Polarized infrared emission from X-ray binary jets

Abstract

Near-infrared (NIR) and optical polarimetric observations of a selection of X-ray binaries are presented. The targets were observed using the Very Large Telescope and the United Kingdom Infrared Telescope. We detect a significant level (3σ ) of linear polarization in four sources. The polarization is found to be intrinsic (at the >3σ level) in two sources; GRO J1655−40 (∼4-7 per cent in the H and K s bands during an outburst) and Sco X-1 (∼0.1-0.9 per cent in the H and K bands), which is stronger at lower frequencies. This is likely to be the signature of optically thin synchrotron emission from the collimated jets in these systems, whose presence indicates that a partially ordered magnetic field is present at the inner regions of the jets. In Sco X−1, the intrinsic polarization is variable (and sometimes absent) in the H and K bands. In the J band (i.e. at higher frequencies), the polarization is not significantly variable and is consistent with an interstellar origin. The optical light from GX 339-4 is also polarized, but at a level and position angle consistent with scattering by interstellar dust. The other polarized source is SS 433, which has a low level (0.5-0.8 per cent) of J-band polarization, likely due to local scattering. The NIR counterparts of GRO J0422+32, XTE J1118+480, 4U 0614+09 and Aql X-1 (which were all in or near quiescence) have a linear polarization level of <16 per cent (3σ upper limit, some are <6 per cent). We discuss how such observations may be used to constrain the ordering of the magnetic field close to the base of the jet in such systems.

Rapid variations of polarization in low-mass X-ray binaries

“…The observations and results are summarized in Table 1; this work was published in [32]. Most sources were in quiescence, and no polarization was detected.…”

“…Very recently, evidence for polarization intrinsic to the emitting photons of LMXBs has been revealed from infrared observations [40,32]. Shahbaz et al (2008) [40] performed infrared spectropolarimetry of three LMXBs and found two of them (the NSs Sco X-1 and Cyg X-2) to be intrinsically polarized, with an increasing linear polarization (LP) at lower frequencies (up to ∼ 2% in Sco X-1 and ∼ 10% in Cyg X-2).…”

Different twins in the millisecond pulsar recycling scenario: Optical polarimetry of PSR J1023+0038 and XSS J12270-4859

“…The radiation emitted from a leptonic jet has a synchrotron nature, which is known to be intrinsically linearly polarised up to tens of per cent. Even in this case, because of the presence of strongly tangled magnetic fields in the jet, radiation is expected to be polarised at a few per cent at most, mainly at lower frequencies (the radio and infrared bands; Russell & Fender 2008). …”

Polarimetric and spectroscopic optical observations of the ultra-compact X-ray binary 4U 0614+091

“…The only evidence of ordered magnetic fields in LMXBs jets has been observed for the persistent system Cyg X-1 (Russell & Shahbaz 2014). Only a few LMXBs have been observed with polarimetric techniques to date, both persistent and transient ones (Charles et al 1980;Dolan & Tapia 1989;Gliozzi et al 1998;Hannikainen et al 2000;Schultz et al 2004;Brocksopp et al 2007;Shahbaz et al 2008a;Russell & Fender 2008;Russell et al 2011;Baglio et al 2014).…”