I'm having trouble understanding the anomalously high values for
nH I'm finding in some cluster data.
The nH routine reports the nH column density in the direction of
my cluster should be 1.6e20 cm^-2.
When fitting a wabs * mekal spectrum to the data in sherpa, I
consistently fit nH to be 8 to 10 times larger. (same thing for
For example, for a region away from the centre of the cluster,
and therefore unaffected by self-absorption within the cluster,
I get these fits:
Emin nH (10^22/cm^2) kT
0.3 0.177+-0.003 3.8*
0.4 0.16+-0.02 3.9+-0.4
0.5 0.13+-0.02 4.3+-0.5
0.6 0.14+-0.02 4.2+-0.5
0.7 0.13+-0.03 4.3+-0.5
0.8 0.14+-0.03 4.2+-0.5
0.9 0.11+-0.05 4.4+-0.6
1.0 0.12+-0.07 4.4+-0.7
1.2 0.05* 4.7*
1.5 0.31+-0.35 3.9+-1.3
2.0 1 +-0.98 3.1+-1.1
The span over which the data were fit is Emin to 9 keV, so each
row of the table progressively fits less of the energy range in
which H absorption is important. That explains why the fits to nH
become less constraining as Emin increases.
( * => +-0, which is a sherpa problem for another day. )
But why are the nH's so high? Indeed, the numbers are very
consistent with there being a decimal shift somewhere in the
code, since 1.6e21 cm^-2 is consistent with the data.
The question could be one of science, in which case great! But
I'm worried that I've messed something up in the data processing.
Has anyone else experienced this?
I'm using the Markevitch no-source background files to estimate
the background spectrum. These have a soft component due to
galactic emission. Could the difference between the estimated
background and the actual be large enough to skew the spectrum at
the same location?
To confuse the issue, fitting a region covering the core of the
cluster gives nH=9e20 cm^-2, which is almost half of that seen in
the outer cluster.
Fixing nH to 1.6e20 cm^2 gives poor fits that are very sensitive
in kT to Emin.
--- Dr. Eric Tittley Post Doctoral Research Associate Joint Center for Astrophysics UMBC
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