Supersoft X-ray nebulae in the Large Magellanic Cloud

Supersoft X-rays sources (SSSs) have been proposed as potential Type Ia supernova (SN Ia) progenitors. If such objects are indeed persistently X-ray luminous and embedded in sufficiently dense interstellar medium (ISM), they will be surrounded by extended nebular emission. These nebulae should persist even long after an SN Ia explosion, due to the long recombination and cooling times involved. With this in mind, we searched for nebular [O III] emission around four SSSs and three SNRs in the Large Magellanic Cloud, using the 6.5-m Baade telescope at Las Campanas Observatory and the IMACS camera. We confirm that, out of the four SSS candidates, only CAL 83 can be associated with an [O III] nebula. The [O III] luminosity for the other objects is constrained to ≲17 per cent of that of CAL 83 at 6.8 pc from the central source. Models computed with the photoionization code CLOUDY indicate that either the ISM densities in the environments of CAL 87, RX J0550.0-7151, and RX J0513.9-6951 must be significantly lower than surrounding CAL 83 or the average X-ray luminosities of these sources over the last ≲10 000 yr must be significantly lower than presently observed, in order to be consistent with the observed luminosity upper limits. For the three SNRs we consider (all with ages <1000 yr), our [O III] flux measurements together with the known surrounding ISM densities strongly constrain the ionizing luminosity of their progenitors in the last several thousand years, independent of the progenitor channel.