A Magnetar Model for Ultraluminous X-ray Sources

A scenario is described in which newly-born, highly-magnetized, spin-powered pulsars radiate >1039 ergs s 1 in the X-ray band. The emission can be detected by a distant observer if the overlying supernova ejecta, coeval with the pulsar birth, becomes transparent to X-radiation on timescales < 50 d which requires an ejecta kinetic energy/mass ratio Ekin=M> YY erg g 1. These conditions are met by massive progenitor stars, M > 40{60 M , which loose most of their mass through stellar winds prior to exploding as Type Ib/c supernovae. It is argued that magnetars are the remnants of these massive progenitor star supernovae and that they can account for XX% of the most luminous Ultra-luminous X-ray sources found in star-forming regions of nearby spiral and interacting galaxies. The predicted X-ray spectrum of these objects is a power law from the pulsar combined with thermal emission from gas shock-heated in the interaction between supernova ejecta and circumstellar material. The relative contributions to the X- ray luminosity from these two components is undetermined because the thermal component depends sensitively on the circumstellar environment at the time of the explosion. If the pulsar emission dominates the X-ray luminosity then these sources should fade quickly and remain undetectable thereafter. If the supernova shock emission is signican t then the spectrum should transition from a hard, non-thermal power-law state to a thermal prole and these objects could be strong soft X-ray emitters for 10's of years.