Hi Doug,
The data is on chips I0-3. The cluster is centred on I1 with the `distant'
region described below on chip I2.
I'll compare the soft background in the S2 chip to that extracted from the
Markevitch BG database. The region is relatively emission-free.
Thanks,
Eric
From: Douglas Burke, Thu, 29 Nov 2001 19:15:45 -0500
>
>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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