Although some SNRs are thought to be asymmetrical, this one was caught within a few weeks, at most, of its detonation (correcting, of course, for the colossal light travel time from the galaxy to us). The blob in the image would have to be, at most, a few light-weeks across, much too small to be resolved at that distance. It is essentially a point source, not the extended object depicted in the image.

If that galaxy is approximately the size of ours (about 100,000 ly) then the light from the SN would have traveled a distance of about 1/5,200,000 the size of the galaxy for every week after the explosion that passed before the image was taken. In fact, if you compare the optical and Xray imagery in the original article, you can see that they are both at the same scale, and show both SNs, one near the nucleus and one in the outer arms. The red blob in the Xray image covers a substantial portion of the galaxy, it would have taken thousands of years for that SN to have illuminated it all.

My question is, why should the Xray image show such a large blob of Xray emission when there simply hasn't been enough time for either SN to have excited that much area? And why is the emission concentrated all at that NW end of the galaxy, centered on the 2008 object? My guess is simply that the Xray detector overexposed due to the extremely strong signal and the adjoining pixels were saturated. Xrays cannot be focussed as sharply as visual light.

Another consequence of light travel time is that the two SNs appear to have occurred just a few weeks apart, however, this is an illusion. Remember, that galaxy is thousands of light years across, and we are seeing it in projection against the plane of the sky. The light from both SNs got here at about the same time, but the two events may have been separated by thousands of years. One SN was just much closer than the other so the arrival of both signals appears to us to be near simultaneous. Actually, we have no way of knowing which one actually blew up first, the dates in their names are the years we saw them, not the years they happened.