 1' in the telescope axis from nominal. This did not affect the astrometry but could have been the reason for some of the flux discrepancies between MOS and PN caused by the vignetting correction which was not adapted to this offset in SAS versions earlier than 6.0.0. The offset was determined and implemented in the corresponding CCF (XMM_MISCDATA_0020). With SAS 6.0.0 and the current available CCF the corrected optical axis position improves the vignetting correction. However the vignetting correction itself has not changed at all, the only difference is that it is now applied for correct off axis angles that could not be calculated correctly before due to the wrong information for the optical axis. This improves differences in flux for off axis sources for each camera from ± 14 % to ± 5 %. Detailed information can be found in XMM-CAL-TN-54 and XMM-CAL-SRN-0156. Figure 1-10 shows the average vignetting measured over 4 azimuths at an off-axis angle of around 10' derived from Calibration measurements of the SNR G21.5-09


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Figure 1-10: Average vignetting measured over 4 azimuths at an off-axis angle of around 10' derived from calibration measurements of the SNR G21.5-09


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1.3

Energy Redistribution

Energy Redistribution means: The energy profile recorded by the detector system in response to a monochromatic input. 1.3.1 MOS The same energy redistribution matrices can be used for all MOS imaging modes. Since SAS 5.4.1, rmfgen supports all MOS modes and takes the observed change in energy resolution with time into account. Also, ready-made redistribution matrices addressing this, are available at http://xmm.vilspa.esa.es/external/xmm_sw_cal/calib/epic_files.shtml.

Figure 1-11: Energy resolution MOS 1 CCD1 in eV. The drop after rev 532 is due to the cooling of the cameras. The red vertical lines indicate major solar flares. 1.3.2 PN The energy redistribution is mode-dependent for the PN camera. That means that different respons