Waseda University Nasu telescope array is a spatial fast Fourier transform ( FFT ) 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 with 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 years .
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 June 21.73 and June 26.71 .
The flux density and time-scale of each flare were 313 \pm 30 mJy and 1.50 \pm 0.49 days , 364 \pm 30 mJy and 1.70 \pm 0.16 days , respectively .
We have also confirmed the extreme variation of radio spectra within a short period by collecting other radio data observed with several radio telescopes .
Such spectral behaviors are 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 .