Absence of nebular He{\sc ii} $\lambda$4686 constrains the UV emission from the Ultraluminous X-ray pulsar NGC~1313~X--2

TL;DR

This study constrains the UV emission of the ULX NGC 1313 X-2 by analyzing the absence of nebular He II emission, providing insights into its accretion and radiation properties compared to other ULXs.

Contribution

It presents the first UV luminosity constraints for NGC 1313 X-2 using nebular emission limits and compares these with other ULXs to explore differences in their accretion states.

Findings

UV luminosity of NGC 1313 X-2 is constrained to less than 1×10^{39} erg/s.

Absence of nebular He II emission suggests intrinsic differences between hard and soft ULXs.

Implication that ULX ionising radiation impacts surrounding nebulae and galaxy environments.

Abstract

While much has been learned in recent decades about the X-ray emission of the extragalactic Ultraluminous X-ray sources (ULXs), their radiative output in the UV band remains poorly constrained. Understanding of the full ULX spectral energy distribution (SED) is imperative to constrain the accretion flow geometry powering them, as well as their radiative power. Here we present constraints on the UV emission of the pulsating ULX (PULX) NGC~1313~X--2 based on the absence of nebular {He{\sc ii} $\lambda$4686} emission in its immediate environment. To this end, we first perform multi-band spectroscopy of the ULX to derive three realistic extrapolations of the SED into the unaccessible UV, each predicting varying levels of UV luminosity. We then perform photo-ionization modelling of the bubble nebula and predict the {He{\sc ii} $\lambda$4686} fluxes that should have been observed based on each of the derived SEDs. We then compare these predictions with the derived upper limit on {\heii} from MUSE data, which allows us to infer a UV luminosity $L_\mathrm{UV} \lesssim 1 \times 10^{39}$ erg/s in the PULX NGC~1313~X--2. Comparing the UV luminosity inferred with other ULXs, our work suggests there may be an intrinsic difference between hard and soft ULXs, either related to different mass-transfer rates and/or the nature of the accretor. However, a statistical sample of ULXs with inferred UV luminosities is needed to fully determine the distinguishing features between hard and soft ULXs. Finally, we discuss ULXs ionising role in the context of the nebular {He{\sc ii} $\lambda$4686} line observed in star-forming, metal-poor galaxies.