Chandra observations of M31 allow the first spatially resolved X-ray image of a supernova remnant ( SNR ) in an external spiral galaxy .
CXOM31 J004327.7+411829 is a slightly elongated ring-shaped object with a diameter of \sim 11 ^ { \prime \prime } ( 42 pc ) .
In addition , the X-ray image hints that the chemical composition of the SNR is spatial dependent .
The X-ray spectrum of the SNR can be well fitted with a Raymond-Smith model or a non-equilibrium ionization model .
Depending on the spectral model , the 0.3–7 keV luminosity is between 3.2 \times 10 ^ { 36 } \hbox { erg } \thinspace \hbox { s } ^ { -1 } and 4.5 \times 10 ^ { 37 } \hbox { erg } \thinspace \hbox { s } ^ { -1 } .
The age of the SNR is estimated to be 3210–22300 years and the number density of ambient gas is \sim 0.003 - 0.3 cm ^ { -3 } .
This suggests that the local interstellar medium around the SNR is low .