No Evidence of Intrinsic Optical/Near-Infrared Linear Polarization for V404 Cygni During its Bright Outburst in 2015: Broadband Modeling and Constraint on Jet Parameters

Y. T. Tanaka, R. Itoh, M. Uemura, Y. Inoue, C. C. Cheung, M. Watanabe, K. S. Kawabata, Y. Fukazawa, Y. Yatsu, T. Yoshii, Y. Tachibana, T. Fujiwara, Y. Saito, N. Kawai, M. Kimura, K. Isogai, T. Kato, H. Akitaya, M. Kawabata, T. Nakaoka, K. Shiki, K. Takaki, M. Yoshida, M. Imai, S. Gouda, Y. Gouda, H. Akimoto, S. Honda, K. Hosoya, A. Ikebe, K. Morihana, T. Ohshima, Y. Takagi, J. Takahashi, K. Watanabe, D. Kuroda, T. Morokuma, K. Murata, T. Nagayama, D. Nogami, Y. Oasa, K. Sekiguchi

We present simultaneous optical and near-infrared (NIR) polarimetric results for the black hole binary V404 Cygni spanning the duration of its 7-day long optically-brightest phase of its 2015 June outburst. The simultaneous R and Ks-band light curves showed almost the same temporal variation except for the isolated (~30 min duration) orphan Ks-band flare observed at MJD 57193.54. We did not find any significant temporal variation of polarization degree (PD) and position angle (PA) in both R and Ks bands throughout our observations, including the duration of the orphan NIR flare. We show that the observed PD and PA are predominantly interstellar in origin by comparing the V404 Cyg polarimetric results with those of the surrounding sources within the 7’x7′ field-of-view. The low intrinsic PD (less than a few percent) implies that the optical and NIR emissions are dominated by either disk or optically-thick synchrotron emission, or both. We also present the broadband spectra of V404 Cyg during the orphan NIR flare and a relatively faint and steady state by including quasi-simultaneous Swift/XRT and INTEGRAL fluxes. By adopting a single-zone synchrotron plus inverse-Compton model as widely used in modeling of blazars, we constrained the parameters of a putative jet. Because the jet synchrotron component cannot exceed the Swift/XRT disk/corona flux, the cutoff Lorentz factor in the electron energy distribution is constrained to be <10^2, suggesting particle acceleration is less efficient in this microquasar jet outburst compared to AGN jets. We also suggest that the loading of the baryon component inside the jet is inevitable based on energetic arguments.