Eric,
Is the cluster on I3 or S7 (ie FI or BI)? (not that knowing this will lead to
an answer from me ...). The soft background component does vary spatially,
so that is definitely one thing to think about (as an example see the
discussion in http://xxx.lanl.gov/abs/astro-ph/0105093 - Merger shocks in
galaxy clusters A665 and A2163 and their relation to radio halos by M.
Markevitch & A. Vikhlinin ).
Doug
On Thursday 29 November 2001 06:57 pm, Eric Tittley wrote:
> Hi All,
>
> 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
> XSPEC)
>
> 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:
>
> nH free
> 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.
>
> Sincerely,
> Eric Tittley
>
> ---
> Dr. Eric Tittley Post Doctoral Research Associate
> Joint Center for Astrophysics UMBC
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