The XSPEC projct model is used to estimate parameters in 3-D space from the 2-D projected spectra. It is used as follows.

Suppose you have a Chandra observation of a cluster of galaxies and you extract three spectra file1.sp, file2.sp, and file3.sp from concentric annuli of width 20 arcsec ie the file1.sp is from the center to a radius of 20", file2.sp is from 20" to 40", and file3.sp is from 40" to 60". For simplicity we will assume that these are all circular annuli. The first step required is to add some keywords to each of the files.

You can do this from the Unix command prompt by 

> fparkey 20.0 file1.sp XFLT0001 add=yes
> fparkey 20.0 file1.sp XFLT0002 add=yes
> fparkey 0.0  file1.sp XFLT0003 add=yes
> fparkey 40.0 file2.sp XFLT0001 add=yes
> fparkey 40.0 file2.sp XFLT0002 add=yes
> fparkey 0.0  file2.sp XFLT0003 add=yes
> fparkey 60.0 file3.sp XFLT0001 add=yes
> fparkey 60.0 file3.sp XFLT0002 add=yes
> fparkey 0.0  file2.sp XFLT0003 add=yes
> fparkey 0.0  file1.sp XFLT0003 add=yes

Now read these files into XSPEC as three separate datagroups 

XSPEC12> data 1:1 file1.sp 2:2 file2.sp 3:3 file3.sp

We will assume that these files have background and response keywords set so these associated files are read in automatically. Now we define the model 

XSPEC12> model projct(phabs*apec)

You will be prompted three times for projct and apec parameter values. You should keep the defaults for the projct parameters and set the apec parameters appropriately. For example : 

Input parameter value, delta, min, bot, top, and max values for ...
              0
1:data group 1::projct:major>
              0
2:data group 1::projct:minor>
              0
3:data group 1::projct:orient>
              1      0.001          0          0     100000      1e+06
4:data group 1::phabs:nH>0.05
              1       0.01      0.008      0.008         64         64
5:data group 1::apec:kT>5
              1     -0.001          0          0          5          5
6:data group 1::apec:Abundanc>0.3 0
              0      -0.01          0          0         10         10
7:data group 1::apec:redshift>0.01
              1       0.01          0          0      1e+24      1e+24
8:data group 1::apec:norm>1e-5

Input parameter value, delta, min, bot, top, and max values for ...
              0
9:data group 2::projct:major>= 1
              0
10:data group 2::projct:minor>= 2
              0
11:data group 2::projct:orient>= 3
           0.05      0.001          0          0     100000      1e+06
12:data group 2::phabs:nH>= 4
              5       0.01      0.008      0.008         64         64
13:data group 2::apec:kT>6.0
            0.3      0.001          0          0          5          5
14:data group 2::apec:Abundanc>= 6
           0.01      -0.01          0          0         10         10
15:data group 2::apec:redshift>= 7
          1e-05       0.01          0          0      1e+24      1e+24
16:data group 2::apec:norm>1e-5

Input parameter value, delta, min, bot, top, and max values for ...
              0
17:data group 3::projct:major>= 1
              0
18:data group 3::projct:minor>= 2
              0
19:data group 3::projct:orient>= 3
           0.05      0.001          0          0     100000      1e+06
20:data group 3::phabs:nH>= 4
              5       0.01      0.008      0.008         64         64
21:data group 3::apec:kT>7.0
            0.3      0.001          0          0          5          5
22:data group 3::apec:Abundanc>= 6
           0.01      -0.01          0          0         10         10
23:data group 3::apec:redshift>= 7
          1e-05       0.01          0          0      1e+24      1e+24
24:data group 3::apec:norm>1e-5

The apec parameters are now for the emission in a sphere of radius 20", a spherical shell from 20-40", and a spherical shell from 40-60", respectively. You can go ahead and fit for the apec parameters just as you would without the projct model. The temperature parameters give the temperature in each shell. To derive the density you need to calculate the volumne in each shell (in cm^3) then use the equation for the norm given in the help.

The projct parameters are used to specify an inner radius for the first shell/annulus. In most cases this will be zero (the default). One case where you would want non-zero values of the parameters is if there is a radio source at the center of the cluster (eg Cygnus-A) displacing the X-ray emitting plasma. In all cases the parameters should be kept frozen. If any of them are allowed to be free parameters the model will be significantly slowed down.

Note that you should try to start the fit with parameters that are not too wrong otherwise the fit may not converge correctly. A good way to do this is to start by fitting all the datasets without using the projct model, for example : 

XSPEC12> phabs*apec

When you have found a good fit to this model add in the projct model by : 

XSPEC12> editmod projct(phabs*apec)

and you will be prompted for the projct model parameters : 

Input parameter value, delta, min, bot, top, and max values for ...
              0
1:data group 1::projct:major>
              0
2:data group 1::projct:minor>
              0
3:data group 1::projct:orient>

Input parameter value, delta, min, bot, top, and max values for ...
              0
9:data group 2::projct:major>= 1
              0
10:data group 2::projct:minor>= 2
              0
11:data group 2::projct:orient>= 3

Input parameter value, delta, min, bot, top, and max values for ...
              0
17:data group 3::projct:major>= 1
              0
18:data group 3::projct:minor>= 2
              0
19:data group 3::projct:orient>= 3

Now change the norm parameters of the apec components to give approximately the right values then start the fit.

projct model (last edited 2008-01-30 13:45:22 by KeithArnaud)