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Probing Jet Launching in Neutron Star X-Ray Binaries: The Variable and Polarized Jet of SAX J1808.4–3658

Authors :
Payaswini Saikia
David M. Russell
N. Masetti
Stefano Covino
T. Muñoz-Darias
D. M. Bramich
Sergio Campana
Rob Fender
S. Crespi
A. Miraval Zanon
T. Shahbaz
Sara Motta
Jeroen Homan
Fraser Lewis
P. D'Avanzo
Adelle J. Goodwin
P. Goldoni
Angad Johar
Maria Cristina Baglio
AstroParticule et Cosmologie (APC (UMR_7164))
Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut National de Physique Nucléaire et de Physique des Particules du CNRS (IN2P3)-Observatoire de Paris
Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)
Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Centre National de la Recherche Scientifique (CNRS)-Université de Paris (UP)
Source :
Astrophys.J., Astrophys.J., 2020, 905 (2), pp.87. ⟨10.3847/1538-4357/abc685⟩
Publication Year :
2020
Publisher :
HAL CCSD, 2020.

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.<br />Comment: Accepted for publication in ApJ

Details

Language :
English
Database :
OpenAIRE
Journal :
Astrophys.J., Astrophys.J., 2020, 905 (2), pp.87. ⟨10.3847/1538-4357/abc685⟩
Accession number :
edsair.doi.dedup.....bcee9508a958a6f71fb29e81b6d3f8db