Peering into the tilted heart of Cyg X-1 with high-precision optical polarimetry
Additional information
Authors
Kravtsov V.,
Veledina A.,
Berdyugin A. V. .,
Zdziarski A. A. .,
Henson G. D. .,
Piirola V.,
Sakanoi T.,
Kagitani M.,
Berdyugina S.,
Poutanen J.
Type
Journal Article
Year
2023
Language
English
Abstract
We present high-precision optical polarimetric observations of the blackhole X-ray binary Cygnus X-1 that span several cycles of its5.6-day orbital period. The week-long observations on twotelescopes located in opposite hemispheres allowed us to trackthe evolution of the polarization within one orbital cycle withthe highest temporal resolution to date. Using the field stars,we determined the interstellar polarization in the sourceensuremathpm5textdegree. The optical polarization angleis aligned with that in the X-rays recently obtained with theImaging X-ray Polarimetry Explorer. Furthermore, it isconsistent within the uncertainties with the position angle ofthe radio ejections. We show that the intrinsic polarizationdegree is variable with the orbital period with an amplitude ofensuremathsim0.2% and discuss various sites of itsproduction. Assuming that the polarization arises from a singleThomson scattering of the primary star radiation by the matterthat follows the black hole in its orbital motion, weconstrained the inclination of the binary orbit i >120textdegree and its eccentricity e < 0.08. The asymmetricshape of the orbital profiles of the Stokes parameters alsoimplies the asymmetry of the scattering matter distribution inthe orbital plane, which may arise from the tilted accretiondisk. We compared our data to the polarimetric observations madein 1975-1987 and find good agrement within 1textdegreebetween the intrinsic polarization angles. On the other hand,the polarization degree decreased by 0.4% over half a century,suggesting secular changes in the geometry of the accretingmatter.
Journal
Astronomy and Astrophysics
Volume
678
Start page number
A58
