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

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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) × 1039 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., Hα, Hβ, He ii λ4686), which suggest that the neutron star primary is photoionizing material in the immediate vicinity of the binary. We compare the He ii λ4686 line luminosity (∼(7–9) × 1035 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.

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