2019ApJ...887..233R

We present a 142 ks Chandra observation of the enigmatic combination supernova remnant G310.6-1.6 consisting of a bright pulsar-wind nebula driven by an energetic pulsar, surrounded by a highly circular, very faint shell with a featureless, probably synchrotron, spectrum. Comparison with an observation 6 yr earlier shows no measurable expansion of the shell, though some features in the pulsar-wind nebula have moved. We find an expansion age of at least 2500 yr, implying a current shock velocity less than about 1000 km s^–1. We place severe upper limits on thermal emission from the shell; if the shell locates the blast wave, a Sedov interpretation would require the remnant to be very young, about 1000 yr, and to have resulted from a dramatically sub-energetic supernova, ejecting ≪0.02M_☉ with energy E <= 3 x 10⁴⁷ erg. Even a merger-induced collapse of a white dwarf to a neutron star, with a low-energy explosion, is unlikely to produce such an event. Other explanations seem equally unlikely.