A Potential Cyclotron Resonant Scattering Feature in the Ultraluminous X-Ray Source Pulsar NGC 300 ULX1 Seen by NuSTAR and XMM-Newton
Harrison, F. A.; Fabian, A. C.; Pinto, C.; Steiner, J. F.; Bachetti, M.; Barret, D.; Brightman, M.; Grefenstette, B. W.; Madsen, K. K.; Webb, N.; Walton, D. J.; Fürst, F.; Lau, R. M.; Middleton, M. J.; Heida, M.; Kosec, P.; Kennea, J.
United Kingdom, Italy, Spain, France, United States
Abstract
Based on phase-resolved broadband spectroscopy using XMM-Newton and NuSTAR, we report on a potential cyclotron resonant scattering feature (CRSF) at E ∼ 13 keV in the pulsed spectrum of the recently discovered ultraluminous X-ray source (ULX) pulsar NGC 300 ULX1. If this interpretation is correct, the implied magnetic field of the central neutron star is B ∼ 1012 G (assuming scattering by electrons), similar to that estimated from the observed spin-up of the star, and also similar to known Galactic X-ray pulsars. We discuss the implications of this result for the connection between NGC 300 ULX1 and the other known ULX pulsars, particularly in light of the recent discovery of a likely proton cyclotron line in another ULX, M51 ULX-8.