Disk outflows and X-ray spectral appearance of neutron star versus black hole ULXs
Abstract
The standard model for super-Eddington accretion flows (based on analytical work and GR-MHD simulations) consists of a lower-density polar funnel and a thick disk outflow. Super-Eddington sources viewed down the funnel are predicted to have harder X-ray spectra and higher apparent luminosity; the same sources viewed at higher inclination would have a softer spectrum (due to Compton down-scattering in the wind) and appear fainter. Although this scenario is probably applicable to some individual sources, we will argue that it is not consistent with the observed statistical properties of the ULX population. It is also difficult to reconcile this collimated scenario with the quasi-spherical ionized bubbles observed around several ULXs.The reason why the funnel/outflow scenario struggles against the observations may be that it is only applicable to black holes and to weakly magnetized neutron stars. No strong outflows and funnels are expected for strongly magnetized, super-Eddington neutron stars, whose X-ray spectra would appear harder even at high inclination. If so, it means that the majority of ULXs are neutron stars (consistent with the shape of the cumulative luminosity distribution of this population). We will briefly discuss other tests to identify a neutron star ULX when X-ray pulsations are not detected.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E3207S