The first polarimetric signatures of infrared jets in X-ray binaries

TL;DR

This study reports the first infrared polarimetric signatures of jets in X-ray binaries, revealing synchrotron emission close to the jet base and providing insights into magnetic field alignment near the accreting compact objects.

Contribution

It presents near-infrared spectropolarimetry of X-ray binaries, identifying jet-related polarization signatures and their implications for magnetic field structure near the jets.

Findings

Polarization levels indicate synchrotron emission from jets.

Magnetic field aligned with the jet in Sco X-1.

Potential to probe magnetic field near accreting objects.

Abstract

We present near-infrared linear spectropolarimetry of a sample of persistent X-ray binaries, Sco X-1, Cyg X-2 and GRS1915+105. The slopes of the spectra are shallower than what is expected from a standard steady-state accretion disc, and can be explained if the near-infrared flux contains a contribution from an optically thin jet. For the neutron star systems, Sco X-1 and Cyg X-2, the polarization levels at 2.4um are 1.3+/-0.10% and 5.4+/-0.7% respectively which is greater than the polarization level at 1.65um. This cannot be explained by interstellar polarization or electron scattering in the anisotropic environment of the accretion flow. We propose that the most likely explanation is that this is the polarimetric signature of synchrotron emission arising from close to the base of the jets in these systems. In the black hole system GRS1915+105 the observed polarization, although high (5.0+/-1.2% at 2.4um), may be consistent with interstellar polarization. For Sco X-1 the position angle of the radio jet on the sky is approximately perpendicular to the near-infrared position angle (electric vector), suggesting that the magnetic field is aligned with the jet. These observations may be a first step towards probing the ordering, alignment and variability of the outflow magnetic field in a region closer to the central accreting object than is observed in the radio band.