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

TL;DR

This study reports on 1.4 GHz radio observations of V404 Cygni during its 2015 outburst, detecting two significant radio flares and analyzing their spectral variations to understand black hole accretion and ejection processes.

Contribution

First detailed radio monitoring of V404 Cygni during its 2015 outburst using the Nasu telescope array, revealing rapid spectral changes linked to ejection activity.

Findings

Detected two radio flares at 1.4 GHz with flux densities around 300 mJy.

Observed rapid spectral variations indicating changing opacity of ejected blobs.

Confirmed extreme radio spectral variability associated with black hole activity.

Abstract

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+/-30 mJy and 1.50+/-0.49 days, 364+/-30 mJy and 1.70+/-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.