Fermi Gamma-ray Space Telescope

No Evidence of Intrinsic Optical Near-Infrared Linear Polarization and Fermi-LAT Observation for V404 Cyg during the Bright Outburst in 2015: Broadband Modeling and Constraint on Jet Parameters

Y. T. Tanaka
R. Itoh Dr., M. Uemura Dr., Y. Inoue Dr., C. C. Cheung Dr., OISTER and Fermi-LAT collaborations

Abstract:

We present simultaneous optical and near-infrared (NIR) polarimetric results for the black hole binary V404 Cyg spanning the duration of its 7-day long optically-brightest phase in the 2015 June outburst. The simultaneous R and Ks-band light curves showed almost the same temporal variation except for an isolated (? 30 min duration) orphan Ks-band flare observed at MJD 57193.54. We did not find any significant temporal variation in the polarization degree (PD) and position angle (PA) in both R and Ks bands during our exposures and even during the NIR flare. We show that the observed PD and PA are predominantly interstellar in origin by comparing the polarimetric results with surrounding sources within the 6'x6' 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 by including quasi-simultaneous Swift/XRT and INTEGRAL fluxes as well as Fermi/LAT result in MeV/GeV gamma-ray band. By adopting a single-zone synchrotron plus inverse-Compton model as widely used in modeling of blazars, we found that the cutoff Lorentz factor in the electron energy distribution is constrained to be <10^2, indicating particle acceleration is less efficient in this microquasar jet outburst than in AGN jets. We also suggest that the loading of the baryon component inside the jet is inevitable based on energetic arguments.