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From: d.a.evans@bristol.ac.uk
Subject: MOS/PN flux discrepancy
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Date: Wed, 25 Jun 2003 13:36:22 GMT
From: d.a.evans@bristol.ac.uk
To: xmmhelp@xmm.vilspa.esa.es
CC: d.a.evans@bristol.ac.uk
Subject: MOS/PN flux discrepancy
Full_Name: Daniel Evans
Submission from: (NULL) (137.222.58.154)


Dear Helpdesk,

I have been analysing a source that I know to be piled-up. My investigations
showed that the pile-up was essentially confined to a circle of radius 20
arcseconds centred on the point source.

Therefore I extracted spectra using an annulus with an inner radius of 20
arcseconds and outer radius 50 arcseconds for the MOS1, MOS2 and pn cameras. I
used FLAG=0 and PATTERN=0 filtering to minimise the extent of any further
pile-up. 

I believe that each of the source and background spectra were extracted OK,
although I did get a warning message backscale::arfgen: warning (zeroSumDetmap),
The filtered detector map has a zero normalisation - returning zero area. Try
using a flat detector map and/or more bins.

I then proceeded to generate rmf and arf files using the syntax:

rmfgen format=var rmfset=mos1_corespectrum_annulus_50_20_pattern0_detpsf.rmf
spectrumset=mos1_corespectrum_annulus_50_20_pattern0_detpsf.ds threshold=1.0e-6

arfgen arfset=mos1_spectrum_annulus_50_20_pattern0_detpsf.arf
spectrumset=mos1_spectrum_annulus_50_20_pattern0_detpsf.ds withrmfset=yes
rmfset=mos1_spectrum_annulus_50_20_pattern0_detpsf.rmf extendedsource=no
modelee=yes detxbins=100 detybins=100

and a similar syntax for the pn camera.

After grouping the spectra, I viewed them in XSPEC. The spectrum of my source is
quite complex, so I decided to limit my analysis to the 4-8 keV band, as I know
that there is just one component that vastly dominates the spectrum here.

The problem I find is that the flux and normalisation of the MOS1 and MOS2
cameras is 30-35% higher than that of the pn.

I note that extracting MOS and pn spectra using a circular extraction region
with a radius of 100 arcseconds does not produce any discrepancy in the flux.

So there's obviously a problem somewhere! Any help would be greatly
appreciated.

Thanks in advance,

Dan Evans.


Reply 1

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From: Nora Loiseau <xmmhelp@xmm.vilspa.esa.es>
To: d.a.evans@bristol.ac.uk
Subject: Re: MOS/PN flux discrepancy (PR#8764)
Date: Mon Jul 14 16:48:20 2003
Dear Dr Evans,

sorry for the delay in replying, we were trying to get some positive 
answer from the experts, although for the moment it seems that there
is no solution. The problem here is that the PSF correction for 
annuli is not accurate enough.

A 20-50" annulus at 6 keV gives 16% counts for PN and 13% for MOS-1
nominally. But, the systematic error on these numbers is probably
~+/-5% giving a systematic error on the flux, which the software 
doesn't include anywhere of ~30%.

There is nothing that we can do about this at the moment. There is
a pile-up correction available, from Molendi and Sembay 2003 which
allows pattern 0 only spectra from mildly piled-up sources to be
corrected. However, in the case where 20" needs to be excised this
is probably not going to work.

With best regards,

Nora Loiseau
XMM-Newton User Support Group

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