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XMM-Newton CCF Release Note
XMM-CCF-REL-189
EPIC-pn spectral response
R. D. Saxton
9 May 2005
1 CCF components
Name of CCF VALDATE Blocks changed XSCS ag
EPN QUANTUMEF 0016.CCF 2000-01-01 FRACTION ENERGY NO
EPN REDIST 0010.CCF 2000-01-01 PARTEVENT PARAMS NO
2 Changes
This release contains an improvement in the low-energy (below 1 keV) response function
which has been made more peaked to better t the spectra of continuum sources (Fig.
1).
These changes lead to a modication of the pattern fractions stored in the quantum
eфciency le (EPN QUANTUMEF 0016.CCF), otherwise there has been no material
change to the quantum eфciency calibration.
3 Scientic Impact of this Update
An ensemble of blazar spectra have been individually t, over the 0.15{1.5 keV energy
band, with a single power-law plus galactic absorption model. The resulting residuals show
a characteristic shape which is repeated across the sample (Fig. 2). While the spectral
model employed may not be perfect for each individual source, due to uncertainties in the
galactic absoption parameterisation or possible spectral breaks within the tted band,
the consistency of the results argues strongly for a problem with the modelling of the
instrumental response. The reason for this is not currently understood in terms of physical
eects in the detector. In this release an empirical tweak is applied to the redistribution
function below 1 keV to atten out the observed residuals (Fig. 2).
While this change has been made without reference to other instruments it is worth
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noting that it brings the ts on continuum sources into better agreement with those of
EPIC-MOS.
A long standing problem with ts to EPIC-pn spectra is that they tend to return absorp-
tion columns which are too low by  7  10 19 cm 2 compared with radio measurements
and ts to Chandra and EPIC-MOS spectra. Perhaps the clearest example of this is the
isolated neutron star RXJ 1856.5-3754, where Chandra LETG and HRC spectra are well
t by a 61 eV black-body model with an inter-stellar absorption NH = 9:5  1:5  10 19
cm 2 [1]. While EPIC-pn and EPIC-MOS both measure a similar temperature to Chan-
dra and MOS returns a similar absorption value (6:7  0:2  10 19 cm 2 ), the pn spectra
yields an NH of 1:8  0:3  10 19 cm 2 [2]. A t with the new calibration presented here
gives NH = 7:1  0:3  10 19 cm 2 (Fig. 3) in much better agreement with the other
instruments.
It should be noted that the change introduced here aects all pn observing modes.
4 Estimated Scientic Quality
Fits to continuum spectra now agree with the MOS from 0.2 to 5 keV to within 5% but
show an excess of  10% at the very lowest energies.
Gross errors in the pn returned absorption columns have been addressed. Remaining
errors have not been quantied but are likely to be at the 1 2  10 19 cm 2 level.
Fits to low-energy spectral lines show an improved chi-squared but are still not perfect.
5 Expected Updates
6 Test procedures
Test 1: The response matrix generation task rmfgen has been tested against canned
matrices produced by separate software running at MPE for single, double and single
plus double pixel spectra.
7 Test results
Test 1: In all cases the matrices produced by SAS were nearly identical to the canned
matrices.
References
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Figure 1: The change in the shape of the pn redistribution function at 0.6 keV. An increase in the peak
height of 6% is visible in the new (red) curve compared with the old one (black).
[1] Drake et al. 2002, ApJ, 572, 996
[2] Burwitz et al. 'The thermal radiation of the isolated neutron star RX J1856.5-3754
observed with Chandra and XMM-Newton' (in prep.)
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Figure 2: The best t and residuals from a t of an absorbed power-law to the 0.15 - 1.5 keV pn spectra of
the blazars, MKN 421, PKS 2155, MKN 501, 1H 1219, PKS 0548, H 1426+428, MKN 180 and 1H 0414.
Top panel: SAS 6.1 responses based on EPN QUANTUMEF 0015.CCF and EPN REDIST 0009.CCF;
Bottom panel: Responses based on EPN QUANTUMEF 0016.CCF and EPN REDIST 0010.CCF.
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Figure 3: Black-body (kT=62.3 eV) t to three thin lter observations of RXJ 1856-3754 taken at
dierent epochs. The mean nH=7:1  10 19 cm 2 .
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