Observations of V404 Cygni during the 2015 outburst by the Nasu telescope array at 1.4 GHz

Abstract

Abstract Waseda University Nasu telescope array is a spatial fast Fourier transform interferometer consisting of eight linearly aligned antennas with 20 m spherical dishes. This type of interferometer was developed to survey transient radio sources with an angular resolution as high as that of a 160 m dish and a field of view as wide as that of a 20 m dish. We have been performing drift-scan-mode observations, in which the telescope scans the sky around a selected declination as the Earth rotates. The black hole X-ray binary V404 Cygni underwent a new outburst in 2015 June after a quiescent period of 26 yr. Because of the interest in black hole binaries, a considerable amount of data on this outburst at all wavelengths was accumulated. Using the above telescope, we had been monitoring V404 Cygni daily from one month before the X-ray outburst, and two radio flares at 1.4 GHz were detected on 2015 June 21.73 and June 26.71. The flux density and timescale of the flares were 313 ± 30 mJy and 1.50 ± 0.49 d, 364 ± 30 mJy and 1.70 ± 0.16 d, respectively. We also confirmed the extreme variation of the radio spectra within a short period by collecting other radio data observed with several radio telescopes. Such spectral behavior is considered to reflect the change in the opacity of the ejected blobs associated with these extreme activities in radio and X-ray. Our 1.4 GHz radio data are expected to be helpful for studying the physics of the accretion and ejection phenomena around black holes.