J. P. W. Stark
X-ray observations of the Cygnus Loop using a crystal spectrometer
1977 (supervisor: J. L. Culhane)

An experiment to search for X-ray spectral lines from the Cygnus Loop SNR is described. The observations were performed using a Bragg crystal spectrometer on board a Skylark sounding rocket. The lines studied were three helium like transitions of O VII at wavelengths 21.6A, 21.8A and 22.1A. These lines were not detected at the sensitivity of the instrument, however the upper limit obtained indicates that less than 6% of the total X-ray flux from the source is in the form of these lines, the 3 sigma upper limit is 1.3 photons cm^-3 s^-1 at the Earth. This flux is substantially below the line flux tentatively postulated from observations using a filter system (Stevens (1973)). This upper limit is used to re-assess the temperature of the emitting region in view of isothermal and adiabatic models for SNR evolution, incorporating in an approximate manner, the ionised state of the Cygnus Loop. The temperatures determined in this way are compared with continuum measurements for the source. This comparison leads to a discussion of the fitting of complex spectra with simple spectral forms. It is found that a self-consistent model to describe the Cygnus Loop is more readily available if the remnant is expanding isothermally, the present temperature being ~ 4.10^6 K.