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

Abstract

Time-resolved optical and infrared polarimetric observations of black hole and neutron star lowmass X-ray binaries are presented. Data were acquired with the VLT, UKIRT and HIPPO on the SAAO 1.9-m. We find that for some sources in outburst, a rapidly variable component of polarization is evident that is stronger in the redder wavebands. We attribute this to the polarimetric signature of synchrotron emission from jets in these systems, the emission of which is known to dominate these redder bands. Such synchrotron emission from jets launched close to black holes and neutron stars can be highly linearly polarized, depending on the configuration of the magnetic field. The variability of the polarization is suggestive of a tangled and turbulent magnetic field at the location of the compact jet. For some sources the position angle of polarization is consistent with a magnetic field that is parallel to the observed radio jet. These are some of the first observational constraints of the geometry and magnetic structure at the inner regions of the outflow. We also present the first ever simultaneous optical polarization and X-ray campaign of an X-ray binary, using data taken simultaneously with HIPPO and RXTE with sub-second time resolution.

Polarized synchrotron emission in quiescent black hole X-ray transients

“…If synchrotron emission from the jet dominates the KS-band polarization, then this implies the magnetic field vector is parallel to the jet axis. This is the same as found in other LMXBs from NIR data Russell et al 2011) and radio data (Curran et al 2015). Whether the synchrotron is optically thin, or partially self-absorbed (i.e.…”

“…The polarization of 8.0±2.5 per cent implies the magnetic field in the region of the jet near its base is moderately tangled. However, the FLP is fairly high for LMXB jets, being comparable to that measured in Cyg X-2 (Shahbaz et al 2008), lower than 4U 0614+09 (if the disc is unpolarized in that source; Baglio et al 2014b) and much lower than Cyg X-1 (Laurent et al 2011;Russell & Shahbaz 2014;Rodriguez et al 2015), but higher than GX 339-4, Sco X-1, GRO J1655-40 and XTE J1550-564 (Shahbaz et al 2008;Russell et al 2011;Chaty et al 2011). The length of the observations implies a continuously launched (stable on long timescales), highly variable (on short timescales) jet in Swift J1357.2-0933, which has a moderately tangled magnetic field close to the jet base.…”

“…Just recently, during the bright outburst of V404 Cyg in 2015, flares of optical polarization were detected and interpreted as synchrotron emission from a variable jet (Lipunov et al 2016;Shahbaz et al 2016). In neutron star LMXBs, intrinsic infrared or optical polarization was detected in Sco X-1, Cyg X-2 and 4U 0614+09, at similar levels as the black hole LMXBs (up to ∼ 10 per cent in Cyg X-2) and short timescale variations of polarization were observed in Sco X-1 Shahbaz et al 2008;Russell et al 2011;Baglio et al 2014b). These measurements have so far suggested that the magnetic fields near the jet base are largely tangled, chaotic and on average, aligned with the jet axis.…”

“…Detections of (in some cases variable) intrinsic polarization on levels of a few per cent have now been confirmed in several LMXBs. Three black hole candidates were found to have intrinsic polarization at infrared wavelengths during outburst, all during the hard state when jets are expected to be launched, at levels of ∼ 4-7 per cent (GRO J1655-40), ∼ 2.4 per cent (XTE J1550-564) and up to 2.8 per cent (GX 339-4; Chaty et al 2011;Russell et al 2011). In the latter source, variability of the polarization was seen on timescales of minutes.…”

Polarimetric observations of the innermost regions of relativistic jets in X-ray binaries

“…In the optical regime, polarization due to the scattering of intrinsically unpolarized thermal emission can be modulated on the orbital period, which places constraints on the physical and geometrical properties of the system [8,15]. At radio, and in some cases at optical/IR frequencies, variable polarization can be due to synchrotron emission from the jets launched via the process of accretion onto the black hole or neutron star in X-ray binaries [5,16,28,29,34].…”