Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Juri Poutanen, Sergey S. Tsygankov, Victor Doroshenko, Sofia V. Forsblom, Peter Jenke
| Summary:
Discovery of pulsations from a number of ULXs proved that accretion onto
neutron stars can produce luminosities exceeding the Eddington limit by several
orders of magnitude. The conditions necessary to achieve such high luminosities
as well as the exact geometry of the accretion flow in the neutron star
vicinity are, however, a matter of debate. The pulse phase-resolved
polarization measurements that became possible with the launch of the Imaging
X-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometry
and its orientation relative to the orbital plane. They provide an avenue to
test different theoretical models of ULX pulsars. In this paper we present the
results of three IXPE observations of the first Galactic ULX pulsar Swift
J0243.6+6124 during its 2023 outburst. We find strong variations in the
polarization characteristics with the pulsar phase. The average polarization
degree increases from about 5% to 15% as the flux dropped by a factor of three
in the course of the outburst. The polarization angle (PA) as a function of the
pulsar phase shows two peaks in the first two observations, but changes to a
characteristic sawtooth pattern in the remaining data set. This is not
consistent with a simple rotating vector model. Assuming the existence of an
additional constant polarized component, we were able to fit the three
observations with a common rotating vector model and obtain constraints on the
pulsar geometry. In particular, we find the pulsar angular momentum inclination
with respect to the line of sight of 15-40 deg, the magnetic obliquity of 60-80
deg, and the pulsar spin position angle of -50 deg, which significantly differs
from the constant component PA of about 10 deg. Combining these X-ray
measurements with the optical PA, we find evidence for at least a 30 deg
misalignment between the pulsar angular momentum and the binary orbital axis.
| Search Query: ArXiv Query: search_query=au:”Herman L. Marshall”&id_list=&start=0&max_results=3