Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Ruth M. E. Kelly, Herman L. Marshall, Silvia Zane, Nabil Brice, Swati Ravi
| Summary:
X-ray polarimetry offers a unique window into neutron star physics and can
provide answers to questions that cannot otherwise be probed. The up-and-coming
REDSoX sounding rocket mission will be the first experiment equipped with a
detector able to explore polarized X-rays below 1 keV, observing in the 0.2-0.4
keV range. Although REDSoX will only be capable of short, one-off observations,
it will crucially test the instrument performance. In this paper we investigate
how a fully-fledged orbital mission with longer lifetime, based on an
instrument design similar to REDSoX, will allow us to study thermal emission
from the X-ray dim isolated neutron stars (XDINSs) and magnetars, probing their
magnetic field and the physics of their outer surface layers, including vacuum
effects and QED mode conversion at the vacuum resonance. We discuss the
potentially observable features for promising values of the star’s surface
temperature, magnetic field, and viewing geometry. Assuming emission from the
whole surface, we find that, for a source with a magnetic field B=5×10^{13} G
and surface temperature T~10^7 K, the instrument can resolve a proton-cyclotron
absorption feature in the spectrum with high significance when collecting
~25,000 counts across a single observation. Similarly, for a source with
B=10^{14} G and T~10^7 K, a switch in the dominant polarization mode, caused by
mode conversion at the vacuum resonance, can be detected by collecting ~25,000
counts, allowing for a long-sought observational test of the presence of QED
effects. We then present two case studies for XDINS targets: RX J1856.5-3754
and RX J0720.4-3125.
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