Absence of nebular He ii λ4686 constrains the UV emission from the ultraluminous X-ray pulsar NGC 1313 X-2

Abstract

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 ultraviolet (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 ii λ4686 emission in its immediate environment. To this end, we first perform multiband spectroscopy of the ULX to derive three realistic extrapolations of the SED into the inaccessible UV, each predicting varying levels of UV luminosity. We then perform photoionization modelling of the bubble nebula and predict the He ii λ4686 fluxes that should have been observed based on each of the derived SEDs. We then compare these predictions with the derived upper limit on He ii λ4686 from the Multi-Unit Spectroscopic Explorer data, which allows us to infer a UV luminosity LUV ≲ 1 × 1039 erg s−1 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 ionizing role in the context of the nebular He ii λ4686 line observed in star-forming metal-poor galaxies.