X-ray spectrum variations in the south-west Cygnus Loop

Abstract

The Cygnus Loop is one of the nearest supernova remnants (440 pc), allowing studies of spatial variations to be carried out at high physical resolution compared with other supernova remnants. Chandra Advanced CCD Imaging Spectrometer (ACIS) observations were obtained covering the bright V-shaped region on the south-west limb. This region has strong 0.2–2 keV spectral variations seen with the ROSAT Position Sensitive Proportional Counter. The nature of the spectral variations is analysed using the ACIS observations. It is found that temperature, abundance variations and column density variations all have a significant contribution to the observed spectral variations. Fits with C, N, O (O group) elements and Ne, Na, Mg, Al, Si, S, Ar (Ne group) elements varied together are better than fits with just O and Ne varied. Large spatial variations in abundances are seen, yet the Ne group to O group ratio and the Fe group (Ca, Fe, Ni) to O group ratio are nearly constant for all of the spectra. The most natural explanation is that the elements in the region are due to supernova explosion of a star of ∼18 M⊙ which predated the Cygnus Loop explosion.