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From: zhuru@mail.ustc.edu.cn
Subject: Cross-corelation problem caused by GTI?
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Date: Mon, 23 May 2005 15:19:00 GMT
From: zhuru@mail.ustc.edu.cn
To: xmmhelp@xmm.vilspa.esa.es
CC: zhuru@mail.ustc.edu.cn
Subject: Cross-corelation problem caused by GTI?
Full_Name: Zhu Ru
Submission from: (NULL) (61.191.195.182)


Dear Sir or Madam:

I encountered a problem concerning timing analysis. After I creat two light
curve of two different energy band from the same event list file and use
'crosscor' to look for time delay, the result is that the peak value is always
located at time delay=0, which is quite strange.

I suspect that this is caused by the removing of background flare event
filtering which was carried out to filter the original event list file. The GTI
file creat some intervals in which there are no count in the filtered event
list. These intervals are identical in two light curves, which plays an
important role in determining the time delay.

If my guess is reasonable, could you please give me some suggestions.

Yours Sincerely,
Zhu Ru


Reply 1

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From: Maria Santos-Lleo <xmmhelp@xmm.vilspa.esa.es>
To: zhuru@mail.ustc.edu.cn
Subject: Re: Cross-corelation problem caused by GTI? (PR#17907)
Date: Tue May 24 16:17:37 2005
Dear Zhu Ru

I am afraid your question about timing analysis is more
related to the scientific interpretation of the data than to
XMM software and we cannot provide this kind of support. 

In particular the task you mention, crosscor, probably a xronos 
task, does not belong to any software package provided by 
XMM and, therefore, cannot be supported by us.  

In any case, if you think you should find a non-cero delay, and this 
is not the case because of the GTI filtering, my suggestion 
would be that you try to use light curves without previous 
background filtering. The energy-band light-curves must 
be, of course, source light curves, after background subtraction.
Otherwise these light curves can be dominated by the background
behaviour, pending on the source/background ratio. The points 
of the light curve with corresponding to periods with higher 
background will have lower signal to noise than other points 
with lower background. This fact has to be translated into higher 
error bars for the points corresponding to high background periods
Therefore, the best would be that you then use a cross correlation 
software that takes into account the error bars in the points of the 
light curve.

Alternatively, if the high background periods result in a too 
noisy light curve and you definitely want to avoid them in 
the light curve, then you need to use a cross correlation 
software that properly takes into account un-evenly spaced
points in the light curve. Unfortunately I do not know 
if crosscor is or is not appropiate for your purposes. 

With best regards

Maria 



Maria Santos-Lleo, 
XMM-Newton SOC 
User Support Group

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