No Strong Geometric Beaming in the Ultraluminous Neutron Star Binary NGC 300 ULX-1 (SN 2010da) from Swift and Gemini

Abstract

We have obtained near-simultaneous Swift /XRT imaging and Gemini GMOS spectroscopy for the ultraluminous X-ray source (ULX) NGC 300 ULX-1 (formerly designated SN 2010da). The observed X-ray emission is consistent with an inhomogeneous wind that partially obscures a central, bright inner accretion disk. We simultaneously fit eleven 0.3–10 keV spectra obtained over a ∼1 year time period (2016 April to 2017 July) using the same partial covering model and find that although the covering fraction varies significantly (from 78% to consistent with 0%), the unabsorbed luminosity remains essentially constant across all observations ((2–6) × 10 39 erg s −1 ). A relatively high 0.3–10 keV fractional variability amplitude ( F var ) of ∼30% is observed in all 11 observations. Optical spectra from Gemini exhibit numerous emission lines (e.g., H α , H β , He ii λ 4686), which suggest that the neutron star primary is photoionizing material in the immediate vicinity of the binary. We compare the He ii λ 4686 line luminosity (∼(7–9) × 10 35 erg s −1 ) to the contemporaneous soft X-ray emission and find the X-ray emission is broadly consistent with the observed He ii line luminosity. The combination of our X-ray observations and optical spectroscopy suggest that geometric beaming effects in the ULX-1 system are minimal, making ULX-1 one of only a few bona fide ULXs to be powered by accretion onto a neutron star.