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XMM-Newton CCF Release Note
XMM-CCF-REL-314 The RGS effective area: up date of the contamination correction
R. Gonzґlez-Riestra a April 11, 2014

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CCF comp onents
Name of CCF RGS1 EFFAREACORR 0009 RGS2 EFFAREACORR 0009 VALDATE 2001-01-01T00:00:00 2001-01-01T00:00:00 List of Blocks changed EFFAREACORR EFFAREACORR XSCS flag NO NO

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Changes

A correction to the RGS effective area for contamination on the detectors was first implemented in SAS in 2007 [1]. Observations of sources assumed to b e constant (the neutron star RX J1856-3754 and the Vela Pulsar) showed a steady decrease in flux that could b e explained by the accumulation of a carb on layer on the detectors, whose thickness was increasing linearly with time. Subsequent observations showed that the flux of these two sources started to increase. This p ointed to an over-estimation of the correction, indicating that the thickness of the contamination layer was growing at a slower rate than at the b eginning of the mission. The contamination model was changed to an exp onential law, with the thickness of the layer parametrised as: Thickness (nm) = slope в T
decay

в (1-e

-t Tdecay

)

with t in revolutions, slope = 0.210 and T

decay

= 1050 revolutions [2].

The purp ose of these new CCFs is twofold: · To up date the contamination model with a refined value of T 1 , 1090 revolutions, as derived

decay


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Figure 1: Evolution of the thickness of the carb on layer. Points neutron star RX J1856-3754 are shown as black circles, and those Vela Pulsar, as red triangles. The red-dotted line is the model decay time of 1050 revolutions). The blue line shows the up dated revolutions. Data have b een kindly provided by C. de Vries

derived from derived from used in prev model, with a

observations of the observations of the ious CCFs (with a decay time of 1090

from all data available until 2014 (See Fig.1). · To extend the extrap olation of the correction until 2030 (while the previous CCFs covered only until March 2014).

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Scientific Impact of this Up date

These CCFs were derived using a new parametrisation of the contamination model. They are applicable to all ep ochs since the b eginning of the mission. It is then imp ortant to quantify the difference b etween this correction and the previous one in order to decide if a given observations need to b e re-calibrated. Fig. 2 shows the ratio b etween old and new correction factors as a function of wavelength and time.

· The difference in the correction never exceeds 2%. · Below 27 °, the difference never exceeds 1%, and ab ove this wavelength, it is larger than 1% A only for data taken after mid-2006.


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Figure 2: Ratio b etween new and previous contamination corrections, as a function of wavelength and ep och. For wavelengths shortward 27 °, the difference in flux using either of the corrections is A less than 1%. For longer wavelengths, it is ab ove 1% only for observations taken after mid 2006.

· Below 32 °, the difference never exceeds 1.5%, and ab ove this wavelength, it is larger than A 1.5% only for data taken after 2009. · Only for wavelengths ab ove 36 ° and for observations taken after 2012, the change in flux A would b e larger than 2%.

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Estimated Scientific Quality

The implementation of the up dated contamination model will led to a more accurate correction of the RGS effective area, as recent data were starting to diverge from the assumed model.

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Test pro cedures & results

The new CCFs have b een fully tested in SASv13.5. Effective areas and fluxed sp ectra have b een generated for all the available observations of RX J1856-3754. Comparison with the results obtained with the previous version of the CCFs shows the exp ected differences, in total agreement with the difference in the contamination models.


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Exp ected Up dates

The Vela Pulsar and the neutron star RX J1856-3754 are regularly observed (one and two times p er year, resp ectively). The depth of the contamination layer is derived from these observations, and compared to the current contamination model. If a significant different is found, the model will b e refined and implemented in new CCFs. So far, there is no evidence for differences in the contamination rates of b oth RGSs. Should b oth instruments start to b ehave differently, indep endent contamination models would b e used.

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References

[1] "An improved model of the RGS effective area based on the build-up of Carb on contamination", A. Pollock, XMM-CCF-REL-238, August 2007 (http://xmm2.esac.esa.int/docs/documents/CAL-SRN-0238-1-0.ps.gz ) [2] "The RGS effective area incorp orating exp onential contamination and a mechanism for rectification", A. Pollock, XMM-CCF-REL-262, April 2010 (http://xmm2.esac.esa.int/docs/documents/CAL-SRN-0262-1-1.ps.gz )