Pulsating ULXs: the most extreme accreting neutron stars
Abstract
Sifting the XMM archive, the project EXTraS revealed that two more ultraluminous X-ray sources (ULXs), NGC7793 P13 and NGC5907 ULX, host accreting pulsars. The existence of these sources, with a isotropic luminosity in excess of 10^{41} erg/s (about 1000 times the Eddington limit for a neutron star), periods shorter than 1.2 s and characteristic ages smaller than 100 yr, challenges the standard accretion models and questions the frequency of occurrence of such objects with respect to the massive black holes often invoked in ULXs. While the assumption of a mild beaming factor is insufficient to account for their luminosity, a strong multipolar magnetic field close to the neutron star surface can explain their properties. Finally, the timing characteristics of the three known pulsar-ULXs have been considered in the design of a project aimed at looking for the presence of other accreting neutron stars in known ULXs, which will make it possible to understand better the census of pulsar-ULXs.
- Publication:
-
The X-ray Universe 2017
- Pub Date:
- October 2017
- Bibcode:
- 2017xru..conf..104I