Tracking the variable jets of V404 Cygni during its 2015 outburst

Abstract

We present multi-frequency monitoring observations of the black hole X-ray binary V404 Cygni throughout its June 2015 outburst. Our data set includes radio and mm/sub-mm photometry, taken with the Karl G. Jansky Very Large Array, Arc-Minute MicroKelvin Imager Large Array, Sub-millimeter Array, James Clerk Maxwell Telescope, and the Northern Extended Millimetre Array, combined with publicly available infrared, optical, UV, and X-ray measurements. With these data, we report detailed diagnostics of the spectral and variability properties of the jet emission observed during different stages of this outburst. These diagnostics show that emission from discrete jet ejecta dominated the jet emission during the brightest stages of the outburst. We find that the ejecta became fainter, slower, less frequent, and less energetic, before the emission transitioned (over 1-2 days) to being dominated by a compact jet, as the outburst decayed toward quiescence. While the broad-band spectrum of this compact jet showed very little evolution throughout the outburst decay (with the optically thick to thin synchrotron jet spectral break residing in the near-infrared/optical bands; ∼ 2 − 5 × 10 14 Hz), the emission still remained intermittently variable at mm/sub-mm frequencies. Additionally, we present a comparison between the radio jet emission throughout the 2015 and previous 1989 outbursts, confirming that the radio emission in the 2015 outburst decayed significantly faster than in 1989. Lastly, we detail our sub-mm observations taken during the December 2015 mini-outburst of V404 Cygni, which demonstrate that, similar to the main outburst, the source was likely launching jet ejecta during this short period of renewed activity.

Accretion and outflow in V404 Cyg

“…A direct comparison with Figure 1 shows that, while the 2015 outburst terminates ∼10 days after the X-ray trigger, the 1989 (and 1938) outburst exhibit a much longer luminosity fall (also seen in radio and X-rays, e.g. Han & Hjellming 1992;Tetarenko et al 2019). The latter follows a classic exponential tail with an e-folding timescale of ∼110 d that takes ∼ 2 yr to bring the system into quiescence.…”

“…Many papers have dealt with the timing and spectral properties of the exceptional flares in different energy bands (e.g. Rodriguez et al 2015;Natalucci et al 2015;Roques et al 2015;Kimura et al 2015;Jenke et al 2016;Martí et al 2016;Gandhi et al 2016;Loh et al 2016;Jourdain et al 2017;Rodi et al 2017;Walton et al 2017;Motta et al 2017a;Sánchez-Fernández et al 2017;Maitra et al 2017;Tachibana et al 2017;Tetarenko et al 2017Tetarenko et al , 2019. Here in this paper we have focused instead on the optical study of the nebular phase and subsequent decay, with the aim of constraining the physics of the outflow.…”

Radio Variability from a Quiescent Stellar-mass Black Hole Jet

“…ARCHIVAL OBSERVATIONS AND DATA ANALYSIS V404 Cygni has undergone two major outbursts during the VLA era, the first discovered on 1989 May 22 (Makino 1989), and the second outburst on 2015 June 15 (Barthelmy et al 2015;Kuulkers et al 2015;Negoro et al 2015;Younes 2015). After the main 2015 outburst ended in July (see next paragraph), renewed X-ray activity was observed on 2015 December 21 (Malyshev et al 2015), and V404 Cygni went through a mini-outburst that lasted ∼30 days (see, e.g., Kimura et al 2017;Muñoz-Darias et al 2017;Tetarenko et al 2019).…”

Evidence of Outflow-induced Soft Lags of Galactic Black Holes

“…The synchrotron break frequency of this object was found to be ∼ 2 − 5 × 10 14 Hz which is in the NIR-optical band. Due to the very high break frequency, Tetarenko et al (2019) also argued that a large fraction of photons originated in the jet region might have inverse Comptonized in the hot corona and contribute in the observed X-ray. The positive lag (soft/negative lag in our nomenclature as the lesser of the two frequency bands is lagging) of ∼ 12 minutes between 26 and 5 GHz indicates light crossing time of 1.5 AU.…”