TAPIR Seminar

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 a 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 x 1039 L☉). This is significantly higher than the variable X-ray emission previously observed with Chandra. A relatively high 0.3-10 keV fractional variability amplitude (Fvar) of ~30% is observed in all eleven 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. By comparing the He~II λ4686 line luminosity (~7-9 x1035 L☉) to the contemporaneous soft X-ray emission, we 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.  [NOTE UNUSUAL TIME -- 3:00pm]