Multi-band polarimetry of V404 Cyg with the Nordic Optical Telescope
ATel #7678; P. Blay (IAC-NOT), T. Munoz-Darias (IAC-ULL), J. Kajava (ESAC), J. Casares (IAC-ULL), S. Motta (Oxford), J. Telting (NOT)
on 20 Jun 2015; 22:11 UT
Credential Certification: Teo Muñoz-Darias (tmd@brera.inaf.it)
Subjects: Optical, Black Hole, Transient
Referred to by ATel #: 7681, 7686, 7696, 7702, 7708, 7709, 7721, 7722, 7734, 7740, 7959
We have observed the black hole transient V404 Cyg, currently in outburst (GCN #17929), with the ALFOSC/FAPOL linear Polarimeter attached to the 2.5m Nordic Optical Telescope (NOT). Observations were performed on Jun 19, 2015 (from 2:34 to 3:46 UT) using the U, B, V, R, I Johnson filters. We obtain:
U: 7.9+-0.1 %
B: 7.0+-0.1 %
V: 8.1+-0.1 %
R: 7.5+-0.1 %
I: 6.5+-0.1 %
Instrumental contributions to the polarization have been ruled out after the observation of a zero polarization standard (HD154892). Measurements were also performed in field stars, finding values < 2%. This is consistent with the maximum polarization expected to be produced by interstellar material in the light-of-sight, which according to Fosalba et al. (2002; ApJ 564, 762) is < 4% (using E(B-V) =1.29). Indeed, values around ~1% are reported for this region of the sky.
Therefore, we conclude this high polarization levels are intrinsic to the source. Our results are consistent with the R-band measurement previously reported in ATEL #7674.