Optical and near-infrared polarization of the black hole X-ray binary A0620-00 in quiescence

TL;DR

This paper investigates the intrinsic optical and NIR polarization of the quiescent BH X-ray binary A0620-00 by removing the interstellar polarization using field stars, combining simultaneous high-precision polarimetry with multiwavelength photometry from NIR to UV. The authors find an orbit-averaged intrinsic polarization of $P_{ m int}\sim0.8$–$1.0\%$ with a wavelength-dependent PA that rotates from $\theta\approx 163^\circ$ in the $R$ band to $\theta\approx 177^\circ$ in the $B$ band, and they report significant orbital-phase–dependent PD variations in the $R$ band consistent with Thomson scattering by matter co-orbital with the black hole. A Faraday-rotation interpretation yields RM $= -0.20\pm0.01\,\text{rad}\,\mu\text{m}^{-2}$ and implies a few Gauss magnetic field in the surrounding plasma, while a two-component model (scattering plus a UV-emitting polarized component) can also reproduce the PA rotation, requiring misalignment between components. The UV excess in the SED and the potential UV polarization component motivate future UV polarimetry to distinguish between scenarios and to better constrain the polarization geometry and emission mechanisms in quiescent BH X-ray binaries.

Abstract

We present simultaneous high-precision optical polarimetric and near-infrared (NIR) to ultraviolet (UV) photometric observations of low-mass black hole X-ray binary A0620-00 in the quiescent state. Subtracting interstellar polarization, estimated from a sample of field stars, we derive the intrinsic polarization of A0620-00. We show that the intrinsic polarization degree (PD) is variable with the orbital period with the amplitude of $\sim0.3\%$ at least in the $R$ band, where the signal-to-noise ratio of our observations is the best. This implies that some fraction of the optical polarization is produced by a scattering of stellar radiation off the matter that follows the black hole in its orbital motion. In addition, we see a rotation of the orbit-average intrinsic polarization angle (PA) with the wavelength from $163°$ in the $R$ to $177°$ in the $B$ band. All of the above, combined with the historical NIR-to-optical polarimetric observations, demonstrates the complex behavior of the average intrinsic polarization of A0620-00: the PA continuously rotates from the infrared to the blue band by $\sim53°$ in total, while the PD of $\sim1\%$ remains nearly constant over the entire spectral range. The spectral dependence of the PA can be described by Faraday rotation with the rotation measure of RM=$-0.2$ rad $μ$m$^{-2}$, implying a few Gauss magnetic field in the plasma surrounding the black hole accretion disk. However, our preferred interpretation of the peculiar wavelength dependence is an interplay between two polarized components with different PAs. Polarimetric measurements in the UV range can help in distinguishing between these scenarios.

Figures (10)

• Figure 1: Polarization map of A0620$-$00 (at the origin) and field stars in the $R$ band. The lines correspond to the observed polarization, with the length of the bars showing the PD, and the direction indicating the PA (measured from north to east).
• Figure 2: Observed orbit-average Stokes parameters of A0620$-$00 (squares) and field stars in the $B$, $V$, and $R$ bands (blue, green, and red markers, respectively). The light blue circle shows the uncertainty of the IS polarization with wavelength $\sigma_{\rm is, \lambda}$; the light red circle shows the systematic error on the IS polarization zero point, $\sigma_{\rm is, sys}$.
• Figure 3: Intrinsic Stokes parameters of the polarization of A0620$-$00 in quiescence measured in different filters from the $K_{\rm s}$ to $B$, as indicated by letters near each data point. The light blue circle at the origin illustrates the possible uncertainty of the IS polarization with wavelength, $\sigma_{\rm is, \lambda}$; the light red circle shows the systematic error on the IS polarization zero point, $\sigma_{\rm is, sys}$. The red and blue arrows indicate the intrinsic polarization vector directions in the $K_{\rm s}$ and $B$ bands, respectively. The dashed curved arrow shows the track left by the intrinsic polarization vector during its rotation from the IR to the blue part of the spectrum. An orange star shows the polarization of A0620$-$00 in $V$ measured during the 1975 outburst by Dolan1976.
• Figure 4: Rotation of the polarization on the sky. Red and blue arrows correspond to the direction of the polarization in the $B$ and $K_{\rm s}$ filters, respectively. The blue region shows the direction of the radio ejections as measured in Kuulkers1999.
• Figure 5: Orbital profiles of the flux and of the normalized Stokes parameters of A0620$-$00. Solid blue and empty red circles correspond to the data from two different orbits. Panel (a): Photometric $R$ magnitudes of A0620$-$00 folded with the orbital period. Panels (b) and (c): Observed normalized Stokes parameters. Panel (d): Intrinsic PD of A0620$-$00 in the $R$ filter folded with the orbital period. Each circle with a 1$\sigma$ error bar shows the 35-minute average polarization.