A measurement of interstellar polarization and an estimation of Galactic extinction for the direction of the X-ray black hole binary V404 Cygni

• Published in: Publications of the Astronomical Society of Japan 2017-04, Vol.69 (2)
• Main Authors: Itoh, Ryosuke, Tanaka, Yasuyuki T., Kawabata, Koji S., Uemura, Makoto, Watanabe, Makoto, Fukazawa, Yasushi, Kanda, Yuka, Akitaya, Hiroshi, Moritani, Yuki, Nakaoka, Tatsuya, Kawabata, Miho, Shiki, Kensei, Yoshida, Michitoshi, Oasa, Yumiko, Takahashi, Jun
• Format: Article
• Language: English
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/psw130

Abstract V404 Cygni is a well-known candidate for the black hole binary thought to have relativistic jets. It showed extreme outbursts in 2015 June, characterized by a large amplitude and short time variation of flux in the radio, optical, and X-ray bands. Not only disk emission but also synchrotron radiation from the relativistic jets was suggested by radio observations. However, it is difficult to measure the accurate spectral shape in the optical/near-infrared band because there are uncertainties of interstellar extinction. To estimate the extinction value for V404 Cygni, we performed photopolarimetric and spectroscopic observations of V404 Cygni and nearby field stars. Here, we estimate the Galactic extinction using interstellar polarization based on the observation that the origin of the optical polarization is the interstellar medium, and investigate the properties of interstellar polarization around V404 Cygni. We found a good correlation between the color excess and polarization degree in the field stars. We also confirmed that the wavelength dependence of the polarization degree in the highly polarized field stars was similar to that of V404 Cygni. Using the highly polarized field stars, we estimated the color excess for the (B − V) color and the extinction in the V band to be E(B − V) = 1.2 ± 0.2 and 3.0