First multicolour polarimetry of TeV gamma-ray binary HESS J0632+057 close to periastron passage

TL;DR

This study presents multicolour polarimetry of the TeV gamma-ray binary HESS J0632+057 near periastron, revealing significant changes in intrinsic polarisation levels and angles, suggesting dynamic circumstellar conditions.

Contribution

First multicolour polarimetric observations of HESS J0632+057 near periastron, analyzing interstellar and intrinsic polarisation components and their wavelength dependence.

Findings

Polarisation increased after periastron passage

Intrinsic polarisation has a flat wavelength dependence

Changes in polarisation suggest perturbations in circumstellar material

Abstract

We present the results of UBVRI polarimetry of the TeV gamma-ray binary HESS J0632+057 obtained on 2015 March 24 (JD 2457106) and 2015 December 12 (JD 2457369). The detected polarisation values of HESS J0632+057, just after periastron passage (March 24), are higher than all previously published values (p(V) ~ 4.2%) and the position angle (theta ~ 171-172 degrees) is also different by ~6-10 degrees from previously published values. The data obtained just before the subsequent periastron passage (December 12) show statistically lower polarisation in all photometric bands (p(V) ~ 3.9%) and a different position angle theta ~ 167-168 degrees. From observations of a nearby field star, the interstellar component of the measured polarisation was estimated as p_is(V) ~ 0.65% and theta_is ~ 153 degrees. This estimate was used with the previous "V"-band estimation by "field-stars method" (p(V) ~ 2% and theta_is ~ 165 degrees) of Yudin (2014) to identify the wavelength dependence of the intrinsic polarisation 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 polarisation in HESS J0632+057 is essentially flat. We propose that the formation of an additional source of polarisation or some perturbation of circumstellar material at this orbital phase can explain the changes in the level of polarisation in HESS J0632+057 close to periastron passage.