The Emissivity of O VII and O VIII

Finding the transitions

• Go to the AtomDB WebGuide at http://www.atomdb.org/Webguide/webguide.php.
  
• Move to the part of the page to "List strong lines in wavelength region at a specific temperature:" and find the lines at "wavelength" 0.6 keV with a width of 0.1 keV at an electron temperature of 0.2 keV. Hit the button for "Get Strong Lines".
  
• You should get a display of "Searching for lines between 0.500 and 0.700 keV at Te 0.200 keV".
• The default listing shows the lines sorted by decreasing wavelength. Click on "Relative Intensity" twice to sort the list by decreasing relative intensity.
• Record all of the lines with relative intensity of at least 3%. For each line, record the ion, the energy in keV (mislabeled as "Wavelength"), the upper and lower levels. Click on the line to get details about the transition and record the electron configuration of the upper and lower levels. 
  
• Make a table to include in your report of the transitions and the information about the transitions. Sort the lines by increasing energy.
• Repeat the process above with electron temperatures of 0.1 keV and 0.3 keV to see if there are any additional lines of interest. If so, add them to your table.
  
• HaloSat will have an energy resolution of around 80 eV (FWHM) in this band.  Therefore groups of transitions separated in energy by less than about 40 eV will appear as unresolved lines in the measured energy spectra. Go through your table of lines and figure out which transitions should be grouped together into a single observed line, but keep lines from different ions separate even if they are close in energy. Record this.

Reading the emissivity data

• Make a directory called atomdb.
• Download the equilibrium line and continuum emissivity data from the AtomDB page at http://www.atomdb.org/download.php.  You want a file with a name like atomdb_v3.0.3.tar.bz2.
  
• Extract the file apec_line.fits to a location of your choice. Remember the location.
• Note that if you have LHEASOFT, you already have apec_line.fits on your computer, so you do not need to download the AtomDB again. However, you do need to figure out where apec_line.fits resides on your computer.
• Look through the Python program emissivity.py. The first part imports pyfits, matplotlib, and some functions needed from numpy.  The second part is a routine called get_emissivity, which reads data from apec_line.fits. This is a slightly edited version of a routine with the same name from pyatomdb. You can skim through the routine down to the return statement at its end.
• The next part of the code sets up some variables and calls get_emissivity.
• Edit the code so that the variable linefile is set to point at your copy of apec_line.fits.
• The variables elem, ion, upper, and lower set the element, ion, and upper and lower levels of the transition. The code is set for a particular transition of O VII.
  
• The last part of the code plots the emissivity for this specific transition as a function of temperature. It uses log scales and has nice axes labels, a plot title, and a legend. The axes labeled could be improved by figuring out how to do superscripts.
• Run the program. You should get a plot of emissivity versus temperature.
• If you get errors complaining about filename, you probably got the path to your file incorrect or maybe you don't have pyfits installed.
  

Finding the emissivity for the observed lines

• The goal of this part is to generate and plot emissivity curves for the lines that we observe in the data. As you found in the first part of the exercise, each observed line is a sum of several transitions. We need to add together the emissivity of each transition contributing tothe observed line to find the total emissivity for that observed line.
• You should use the groupings that you found above and write Python code that retrieves the emissivity for each transition and adds together all of the transitions in each group.
• Note that the emissivity data may be tabulated at different kT values for each transition. Be careful that you do not add emissivites from different temperatures for different transitions. The get_emissivity routine has a parameter kT. If kT=[-1] then it returns emissivities at the temperatures in the data file. If kT is set to a numpy array, then it will interpolate the emissivity data for the temperature values in the input array.  Thus, you may wish to generate your own array of temperatures and use it as input to get_emissivity in order that all of your emissivities cover the same set of temperature values.
  
• Plot the emissivity versus temperature for each group of transitions. You should have groups for O VIII (654 eV), O VII (569 eV), and O VII (666 eV) where the energy is the approximate energy centroid of the line.  You may wish to also consider other groups of transitions leading to other observed lines.
• Plot the ratio of O VIII (654 eV) to O VII (569 eV) versus temperature.
• For each group, calculate the expected line centroid by finding the average of the transition energies weighted by their relative intensities.  Plot the centroid versus temperature for O VII (569 eV) and O VIII (654 eV).
• In your report, discuss any resulting thoughts that you have regarding this work.