Relation between the hot gas pressure inside a supernova remnant and its size Original Research Article

We have systematically analyzed the X-ray data of supernova remnants (SNRs) showing thin thermal emission using the ASCA gas imaging spectrometer (GIS) data. Focusing on the thermal emission, we selected 19 SNRs by the criteria: it shows a thermal emission; there is no bright point source inside; an entire region is observed to measure the X-ray extent. Source distances are known to 10 sources out of 19 based on various methods. Those for 5 out of 19 are estimated by using a Σ−D relation. We find that the temperature, kTe gradually reduces as the source size, DX increases. The density of the hot gas also shows a decrease as an increase of DX. However, we find a clear anti-correlation between the hot gas pressure, PX and DX:PX∼DX−3.2±0.2. The correlation coefficient is about 0.98. We introduce a new parameter, PT=PXDX, that depends only on the observables, showing a relation of PT∼DX−2.7±0.2. This relation estimates the distance, d, to the source with an accuracy of 14% while the Σ−D relation in radio shows about 41%. This relation suggests that the thermal energy, ∼PXDX3, is almost constant for various SNRs. We discuss why it can be conserved for various SNRs.