The youngest Galactic supernova remnant ( SNR ) \object G1.9+0.3 , produced by a ( probable ) SN Ia that exploded \sim 1900 CE , is strongly asymmetric at radio wavelengths , much brighter in the north , but bilaterally symmetric in X-rays .
We present the results of X-ray expansion measurements that illuminate the origin of the radio asymmetry .
We confirm the mean expansion rate ( 2011 to 2015 ) of 0.58 % yr ^ { -1 } , but large spatial variations are present .
Using the nonparametric “ Demons ” method , we measure the velocity field throughout the entire SNR , finding that motions vary by a factor of 5 , from 0 \farcs 09 to 0 \farcs 44 yr ^ { -1 } .
The slowest shocks are at the outer boundary of the bright northern radio rim , with velocities v _ { s } as low as 3,600 km s ^ { -1 } ( for an assumed distance of 8.5 kpc ) , much less than v _ { s } = 12 , 000 – 13 , 000 km s ^ { -1 } along the X-ray-bright major axis .
Such strong deceleration of the northern blast wave most likely arises from the collision of SN ejecta with a much denser than average ambient medium there .
This asymmetric ambient medium naturally explains the radio asymmetry .
In several locations , significant morphological changes and strongly nonradial motions are apparent .
The spatially-integrated X-ray flux continues to increase with time .
Based on Chandra observations spanning 8.3 yr , we measure its increase at 1.3 \% \pm 0.8 \% yr ^ { -1 } .
The SN ejecta are likely colliding with the asymmetric circumstellar medium ejected by the SN progenitor prior to its explosion .