2017MNRAS.464.4325Y

We present the results of UBVRI polarimetry of the TeV γ-ray binary HESS J0632+057 obtained on 2015 March 24 (JD 245 7106) and 2015 December 12 (JD 245 7369). The detected polarization values of HESS J0632+057, just after periastron passage (March 24), are higher than all previously published values (p_V ∼ 4.2 per cent), and the position angle (Θ_obs ∼ 171°-172°) is also different by ∼6°-10° from previously published values. The data obtained just before the subsequent periastron passage (December 12) show a statistically lower polarization in all photometric bands (p_V ∼3.9 per cent) and a different position angle Θ_obs ∼ 167°-168°. From observations of a nearby field star, the interstellar component of the measured polarization was estimated as p_ is_^V ∼ 0.65 per cent and Θ_is ∼ 153°. This estimate was used with the previous 'V'-band estimation by the 'field-stars method' (p_ is_^V ∼ 2 per cent and Θ_is ∼ 165°) of Yudin to identify the wavelength dependence of the intrinsic polarization in HESS J0632+057. It was found that after subtraction of the interstellar component (for both p_is estimates), the wavelength dependence of the intrinsic polarization in HESS J0632+057 is essentially flat. We propose that the formation of an additional source of polarization or some perturbation of circumstellar material at this orbital phase can explain the changes in the level of polarization in HESS J0632+057 close to periastron passage.