Optical polarimetric observations of the black hole X-ray binary V4641 Sgr

ATel #15023; V. Kravtsov, A. Berdyugin, A. Veledina, J. Poutanen, V. Piirola (University of Turku, Finland), S. Berdyugina (Leibniz Institute for Solar Physics, Freiburg, Germany), T. Sakanoi, M. Kagitani (Tohoku University, Sendai, Japan)
on 8 Nov 2021; 21:06 UT
Credential Certification: Alexandra Veledina (alexandra.veledina@utu.fi)

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Following the renewed activity of the black hole X-ray binary V4641 Sgr (ATel #14968, #14971), we performed optical polarimetric observations of the source.
We have observed V4641 Sgr for 3 nights (JD 2459519.7, 2459521.7, 2459522.7) with DIPol-2 BVR polarimeter installed on the remotely controlled 60 cm Tohoku telescope (T60, Haleakala Observatory, Hawaii). Each night the source has been observed for about 1 hour allowing us to make 15 to 24 measurements of normalized Stokes parameters q and u simultaneously in B, V and R bands. The table below contains nightly average values of the linear polarization degree (PD) and polarization angle (PA), along with their measurement errors.
Using observations of 6 field stars located close to the source we determined the value of interstellar polarization in the direction of V4641 Sgr, which is consistent, within errors, with our measurements of the source polarization, indicating absence of high intrinsic polarization. These values are also consistent with the previously measured quiescent-state polarization of the source (see Table 3 in Piirola et al. 2021).
JD PD(%) error PA(deg) error
B-band
2459519.710 0.559 0.079 38.5 4.0
2459521.714 0.538 0.079 41.8 4.2
2459522.710 0.470 0.080 42.1 4.8
V-band
2459519.710 0.362 0.145 31.9 10.9
2459521.714 0.374 0.197 45.7 13.9
2459522.710 0.484 0.127 38.6 7.4
R-band
2459519.710 0.290 0.119 46.9 11.2
2459521.714 0.581 0.094 49.5 4.6
2459522.710 0.500 0.100 44.4 5.6