A VLT VIMOS IFU study of the ionisation nebula surrounding the supersoft X-ray source CAL 83

Context. CAL 83 is a prototype of the class of Super Soft X-ray Sources (SXS). It is a binary consisting of a low mass secondary that is transferring mass onto a white dwarf primary and is the only known SXS surrounded by an ionisation nebula, made up of the interstellar medium (ISM) ionised by the source itself. We study this nebula using integral field spectroscopy. Aims. The study of ionised material can inform us about the source that is responsible for the ionisation, in a way that is complementary to studying the source directly. Since CAL 83 is the only SXS known with an ionisation nebula, we have an opportunity to see if such studies are as useful for SXSs as they have been for other X-ray ionised nebulae. We can use these data to compare to models of how CAL83 should ionise its surroundings, based on what we know about the source emission spectrum and the physical conditions of the surrounding ISM. Methods. With the VIMOS integral field spectrograph we obtained spectra over a 25x25 '' field of view, encompassing one quarter of the nebula. Emission line maps - HI, HeII lambda 4686, [O III] lambda lambda 4959, 5007, [N II] lambda lambda 6548, 5683, and [S II] lambda lambda 6716, 6731 - are produced in order to study the morphology of the ionised gas. We include CAL 83 on diagrams of various diagnostic ion ratios to compare it to other X-ray ionised sources. Finally we computed some simple models of the ionised gas around CAL 83 and compare the predicted to the observed spectra. Results. CAL 83 appears to have a fairly standard ionisation nebula as far as the morphology goes: the edges where H is recombining are strong in the low stage ionisation lines and the central, clumpy regions are stronger in the higher stage ionisation lines. But the He II emission is unusual in being confined to one side of CAL 83 rather than being homogeneously distributed as with the other ions. We model the CAL 83 nebula with cloudy using model parameters for SXSs found in the literature. The He II emission does not fit in with model predictions; in fact none of the models is able to fit the observed spectrum very well. Conclusions. The spectral line images of the region surrounding CAL 83 are revealing and instructive. However, more work on modelling the spectrum of the ionised gas is necessary, and especially for the high-ionisation level emission from CAL83. In particular, we wish to know if the He II emission and the other nebular lines are powered by the same ionising source.