A VLT VIMOS IFU study of the ionisation nebula surrounding the Supersoft X-ray Source CAL 83

TL;DR

This study uses VLT VIMOS IFU spectroscopy to analyze the ionisation nebula around CAL 83, a prototype supersoft X-ray source, revealing its morphology and challenging existing models of its ionising processes.

Contribution

First detailed spatially-resolved spectral analysis of CAL 83's nebula, highlighting unusual He II emission distribution and testing photoionisation models against observations.

Findings

The nebula shows standard morphology with recombination edges and central high-ionisation regions.

He II emission is confined to one side, unlike other ions.

Models do not fully reproduce the observed spectra, especially He II emission.

Abstract

CAL83 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. With the VIMOS IFU we obtained spectra over a 25\times25" field of view, encompassing one quarter of the nebula. Emission line maps - H I, He II, [O III], [N II], and [S II] - are produced in order to study the morphology of the ionised gas. We include CAL83 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 CAL83 and compare the predicted to the observed spectra. CAL83 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 CAL83 rather than being homogeneously distributed as with the other ions. We model the CAL83 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. The spectral line images of the region surrounding CAL83 are revealing and instructive. However, more modelling of 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.