Preparing to Run ChaRT

Energy & Density

Energy: a list of up to ten monochromatic energies between 0.2 and 10 keV. The values must be given in [keV] as well.

Density: a single value indicating the density of rays that will be generated at the entrance aperture. The allowed range is 0.001 to 10 rays/mm². The number of rays that arrive at the focal plane is determined by the HRMA effective area at the input energies.

This option is useful when assembling a composite spectrum from several mono-energetic simulations, and it is the preferred method for the HRC, because the construction of a spectral file for the HRC is not straightforward.

Using the "energy & density" option

1. The purpose of making the PSF

   1. To make a PSF with approximately the same number of counts as the source under investigation. For example, you may want to subtract the PSF at the source position in order to look for extended, low brightness emission around the source.

   2. To define the PSF with a much higher S/N than the data.

2. Select an energy

   HRC: for most sources, a monochromatic PSF will suffice, with an energy input of 1 keV. For some sources, e.g. super soft sources, another monoenergy - maybe 0.2 keV - would be a better choice, or perhaps the simulation should include both 0.2 and 1 keV.

   ACIS: the user will have to decide if a single energy PSF is sufficient, and if not, the known or suspected source spectrum will dictate the choice of energies.

3. Determine the necessary density

   Now we need to estimate density to input. Looking at the Effective Area (EA) curve for HRC-I in the POG, we can estimate the EA to be 200 cm² at 1 keV. Since the ChaRT input requires units of rays/mm², we can calculate a source density which is approximately

   (# counts in your source)/(20000 mm^2).
   

   If you want to create a PSF with a much higher S/N, you can increase the number of rays by some factor like 10 or 100 (so long as you don't exceed the maximum of 10 rays/mm²).

Spectrum & Exposure Time

Exposure Time: the desired exposure time for the simulation. It must be given in [ks] and be within the range of 0 to 200 ks.

The value is usually chosen to match the exposure time of the observation, which is stored in the EXPOSURE header keyword of the event file:

unix% dmkeypar acisf00942N003_evt2.fits exposure echo+
49215.214915137

The exposure time is returned in [s], so this observation is 49 ks long.

Spectrum: a file containing the definition of the source spectrum at the entrance aperture of the HRMA for which you would like a PSF to be simulated.

A common user error has proven to be using a file with incorrect units, which often results in ChaRT producing a PSF model that appears much too bright. The file must be energy [keV] vs flux [photons/cm²/s]. Currently, ChaRT will not complain if it appears that the file has incorrect units, so it is up to the user to check that the file is accurate.

For directions on creating a spectrum for your source, see the CIAO Imaging Spectroscopy threads. Then fit the data as described in the Fitting PHA Spectra Sherpa thread.

For this example, the spectrum is fit with a one-dimensional power law multiplied by a photoelectric absorption model.

unix% sherpa
-----------------------------------------------------
Welcome to Sherpa: CXC's Modeling and Fitting Package
-----------------------------------------------------
CIAO 4.2 Monday, November 30, 2009

sherpa> load_pha("3c273.pi");
sherpa> subtract;
sherpa> notice(0.4, 6.0);
sherpa> set_source(xsphabs.abs1 * powlaw1d.p1);
sherpa> abs1.nh = 0.07;
sherpa> freeze(abs1);
sherpa> guess(p1);
sherpa> fit;
Dataset               = 1
Method                = levmar
Statistic             = chi2gehrels
Initial fit statistic = 71.8133
Final fit statistic   = 23.6368 at function evaluation 16
Data points           = 42
Degrees of freedom    = 40
Probability [Q-value] = 0.981499
Reduced statistic     = 0.590919
Change in statistic   = 48.1766
   p1.gamma       2.13334
   p1.ampl        0.000220644

The script must be loaded into Sherpa before it can be run (this step is only necessary once per session):

sherpa> require("sherpa_chart");

If you would like to plot your spectrum, use the function plot_chart_spectrum(); the optional elow and ehigh values specify the minimum and maximum energies to include in the plot:

sherpa> plot_chart_spectrum();
sherpa> plot_chart_spectrum(;elow=1.0, ehigh=8.0);

Figure 2 shows the resulting plot.

Figure 2: A plot of the ChaRT spectrum

The source model is plotted in units of photons per cm² per s against energy (keV), as required by ChaRT.

The save_chart_spectrum() command is used to create the output file. In this example, we use the optional elow and ehigh arguments to restrict the output to the 1-8 keV range. If the bounds are omitted, the full energy range as determined by Sherpa is used.

sherpa> save_chart_spectrum("source_flux_chart.dat"; elow=1.0, ehigh=8.0);
Created: source_flux_chart.dat
sherpa> quit

The output produced by save_chart_spectrum is an ASCII file that contains two columns. The first column gives the center of the energy bin in [keV]. The second column is the flux in [photons/cm²/s].

unix% more source_flux_chart.dat
1.005   1.82537e-06
1.015   1.7956e-06
1.025   1.76639e-06
1.035   1.73775e-06
1.045   1.70966e-06
1.055   1.68211e-06
1.065   1.6551e-06
1.075   1.62822e-06
1.085   1.60202e-06
1.095   1.57659e-06
...
7.935   2.65464e-08
7.945   2.64753e-08
7.955   2.64045e-08
7.965   2.63339e-08
7.975   2.62637e-08
7.985   2.61937e-08
7.995   2.6124e-08