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XMM-EPIC status of calibration and data analysis XMM-Newton Science Operation Centre XMM-SOC-CAL-TN-0018 Page: - 1 ­ Issue: 2.1 M. Kirsch Date: 04.04.03

EPIC status of calibration and data analysis
Marcus Kirsch with the inputs from the whole EPIC Consortium please send all comments to mkirsch@xvsoc01.vilspa.esa.es

This document reflects the status of the calibration of the EPIC camera as implemented in SAS 5.4.1. 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 ________________________________ ________________________________ ___ 4
1.1.1 1.1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.3.1 1.3.2 Astrometry ________________________________ ________________________________ ______ 4 Point Spread Function and Encircled Energy ________________________________ _____________ 6 Mirror collecting area________________________________ _______________________________ Filter transmission ________________________________ ________________________________ _ CCD Quantum efficiency________________________________ ____________________________ Vignetting ________________________________ ________________________________ _______ 10 10 11 12

1.2 Effective Area ________________________________ _____________________________ 10

1.3 Energy Redistribution ________________________________ _______________________ 14
MOS ________________________________ ________________________________ ___________ 14 PN ________________________________ ________________________________ _____________ 15

1.4 CTI/Gain ________________________________ ________________________________ _ 15 1.5 Background ________________________________ _______________________________ 16 1.6 Timing ________________________________ ________________________________ ___ 17 1.7 Examples of spectra ________________________________ _________________________ 19
1.7.1 1.7.2 MOS2 PKS 2155-304 ________________________________ ______________________________ 19 3C273 ________________________________ ________________________________ __________ 19

1.8 Cross Calibration with RGS ________________________________ __________________ 21 1.9 Cross Calibration with other Satellites ________________________________ __________ 22

2

Data Analysis _______________________________________________________________ 24
2.1 New features in SAS ________________________________ ________________________ 24
2.1.1 2.1.2 2.1.3 2.2.1 2.2.2 SAS 5.3.0________________________________ ________________________________ ________ 24 SAS 5.3.3________________________________ ________________________________ ________ 24 SAS 5.4.1________________________________ ________________________________ ________ 24 MOS ________________________________ ________________________________ ___________ 25 PN ________________________________ ________________________________ _____________ 26

2.2 Data Analysis ________________________________ _____________________________ 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, CTI). 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). Where applicable, corrections which are needed for these time-variable changes will be applied to the Current Calibra tion 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/ One of the most important outstanding problems of the calibration is a possible offset of around 1 arcmin in the telescope axis from nominal. This does not affect the astrometry but could be the reason for flux discrepancies between MOS and PN caused by the vignetting correction which has not yet been adapted to this offset. This problem is under intensive investigation. 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 Relative Astrometry Absolute Astrometry PSF Relative Effective Area Absolute Effective Area Line Energies Relative Timing Absolute Timing

Max. Error 1.5''(r.m.s.) 2-3'' (r.m.s) 2% ±5% ± 10 % ± 10 eV P/P<1E-8 500 µs

Energy dependent NO NO YES YES YES YES NO NO

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

Improvements since the last Issue: · MOS CTI correction has been modified in order to compensate the stepwise degradation after solar flares · · · · · · Refined calculation of the MOS gain as a function of observation epoch has been performed. Degradation of the MOS intrinsic spectral resolution due to the larger noise component of the degraded CTI has been modelled in new response matrices. Filter Transmission of the MOS thick filter has been revised and will be soon implemented in CCFs. arfgen and rmfgen now support all modes (incl. Timing and Burst modes). PN QE has been modified to account for the newly determined thickness of the SiO2 entrance window of the CCD (see also in 1.2.3: PN QE) Refined spectral redistribution for PN (concerns all readout modes) at energies below 600 eV.


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Important ongoing calibration topics: · Possible offset of around <1 arcmin in the telescope axis from nominal for all cameras has been under investigation with different analysis methods. The results are implemented in development CCFs and will be released in the near future. Recent observations of calibration targets confirm a significant difference in the low energy redistribution characteristics of the MOS cameras with respect to early observations. This change may be due to an increase in the surface charge loss property of the CCDs which degrades the low energy resolution. At the moment we can detect what appears to be a gradual change in MOS1 starting from around Revolution 200. Similar, but NOT exactly contemporaneous, changes occur in MOS2. The greatest change, however, appears to occur from around Revolution 450 onwards. The change in response is currently under investigation with a view to providing to the user, as soon as possible, epoch-dependent response matrices taking the variation in low energy response into account. It may take some time however to fully understand the observed behaviour. For the time being the user should respect the following rules:: o o o · before Revolution 200 the calibration was good down to ~200 eV with typically an uncertainty in derived column densities of < 5E19 between Revolutions 200 and about 450 the calibration below ~350 eV is suspect. from approximately Revolution 450 onwards the calibration below 500 eV is suspect.

·

A common strategy adopted to analyse spectra of piled up sources has been to excise the core of the PSF. Recent investigations have shown that, at least in some cases, this strategy can yield incorrect results. The problem is most likely related to an inaccurate modelling of the energy dependence in the PSF model used by arfgen to evaluate encircled energy fractions. The user who suspects that pile-up may be affecting their observation is advised to first use the epatplot tool to asses the presence and level of pile-up. In case of moderate pile-up (*) the user is advised to make use of pattern 0 spectra, which are less sensitive than other patterns, for both MOS and PN cameras and use the new method for pile-up correction provided by S. Molendi & S. Sembay http://xmm.vilspa.esa.es/external/xmm_sw_cal/calib/documentation.shtml#EPIC. For MOS-Timing mode a special CTI correction will be developed

·


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1.1

Imaging

1.1.1 Astrometry Astrometry means: The precision with which astronomical coordinates can be assigned to source images in the EPIC focal plane. The XMM absolute astrometry accuracy is limited by the Attitude Measurement System. This allows an Abso lute Pointing Accuracy reconstruction of better than 3 arcsec (r.m.s.). The relative astrometry within each camera is accurate to better than 1.5 arcsec for all cameras and over the full field of view. Among all three EPIC cameras the relative astrometry is also better than 1-2 arcsec across the whole field of view.

Figure 1 -1 : Offset from optical to X-ray frame for 677 XMM pointings. The averaged central position is indicated with a thick cross.


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Note that for faint MOS sources near the detection limit the statistical accuracy of the measurement limits the 90% confidence contours to 2-4 arcsec.

Figure 1 -2 : Overlay of PN and MOS positions


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Point Spread Function and Encircled Energy

Point Spread Function means: 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 means: fraction of the energy of a point source collected within a certain radius. A broad set of in-orbit calibration data has been considered to analyse the on-axis and off-axis (PSF) and the Encircled Energy Fraction (EEF) for the PN and for the two MOS cameras. Data point sources performed in different operating modes in order to inspect both the core and the shape of the PSF is quite complex but the radially averaged profile can be suitably represented by Point Spread Function include observations of wings of the PSF. The an analytical function.

PSF

=

A

1 r 1 + r c
2







The King function used to fit the PSF radial profile is characterised by two shape parameters, the core radius rc and the slope , both depending on the energy and the off-axis angle. For in orbit observations an additional background constant describes a more extended and diffuse component of the PSF. It is worth noting that both this function and its integral are analytic. Correspondingly, both the PSF and the EEF are analytically characterised. For a detailed description of the results see Ghizzardi S, 2001: http://xmm.vilspa.esa.es/external/xmm_sw_cal/calib/documentation.shtml#XRT The PSF is well-determined in orbit for small off axis angles and a few keV. The following two plots show the details for the reliability:

Figure In the green region: For low energies and nearly statistics for these measurements is good. Error evaluations for these curves are not very "far" from

1 -3 : Reliability for PSF on-axis positions a large quantity of data is available and in general bars are in general small (/rc (or ) <~ 1 %) and the rc and 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 errors ( /rc ~ 10%) and the rc 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.


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The following three plots show a comparison of the SAS template PSF and radial profiles created by co-adding several bright on-axis sources:

Figure 1 -4 : A comparison of the SAS PSF and radial profiles created by co-adding several bright non-piled-up on-axis sources :


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Figure 1 -5 : Encircled energy against energy for different annuli (numbers in arcsec) . The red data points are based on the CCF; the black data points are based on radial profiles created by combining data from several on-axis sources.


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Note that the King function is a good but not perfect fit to the PSF of the telescopes. The deviations from the meas ured in-orbit radial profile introduce a systematic error of ~2% of total counts. This gets smeared out in the 4 arcsec binned plots in figure 1-4 but can be seen in Figure 1-6. This equates to: Shape 60" circle 20" circle 5-40" annulus 15-60" annulus Error 2% 3% 4% 8%

Figure 1 -6 : The radial profile binned at 1 arcsecond shows an underprediction in the core. This gets smeared out in the 4 arcsec binned plots in figure 1-4 Addendum: Off-axis PSF 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 (ref 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 arcseconds at an off-axis angle of 7 arcminutes.


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1.2

Effective Area

Effective Area means: the effective collecting area of the optical elements and detector system of the EPIC cameras as a function of energy. 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 cross calibration agrees to 10 % from 0.4 keV to 12 keV. In the range from 0.3-1.0 keV the PN camera shows a up to 10 % higher flux than the MOS, while for energies above 1.5 keV the MOS flux is up to 10 % higher than the PN (see 1.7.2). This is currently under intensive 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 the ground and verified in orbit. 1.2.2 Filter transmission Filter transmission means: the fraction of x-ray photons that pass the filter The filter transmission has been measured on-ground. The following plot shows filter transmission 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. Later the CCFs will be refined to reflect the small differences among the three thick filters.

Figure 1 -7 : Filter transmissions in CCF and ground calibration filter measurements 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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Quantum Efficiency means: the fraction of photons that generate an event in the CCD. Ground calibration measurements have shown that the quantum efficiency of MOS CCDs is 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, lowe r PN)


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The overall discrepancies in the cross-calibration on-axis are probably due to uncertainties in the detector quantum efficiencies and uncertainty in the location of the on-axis position (see also 1.2.4).

Figure 1 -9: QE spatial inhomogeneities within MOS1 appearing only at very low energies (left 150 eV, right 400 eV). The regularly spaced "hot pixels" seen in the 400 eV image are not hot pixels at all, they have been interpreted as false events generated by the reset-on-demand mechanism. At the time this work has been done, this could not yet be suppressed.

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 arcmin . It should be noted that deviations in the data are best explained by assuming that the telescope axis intersects the fov around < 1 arcminute away from the position where it is assumed to do so (see table below). This could partly account for some (5 %) of the observed flux differences between MOS and PN (see 1.7). The results are implemented in development CCFs and will be released in the near future.

MOS1X shift in pixel shift in arcsec -5 -5.5

MOS1Y 9 9.9

MOS2X MOS2Y PNX PNY -25 57 16 5 -27.5 62.7 65.6 20.5


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Figure 1 -10: Average vignetting measured over 4 azimuths at an off-axis angle of around 10 arcmins (a telescope axis shift of 1 arcminute, as described in the text, has been assumed)


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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. In SAS 5.4.1, rmfgen supports 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 in eV. The red points show the energy resolution after cooling


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The energy redistribution is mode dependent for the PN camera. That means that different response matrices are needed for the different modes. In SAS 5.4.1 rmfgen supports all modes. Also ready-made matrices can be used, which can be obtained at: http://xmm.vilspa.esa.es/external/xmm_sw_cal/calib/epic_files.shtml . The response is mainly studied for on-axis sources. The response matrices are for all CCDs (note that for SW, TIMING and BURST only the CCD containing the focal point is used); the only dependence of the redistribution (i.e. the energy resolution) is the RAWY dependence (see 2.2.2). Other off-axis (radial) dependencies do not exist in the re-distribution, vignetting is part of the effective areas created by arfgen. More information on rmfgen and arfgen are available at: http://xmm.vilspa.esa.es/sas/current/doc/arfgen/index.html http://xmm.vilspa.esa.es/sas/current/doc/rmfgen/index.html.

We do not see any significant degradation of energy resolution with time in PN.

1.4

CTI/Gain

CTI means: Charge Transfer Inefficiency i.e. the imperfect transfer of charge as it is transported through the CCD to the output amplifiers. Gain means: Amplification of the charge signal deposited by a detected photon. In normal conditions Gain and CTI are known well enough so that line energy can be determined with an uncertainty of 10 eV over the full energy range and for all modes except MOS Timing mode. The new epoch dependent CTI and Gain correction in SAS 5.4.1 reduced the uncertainty from 20 to 10 eV for the MOS cameras. The reconstructed SAS line energies show however a small increasing trend of over correction especially at high energies (Mn-K) up to 5-8 eV too high. This may indicate a long-term change in the parameters of the MOS gain conversion, or an inadequacy in the complexity of the CTI correction. During the first eclipse season and during occasions when the RGS was first turned off (to diagnose a CCD chain failure), the platform temperature changed significantly and 10 - 30 eV discrepancies were occasionally observed in the PN. Such events occurred on the following few occasions:

Revolution 60 ­ 80 136-146 149-150

Problem 1st Eclipse RGS off RGS off

Subsequently the PN team has established a good correlation to implement a temperature dependent gain correction, but this could not yet be implemented in the SAS 5.4.1. Fortunately the temperature excursions are now dramatically reduced. For MOS no correction has been achieved yet, but the effect of the EMAE (EPIC MOS Analogue Electronics) temperature excursions is usually small. (MOS1: < 10eV at Mn-K and < 5eV at Al-K) Note that the CTI correction for MOS timing mode is currently over-correcting the data by ~10 eV.


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Figure 1 -12: Cas-A spectra in different PN-Modes (count rate spectra are scaled for clarity)

1.5
1.

Background
The astrophysical background dominated by thermal emission at lower energies (E<1 keV) and a power law at higher energies (primarily from unresolved cosmological sources). This background varies over the sky at lower energies. Soft proton flares where the spectrum varies from flare to flare. For weak sources the only option is to select quiet time periods from the data stream for analysis. To identify intervals of flaring background the observer should generate a light curve of high energy (E > 10 keV) single pixel (PATTERN = 0) events. To identify good time intervals use the selection criteria: · · MOS: < 0.35 cts/s (#XMMEA_EM && (PI>10000) && (PATTERN==0) on the full FOV PN: < 1.0 cts/s (#XMMEA_EP && (PI>10000) && (PATTERN==0) on the full FOV

There are three different types of background:

2.

3.

The high-energy proton induced background. These events are created directly by the protons penetrating the CCDs and indirectly by the fluorescence of satellite material to which the detectors are exposed. This background is being actively investigated and there are sample background event files available at


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http://xmm.vilspa.esa.es/external/xmm_sw_cal/calib/epic_files.shtml for use as templates. The goal is to provide a generic-modelling tool for all observation scenarios, but this will require very extensive work. The following two images show the strong metal line features that make the background subtraction complex, especially for large clusters and radial temperature determination. Explanatory notes for background subtraction are given @: http://xmm.vilspa.esa.es/external/xmm_sw_cal/calib/epic_files.shtml .

Figure 1 -13: MOS-Al-K (left) and PN Cu-K (right) internal background caused by X-ray fluorescence lines correlated with the structures of the electronic board (pn-Cu-K) and the more distant camera itself (MOS Al-K)

1.6

Timing

Absolute timing means: Locating events in time with reference to standard time defined by atomic clocks or other satellites. Relative timing means: The capacity to measure time intervals and periodicity reliably The release of SAS 5.3.3 included for the first time all necessary components to support timing analysis with outstanding time resolutions down to 29 and 7 microseconds for PN Timing and Burst modes respectively. Tests on data from the Crab pulsar taken during XMM-Newton's performance verification campaign in early 2000 indicate that the relative deviation in the observed pulse period w.r.t. the most accurate radio data available (P/P) is now considerably less than 10-8, with an absolute timing accuracy of < 500 microseconds. For the Crab pulsar the new results now conform with estimates of the theoretically attainable accuracy with XMMNewton and the expected statistical errors. Further investigations of periodicity of other objects are currently underway. Because of another improvement in converting the onboard time (running counter, kept by the CDMU and synchronised with all the Data Handling units. 48 bits, resolution (LSbit) is 1/65536 secs) to the event time (time in a counter internal to the EPEA that timestamps each frame. This timer is reset to 0 at the beginning of each observation, at a sharp (integer) second of OBT), the user should make sure that the data are fully reprocessed with the SAS higher than SAS 5.3.3 and not only with the new "barycen" task of SAS 5.3.3 in order to achieve the highest timing accuracy.


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Figure 1 -14: Best period and folded light curve for the Crab


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1.7

Examples of spectra

1.7.1 MOS2 PKS 2155-304 The following plot shows the ratio (data/model) of the best-fit broken power-law model to the MOS1 spectrum of PKS2155-304. Residual features generally lie within +/- 3%. Larger residuals below 400 eV are due to uncertainties arising from the complicated shape of the redistribution function and the low energy effective area. Above 7 keV the residuals probably arise form uncertainties in the background subtraction method. The source is bright, but with a steep spectrum and being observed in small window mode we have used an extrapolated background from the outer CCDs.

Figure 1 -15: MOS PKS2155 1.7.2 3C273 Figure 1-16 shows residual fits (expressed as a ratio, DATA/MODEL, with error bars removed for clarity) to the EPIC spectra of 3C273. The observations were done in SW Mode/Medium filter for all cameras in Rev 277. Pattern 0 spectra have been extracted from all cameras. The MOS pattern 0 spectrum is corrected for pile-up according to the method of Molendi and Sembay (see 1.0) as the source is somewhat piled-up. This method avoids the uncertainties associated with excluding cores and the appropriate PSF correction. In these modes, pile-up in the PN is less significant than in the MOS and we have not attempted any correction to the PN spectrum. A simultaneous fit has then been performed to all the data. The ratio plot gives the relative flux difference between the cameras at specific energies. This can be seen to vary between 0 and ~10%.


XMM-EPIC status of calibration and data analysis XMM-Newton Science Operation Centre Note: 1) In the MOS when the pile-up correction method (see 1.0) is performed on bright sources, as opposed to excluding the core, the fit around the instrumental edges is degraded. The mechanism for this is probably a reduction in the nominal CTI for columns associated with the core because of trap filling by the higher flux rate of events. A gain fit in Xspec can be used to improve the fit somewhat. 2) MOS1 and MOS2 diverge below 1 keV by up to 5% at low energies. This difference is actually larger than in previous analyses of bright sources where the method of pile-up correction used was extracting the core. This difference probably reflects a systematic uncertainty in the description of the energy dependent point-spread function which may have masked a greater underlying difference in the telescope effective area of each MOS. XMM-SOC-CAL-TN-0018 Page: - 20 ­ Issue: 2.1 M. Kirsch Date: 04.04.03

Figure 1-1 6: 3C 273: blue: PN black: MOS1, red: MOS2. Expressed as a ratio, DATA/MODEL, with error bars removed for clarity In addition a detailed analysis of 3C273 in the hard energy band was performed with the following results:

· ·

In the 3-10 keV band MOS2, PN-single and PN-double measurements are all found within a of ~0.03. Only MOS1 returns a significantly flatter slope. EPIC measurements, with the exception of the MOS1 3-10 keV measurement, are in good agreement with the BeppoSAX MECS.

For a detailed description of the results see S.Molendi & S. Sembay, 2003: http://xmm.vilspa.esa.es/external/xmm_sw_cal/calib/documentation.shtml#EPIC


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1.8

Cross Calibration with RGS

This section presents a comparison of EPIC results with RGS results for the radio-loud narrow-line Seyfert 1 galaxy PKS0558-508. Figure 1-17 shows the simultaneous fits to EPIC and RGS data where the only independent parameter was a scale factor between the different instruments. The EPIC cameras agree within ± 10-13 % in the normalisation. EPIC and RGS agree within ± 20 % in the normalisation. Individual fitting shows a significant steeper slope for the EPICs.

Figure 1 -17: Simultaneous spectral fits to PKS0558-508. Back: pn, red, MOS1, green: MOS2 and blue RGS1, light blue RGS2. The zoomed ratio (lower panel) has been binned more for clarity.


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1.9

Cross Calibration with other Satellites

This section presents a comparison of EPIC results with other satellites results for 1E0102-72 and G21.5-0.9 Figure 1-18 shows simultaneous fits to EPIC, RGS, ACIS-S, ASCA, and ROSAT PSPC data of the supernova remnant 1E0102.2-7219 where the only independent parameter was a scale factor between the different instruments. The agreement in the measured flux is quite good with a normalised range of 0.85 to 1.0 for the imaging detectors. The poorer agreement with the RGS is likely due to an incorrect treatment of the extended nature of the source. The fits were

Figure 1-18: Simultaneous s pectral fits to various data of the SNR 1E0102.2-7219. The colour coding for the different spectra are given in the figure. The fitted flux is in units of 10-11 erg cm-2 s -1 and the flux ratios are also listed in the figure.

normalised to the PN. Note that while the spectrum is very line rich and the model insufficiently represents the true spectrum, the model is sufficient for this comparison. The top panel of Figure 1-19 shows simultaneous fits to EPIC, ACIS-S, ASCA, BeppoSAX LECS and MECS and ROSAT PSPC data of the supernova remnant G21.5-0.9. The spectrum is a heavily absorbed hard power law which provides a good cross-calibration source at high energies. As for the 1E0102.2-7219 spectrum, the agreement in the measured fluxes is better than +/- 10 %. The bottom panel shows the measured confidence contours for the power-law index and absorption column density. The agreement is good to ~10 % in the column density and ~5 % (except for the ASCA GIS) for the power law index. For the Chandra fits acisabs in Xspec was used to correct for the lower QE caused by contamination.


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Figure 1 -19: Simultaneous spectral fits (top panel) and parameter confidence contours (bottom panel) to various data of the SNR G21.5-0.9. The colour coding for the different spectra are given in the figure. The fitted flux is in units of 10-11 erg cm-2 s -1 and the flux ratios are also listed in the figure.


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2

Data Analysis

This section provides an overview of what the SAS is able to do with the V5.4.1 release. Also included is a guideline of how to work with the different modes of the cameras.

2.1
2.1.1 · · · · · 2.1.2 · · 2.1.3 · · · · ·

New features in SAS
SAS 5.3.0 The MOS CTI correction has been improved to take into account the changes which have occurred since launch. A new task, evigweight , assigns a vignetting correction to each individual event. This allows extraction of vignetting-corrected images and spectra directly. A new task, epatplot , is available to identify pile up. The PN background rejection and CTI correction have been significantly improved. The tasks rmfgen and arfgen now reproduce the canned matrices to within 1%. Also, the full range of event patterns is supported. Further, rmfgen supports the major observing modes. SAS 5.3.3 The latest CTI correction for PN SW and LW mode has been implemented. EPIC-PN Timing and Burst mode response files are available SAS 5.4.1 MOS CTI correction has been modified in order to compensate the stepwise degradation after solar flares Degradation of the MOS intrinsic spectral resolution due to the larger noise component of the degraded CTI has been modelled. Refined calculation of the MOS gain as a function of observation epoch has been implemented. epatplot now also indicates the distribution and amount of invalid PN patterns to ease flux comparison in the case of pattern pile-up. New PN quantum efficiency function which is based on measurements of the thickness of the SiO2 layer on top of the CCD.

Figure 2 -1 : New (red) and old ( green) PN QE


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Refined spectral redistribution for PN (concerns all readout modes) at energies below 600 eV. The redistribution was adjusted to achieve agreement in column density derived from PN and Chandra LETG spectra of RXJ1856.3-3754. calpnalgo now receives all quantities from CCFs. The SW/LW mode CTI-correction function for PN has been modified. arfgen and rmfgen now support all modes (incl. TI and BU modes).

· · ·

2.2

Data Analysis

In the next section some general recommendations for conservative data analysis are provided. This includes: · · · · Where should data be taken from the CCD Which energy range should be used Which pattern range should be used Which response matrix should be used

For detailed guidelines of XMM data analysis please use the SAS Users' Guide at http://xmm.vilspa.esa.es/external/xmm_sw_cal/sas_frame.shtml

2.2.1 MOS Imaging modes Source region: where appropriate Background region: · · point source - From the same observation another region of the same area off-axis, away from source counts. extended source - This is more complicated. Please have a look at explanatory notes available on the XMM web site at http://xmm.vilspa.esa.es/ccf/epic/#background .

Energy range: 0.2-10.0 keV (However, because of calibration uncertainties, care must be taken when interpreting data below 0.3 keV.) In general the user should use patterns 0-12. (see XMM Users Handbook section 3.3.10 available at http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/index.shtml) . However, pattern 0 events can be used to minimise the effects (e.g. spectral distortion) of pile-up. Pattern 0 events can also be used for observations in which the best-possible spectral resolution is crucial and the corresponding loss of counts is not important. In addition the user should only use events flagged as "good" by using (#XMMEA_EM) in the selection expression win dow of xmmselect. When analysing spectra the user should use arf files produced by the SAS (version 5.3.0 or above) task arfgen in conjunction with canned redistribution matrices. Timing modes Source region: where appropriate Background region: background subtraction will not usually be an issue for sources observed in timing mode. However because the timing strip is only 100 pixels wide background regions should be taken from the outer CCDs. Energy range: 0.3-10.0 keV Pattern 0 only. As for imaging mode, canned redistribution matrices valid for timing mode will be made available and should be used with arf files produced by the SAS.


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Imaging modes Source region: where appropriate Background region: · point source - From the same observation but away from source. Ideally the region should have the same distance to the readout node (RAWY) as the source region. This ensures that similar low-energy noise is subtracted, because it increases towards the readout-node. Do not use the columns passing the source to avoid out-of-time events from the source, i.e. do not use a ring around the source region. extended source - see 2.2.1

·

Energy range: 0.15 keV - 15 keV, however both limits depend on readout mode and aim of the analysis. For imaging purposes pattern 0-12 can in principle be used. Since doubles (1-4), triples (5-8) and quadruples (9-12) (see XMM Users Handbook section 3.3.10 available at http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/index.shtml) are only created above twice, three and four times the low-energy threshold, respectively, cleanest images are produced by excluding the energy range just above the thresholds. E.g. to produce a 0.2 - 10 keV image one may select singles from the whole energy band and doubles only from 0.4 keV. FLAG == 0 omits parts of the detector area like border pixels, columns with higher offset, etc. For spectral analysis, response matrices are available only for singles, doubles and singles+doubles. Higher order pattern types are of low statistical significance, have degraded spectral resolution and are therefore not useful. Best spectral resolution is reached by selecting singles for the spectrum. FLAG == 0 should be used for high accuracy to exclude border pixels (and columns with higher offset) for which the pattern type and the total energy is known with significantly lower precision. At high energies the fraction of doubles is however almost as high as that of singles and to include doubles is recommended to increase the statistics. If a sufficient number of counts is available single- and double-spectra can be created separately and fitted simultaneously in XSPEC (with all parameters including norm linked together). One exception is the timing mode (see below). To choose the valid energy band for the spectral fit it is highly recommended to use the task epatplot. It uses as input a spatially selected (source region) event file and plots the fractions of the various patterns as function of energy. Spectral analysis should only be done in the energy band(s) where single- (and double-) fractions match the expected curves. In some observations the low-energy noise can be high, restricting the useful band at low energies. Deviations at medium energies indicate pile-up (more doubles as expected) and in such cases the inner part of the PSF in the source region should be excluded for spectral analysis. (More information on that topic is available in the XMM Users Handbook at http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/index.shtml). The user should use arf files produced by the SAS (version 5.3.3 or above) task arfgen in conjunction with canned redistribution matrices (which are compatible to the CTI correction used in 5.3.3 or above). For each readout mode of the PN a set of rmf files is available (for singles, doubles, singles+doubles. Except timing mode where only singles+doubles must be used). The CTI causes a dependence of spectral resolution with distance to the readout node. Therefore the 200 lines of each CCD are divided into areas of 20 lines each (Y0 at readout, Y9 at opposite side, which includes the nominal focus point) and for each area an rmf file is available Timing and Burst modes Source region: Columns around source position Background region: Columns away from source The RAWY coordinate is related to a fine-time, selection on RAWY will therefore exclude certain time periods. For timing mode no such selection is recommended; in burst mode use RAWY < 160 to avoid direct illumination by the source. For timing mode only sin gles+doubles should be selected for a spectrum and the fit restricted to energies > 0.5 keV to avoid the increased noise. For burst mode energies > 0.4 keV can be used with combinations of singles/doubles as in the window modes.