Probing jet launching in neutron star X-ray binaries: the variable and polarized jet of SAX J1808.4-3658

TL;DR

This study presents optical polarimetric observations of the neutron star X-ray binary SAX J1808.4-3658 during outburst, revealing variable polarization indicative of a jet with tangled magnetic fields, demonstrating polarimetry's utility in jet magnetic field studies.

Contribution

First detailed optical polarimetric campaign on SAX J1808.4-3658 during outburst, linking variable polarization to jet emission and magnetic field structure.

Findings

Detection of a red low-frequency excess indicating jet synchrotron emission.

Variable linear polarization consistent with a jet origin.

Episodes of stronger polarization suggesting magnetic field tangling.

Abstract

We report on an optical photometric and polarimetric campaign on the accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658 during its 2019 outburst. The emergence of a low-frequency excess in the spectral energy distribution in the form of a red excess above the disc spectrum (seen most prominently in z, i and R-bands) is observed as the outburst evolves. This is indicative of optically thin synchrotron emission due to a jet, as seen previously in this source and in other AMXPs during outburst. At the end of the outburst decay, the source entered a reflaring state. The low-frequency excess is still observed during the reflares. Our optical (BVRI) polarimetric campaign shows variable linear polarization (LP) throughout the outburst. We show that this is intrinsic to the source, with low-level but significant detections (0.2-2%) in all bands. The LP spectrum is red during both the main outburst and the reflaring state, favoring a jet origin for this variable polarization over other interpretations, such as Thomson scattering with free electrons from the disc or the propelled matter. During the reflaring state, a few episodes with stronger LP level (1-2 %) are observed. The low-level, variable LP is suggestive of strongly tangled magnetic fields near the base of the jet. These results clearly demonstrate how polarimetry is a powerful tool for probing the magnetic field structure in X-ray binary jets, similar to AGN jets.