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From: phumphre@uci.edu
Subject: arfgen and extended sources
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Date: Wed, 7 Apr 2004 20:15:31 GMT
From: phumphre@uci.edu
To: xmmhelp@xmm.vilspa.esa.es
CC: phumphre@uci.edu
Subject: arfgen and extended sources
Full_Name: Philip Humphrey
Submission from: (NULL) (128.200.44.37)


Dear all,
         Hi. I'm using SAS 6.0.0 and am interested in doing spectral analysis of
extended sources. My question relates to whether arfgen is able to generate
appropriately-weighted responses for such sources. The documentation for this
task is a little ambiguous over the role of the detmap in this regard; according
to parts of the documentation the detmap is used to compute weighted responses
based on how the ARF varies over the detmap pixels. In constrast, however, in
the  algorithm, the mirror area seems to be computed from (e.g.) the RA_PNT, the
DSS geometrical centroid or the sourcex parameters, so that any variation of the
mirror-area with position over the extended source is not factored in, and only
the QE/Filter transmission is taken into account. Have I understood correctly
the manner in which the program works, or does the weighting actually allow for
the variation in mirror area?
         Thanks very much for any light you may be able to shed on this! 

         Cheers,
                 Fill Humphrey


Reply 1

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From: Nora Loiseau <xmmhelp@xmm.vilspa.esa.es>
To: phumphre@uci.edu
Subject: Re: arfgen and extended sources (PR#10579)
Date: Mon Apr 12 12:23:17 2004
Dear Fill,

I am consulting SAS experts to give you a more presice reply.
I come back to you as soon as I have their input.
Best regards,

Nora

----
Dr. Nora Loiseau
XMM-Newton User Support Group


Reply 2

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From: Nora Loiseau <xmmhelp@xmm.vilspa.esa.es>
To: phumphre@uci.edu
Subject: Re: arfgen and extended sources (PR#10579)
Date: Mon Apr 12 13:06:53 2004
Dear Phill,

here follows the reply by our expert:
 
   "thanks for pointing out the inconsistency in the arfgen documentation.

In practice arfgen averages over the chosen detmap using the following
formula:

If  the parameter extendedsource=no  (i.e. a point source - the default)

   calculate the ARF by taking:

   1. The mirror area at the position of the point source
   2. The quantum efficiency averaged over the detector map
   3. The filter transmission (the XMM calibration currently assumes no
       variation of filter transmission with spatial position.

If the parameter extendedsource=yes

   calculate the ARF as above but also average the mirror area over the detmap.


The distinction in the treatment of point and extended sources is simply that
for point sources X-rays have a single path through the mirrors and experience
a single vignetting effect regardless of the spatial region used to accumulate
the spectrum."

Regards,

Nora
----
Dr. Nora Loiseau
XMM-Newton User Support Group

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