The multiwavelength polarization of Cygnus X–1

Abstract

Polarization measurements of the microquasar Cygnus X-1 exist at γ-ray, X-ray, UV, optical and radio frequencies. The γ-ray emission has been shown to be highly linearly polarized. Here, we present new infrared polarimetric data of Cygnus X-1 taken with the 10.4-m Gran Telescopio Canarias and the 4.2-m William Herschel Telescope. We show that the broadband, radio to γ-ray flux spectrum and polarization spectrum in the hard state are largely consistent with a simple phenomenological model of a strongly polarized synchrotron jet, an unpolarized Comptonized corona and a moderately polarized interstellar dust component. In this model, the origin of the γ-ray, X-ray and some of the infrared polarization is the optically thin synchrotron power law from the inner regions of the jet. The model requires the magnetic field in this region to be highly ordered and perpendicular to the axis of the resolved radio jet. This differs to studies of some other X-ray binaries, in which the magnetic field is turbulent, variable and aligned with the jet axis. The model is able to explain the approximate polarization strength and position angle at all wavelengths including the detected X-ray (3-5 keV) polarization, except the observed position angle of the γ-ray polarization, which differs to the model by ∼ 60 • . Past numerical modelling has shown that a curved synchrotron spectrum can produce a shift in position angle by ∼ 60 • , which may account for this.

Polarized synchrotron emission in quiescent black hole X-ray transients

“…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.…”

“…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. One exception is Cyg X-1, which appears to have a highly ordered, stable magnetic field near its jet base that is perpendicular to the axis of the jet (Russell & Shahbaz 2014;Rodriguez et al 2015), and V404 Cyg which had flares of low level polarization, with the magnetic field also perpendicular to the jet axis (Shahbaz et al 2016). These levels of polarization are similar to those often seen from LMXB jets at radio frequencies during the hard state, although higher levels (tens of per cent) of radio polarization have been detected, especially during spectral state transitions when discrete ejections are launched (e.g.…”

“…Repeated sequences were combined and aperture photometry was then performed on the resulting images, and the normalized Stokes parameters q and u, and the fractional linear polarization (FLP) and position angle (PA) were measured using equations (11-13) in Alves et al (2011). Errors on FLP and PA were computed taking into account the errors associated with the raw counts at each polarization angle and polarization bias for low S/N data (Wardle & Kronberg 1974) using the same method as Russell & Shahbaz (2014). The instrumental polarization is known to be very small for LIRIS; < 0.1 per cent (Alves et al 2011).…”

Polarization Radiation with Turbulent Magnetic Fields from X-Ray Binaries

“…, Fender 2006), where a relatively low level of polarization is associated with synchrotron process in the jet. Similar to the studies for multi-wavelength spectral energy distributions (SEDs), it is more inter-esting to study multi-wavelength polarization properties (e.g., Russell & Shahbaz 2014 for a recent work on the microquasar Cygnus X-1). Recently, Laurent et al (2011) observed strong polarization, 67 ± 30%, of the high-energy radiation of Cygnus X-1, between 400 keV and 2 MeV (so-called MeV tail), using the International Gamma-Ray Astrophysics Laboratory Imager on board the Integral satellite (INTEGRAL/IBIS).…”

“…Producing such a high polarization at MeV tail was claimed as the synchrotron emission of relativistic electrons in the jet due to a uniform (ordered), large-scale magnetic field configuration (Laurent et al 2011;Jourdain et al 2012;Russell & Shahbaz 2014;Rodriguez et al 2015). Alternatively, it has been proposed that a hot accretion flow, which requires monodirection motion of relativistic electrons along highly ordered magnetic fields in the inner regions of the accretion flow (Veledina et al 2013), or a hot, highly magnetized plasma corona around the black hole, where hadron and lepton interacting with matter and magnetic fields are invoked, may produce the highly polarized emission at MeV tail (Romero et al 2014).…”