SBS 0335-052E , one of the most metal-poor ( Z \sim 3-4 \% Z _ { \odot } ) HeII-emitter starbursts known in the nearby universe , is studied using optical VLT/MUSE spectroscopic and Chandra X-ray observations .
We spatially resolved the spectral map of the nebular HeII \lambda 4686 emission from which we derived for the first time the total HeII-ionizing energy budget of SBS 0335-052E .
The nebular HeII line is indicative of a quite hard ionizing spectrum with photon energies > 4 Ryd , and is observed to be more common at high-z than locally .
Our study rules out a significant contribution from X-ray sources and shocks to the HeII photoionization budget , indicating that the He ^ { + } excitation is mainly due to hot stellar continua .
We discovered a new WR knot , but we also discard single WR stars as the main responsible for the HeII ionization .
By comparing observations with current models , we found that the HeII-ionization budget of SBS 0335-052E can only be produced by either single , rotating metal-free stars or a binary population with Z \sim 10 ^ { -5 } and a ’ top-heavy ’ IMF .
This discrepancy between the metallicity of such stars and that of the HII regions in SBS 0335-052E is similar to results obtained by ( 48 ) for the very metal-deficient HeII-emitting galaxy IZw18 .
These results suggest that the HeII ionization is still beyond the capabilities of state-of-the-art models .
Extremely metal-poor , high-ionizing starbursts in the local universe , like SBS 0335-052E , provide unique laboratories for exploring in detail the extreme conditions likely prevailing in the reionization era .