Exercise: Spectra of Cas A and its Compact Central Object Introduction: Cas A is a young, ejecta-dominated supernova remnant located at a distance of about 3.4 kpc. It is thought to be the result of a stellar explosion in about the year 1680. In the X-ray band, Cas A is characterized by a complex shell of X-ray emitting material that is dominated by shock-heated stellar ejecta. A thin shell of hard X-ray emission surrounds the remnant, and a compact neutron star resides at its center. In this exercise, you will use Chandra data to derive rough estimates of the dynamical properties of Cas A, investigate variations in the composition of the shock-heated ejecta, determine the nature of the emission from the thin shell, and investigation the emission properties of the central compact object. 1. From the Chandra archive, download data from a Cas A observation carried out with ACIS-S. An observation of about 20 ks is more than adequate for this investigation. 2. Using ds9, create a 3-color image of Cas A. Reasonable energy bands are the following: red: 0.6-1.65 keV green: 1.65-2.25 keV blue: 2.25-7.5 keV a) What is the physical radius of Cas A? b) Given its age, what is the mean expansion velocity of Cas A? What is the corresponding proton temperature? c) Assuming the Galactic average of n_0 ~ 0.3 cm^{-3} for the ambient density, how much mass has Cas A swept up? Do you expect the X-ray emission to be dominated by the CSM/ISM or the ejecta? 3. Using the 3-color image, identify four spectral regions for study: i) The tip of one of the red-colored fingers in the outer southeastern region. ii) A compact blue-green emission interior to the southeastern shell. iii) A blue filament along the outer southeastern boundary. iv) The central compact object. Choose an appropriate region outside the SNR for a background spectrum. 4) Extract the spectrum of the central compact object. Use an absorbed blackbody model (tbabs*bbodyrad) to fit the spectrum. a) What is the temperature of the neutron star? b) Using the distance to Cas A, and the normalization from the best-fit spectrum, what is the radius of the emitting area on the neutron star? How does this compare to the expected size of a neutron star? c) Suppose the neutron star is rotating, and that the magnetic axis is inclined relative to the spin axis. What does the size measured above suggest about the possible pulsed fraction for emission from the source? How could you test this with an X-ray observation? 5) Extract spectra from the red and blue-green knots in the southeastern region of Cas A. Compare the two spectra. They are complex, and you will not be able to get good fits with simple models, but some key features are discernible: a) What dominates the emission of the red knot? b) What elements are particularly over-abundant in the blue-green knot? c) In the nucleosynthesis process of the progenitor and explosion, iron is formed nearest the center of the explosion, interior to where Si is formed, for example. Comment on the spatial location of the two knots in this context. d) What is the approximate temperature of the plasma? How does this compare with the estimated proton temperature above? Comment. 6) Extract the spectrum from thin filament region in southeast. a) Does the spectrum look like ejecta-dominated material? b) Use an absorbed power-law to fit the spectrum. What is the spectral index? Based on these data, estimate a lower limit to the maximum energy of the synchrotron emission. By some estimates, the magnetic field in Cas A may be as high as 1 mG. If this is the case, what is the mean electron energy corresponding to the highest-energy (observed) synchrotron photons? What might this suggest about gamma-ray emission from Cas A? References: Pavlov et al. 2000, ApJ, 531, L53 Hughes et al. 2000, ApJ, 528, L109 Uchiyama & Aharonian 2008, ApJ, 677, L105 Ghavamian et al. 2007, ApJ, 654, L69 Murray et al. 2002, ApJ, 566, 1039