X-Ray Polarization from the Gamma-Ray Binary LS I +61° 303

The gamma-ray-emitting binary stellar system LS I +61° 303 was observed with the Imaging X-ray Polarimetry Explorer on two successive orbits over orbital phases of 0.74–1.05. Polarization is detected at a significance of 4.2 σ with an average polarization degree of 13.1% ± 3.0% in the 2–8 keV band after background subtraction. This is the second detection of polarization of the X-ray synchrotron emission from a gamma-ray binary and, again, suggests that the magnetic field in the particle acceleration region has a significant ordered component. The orbital motion on the sky of LS I +61° 303 is not well determined, which leads to ambiguity in interpretation of the X-ray electric vector position angle (EVPA) measurement. Use of orbital elements determined via radial velocity measurements combined with radio imaging of variable nebular emission suggests an offset between the X-ray EVPA and the compact object-massive star axis on the order of ∼30°. Such an offset could be produced by Coriolis forces due to binary motion. Use of two different sets orbital elements determined via optical polarimetry suggest either no offset or a perpendicular orientation but require an unexpectedly high inclination, while another set based on the same data but modeled assuming a moderate inclination suggests an offset of 15°. Use of orbital elements derived from modeling of the keV/TeV light curves suggests good alignment between the X-ray EVPA and the compact object-massive star axis. Such alignment was found for the gamma-ray binary PSR B1259-63. If the same physical situation holds for LS I +61° 303, that would favor the orbital elements derived from the keV/TeV light curves.