Date: Wed, 23 Jun 2010 23:55:22 -0400 (EDT)
From: AMRUTA DESHPANDE <firstname.lastname@example.org>
Subject: OBSID 0112980201
I'm having trouble analyzing data in obsid 0112980201 (specifically the
I want to analyze spectra. My extractions seem to come out just fine, but
when I go to fit them, there are major problems. I am fitting using xspec
phabs*mekal model. I give initial guess of 4.8e-2 (Nh), 10 (kT), 1e-2
(norm) and 0.296 (redshift). Fit statistics that appear after I input
these inital guess values are good. But after applying the fit command,
the norm drops by a third of the initial guess (while other parameters
remain near by) and the reduced chi squared goes down to a few tenths,
while the null hypothesis probability jumps to 1.
Another interesting problem is that I have extracted spectra for
consecutive regions that are scaled multiples of one another. As I fit
data from larger regions, the fits get worse, the count rates decrease,
and so does luminosity. The regions increase 10* in size, and luminosity
decreases by between 5 % and 14 % between consecutive regions.
Here's what I have confirmed so far:
I am using a calibration index file that I downloaded from the cifbuild
online utility, and have made sure that I have all the calibration files
necessary for an analysis date of "now"
I have also tried a different analysis date (sometime in the year 2006),
and extracted arfs and rmfs using that ccf. But i get teh same bad fit,
with the bad null hypothesis probability.
I am using a detector map with the same bin size as the images provided by
pps (80 physical units), which I think is the same spatial binning that
appears when you view a filtered events list. My detector map covers the
source region just fine (although it does not cover the background
region, which I believe it need not).
I was able to notice that fits improve as I group 2000 channels (minimum),
but over multiple regions, I still see the opposite trend (of luminosity
decreasing with increasing region size). (initially my minimum group size
was 20, which is sufficient for almost every other XMM spectrum I've fit).
I have redownloaded the data, and run my extractions again, and the
I thank you for your patience in reading my description above. I am at a
bit of a loss for ideas at the moment.
Could you please help?