Furiously fast and red: sub-second optical flaring in V404 Cyg during the 2015 outburst peak

Gandhi, P.; Littlefair, S. P.; Hardy, L. K.; Dhillon, V. S.; Marsh, T. R.; Shaw, A. W.; Altamirano, D.; Caballero-Garcia, M. D.; Casares, J.; Casella, P.; Castro‐Tirado, A. J.; Charles, P. A.; Dallilar, Y.; Eikenberry, S. S.; Fender, R. P.; Hynes, R. I.; Knigge, Christian; Kuulkers, E.; Mooley, K. P.; Muñoz-Darias, T.; Pahari, Mayukh; Rahoui, Farid; Russell, D. M.; Santisteban, J. V. Hernández; Shahbaz, T.; Terndrup, D. M.; Tomsick, John A.; Walton, Dom

doi:10.1093/mnras/stw571

Cited by 68 publications

(67 citation statements)

References 70 publications

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“…These sizes are longer than the characteristic timescales associated with the sub--second flaring, arguing against reprocessing in this wind for their origin. It is worth noting that the bluer colour and the longer timescales associated with the slower variations may be compatible with an outer disc or wind reprocessing scenario, though a non--thermal origin is also plausible at present 5 .…”

Section: Power Spectramentioning

confidence: 72%

“…Firstly, the coherence peaks at low Fourier frequencies at and below 0.01 Hz, corresponding to the 'slow variations' reported in ref. 5. The optical in this case shows long (but variable) lags with respect to X--rays, ranging over tens of seconds 10 .…”

Section: Spectral Energy Distributionsmentioning

confidence: 75%

“…It may also be related to the fact that our observations were carried out very soon after transition, when the source had probably not yet entered a 'canonical' hard state. In fact, on the following day of June 26, both the optical and the X--ray PSDs were significantly flatter with a large rise in the strength of high frequency optical 5 and X--ray variations 15 . Summary of known source parameters and related inferences V404 Cygni is known to have a mass function of 6.08 M☉ and an orbital period of 6.47 days 16 .…”

Section: Power Spectramentioning

confidence: 96%

“…2, ref. 10), as well as the fact that the optical variability power spectra of V404 Cygni are much steeper 5,36,37 than GX 339--4 and other sources 38 . The reasons for this difference are unclear, but V404 Cygni stands out in terms of its long orbital period (6.5 days 16 ; as compared to 1.7 days for GX 339--4 39 ) and hence larger disc and/or extended corona, which plausibly implies longer variability timescales.…”

Section: Comparison To Other Sources With Fast Timing Datamentioning

confidence: 99%

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An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

et al. 2017

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Relativistic plasma jets are observed in many accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched 1-3 . Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot be easily disentangled from other accreting components. Here, we show that rapid optical flux variations from a Galactic black--hole binary are delayed with respect to X--rays radiated from close to the black hole by ~0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these sub--second optical variations has hitherto been controversial 4-8 . Not only does our work strongly support a jet origin for the optical variations, it also sets a characteristic elevation of ≲10 3 Schwarzschild radii for the main inner optical emission zone above the black hole 9 , constraining both internal shock 10 and magnetohydrodynamic 11 models. Similarities with blazars 12,13 suggest that jet structure and launching physics could potentially be unified under mass--invariant models. Two of the best--studied jetted black hole binaries show very similar optical lags 8,14,15 , so this size scale may be a defining feature of such systems. In June 2015, the Galactic X--ray binary V404 Cygni underwent the brightest outburst of an X-ray binary so far this century. We coordinated simultaneous optical observations from the William Herschel Telescope with X--ray observations from the NuSTAR space observatory on the morning of June 25. These were high frame--rate optical observations taken by the ULTRACAM instrument, sampling timescales down to 35.94 milliseconds (ms). Both optical and X--ray light curves show variability on a broad range of timescales characteristic of this source 15,16 (Fig. 1). The AMI telescope provided contiguous radio coverage throughout this period. Details of the observations may be found in Methods. These coordinated observations occurred on June 25, the day preceding the peak of the 2015 outburst. When the optical observations began, the X--ray intensity was two orders of magnitude below peak, and the spectrum was dominated by low--energy X--rays (i.e., it was in a state characterised as being relatively 'soft'). Steady, compact jet activity is not expected in such a state, and consistent with this, the radio spectral index is negative, as is typical of emission from discrete optically--thin ejecta. NuSTAR observations were interrupted about 2000 seconds later due to a period of Earth occultation, which separates the two halves (hereafter, 'epochs') of the sequence under consideration. At some point during this occultation, the source underwent a dramatic and very rapid change in its X--ray spectral state. When NuSTAR emerged from Earth occultation, the spectrum was instead found to have pivoted towards high energies, with a larger fraction of X--ray counts above 1...

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Section: Power Spectramentioning

confidence: 72%

Section: Spectral Energy Distributionsmentioning

confidence: 75%

Section: Power Spectramentioning

confidence: 96%

Section: Comparison To Other Sources With Fast Timing Datamentioning

confidence: 99%

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An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

et al. 2017

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“…The clipped-mean value in an annulus centered on the target is used to determine the sky level (see ref. (12) for details).…”

Section: Optical Observationsmentioning

confidence: 99%

A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni

Dallilar

Eikenberry

Garner

et al. 2017

Science

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Conditions in a black hole outburst The binary system V404 Cygni consists of a red giant star orbiting a black hole. In 2015, a surge of accretion by the black hole caused the surrounding plasma to brighten suddenly for the first time since 1989, briefly becoming the brightest x-ray source in the sky. Dallilar et al. combined observations from radio, infrared, optical, and x-ray telescopes taken during the outburst. They compared how fast the flux decayed at each wavelength, which allowed them to constrain the size of the emitting region, determine that the plasma within it cooled through synchrotron radiation, and measure the magnetic field around the black hole. Science , this issue p. 1299

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Future Exoplanet Research: Science Questions and How to Address Them

Schneider

2017

Handbook of Exoplanets

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Furiously fast and red: sub-second optical flaring in V404 Cyg during the 2015 outburst peak

Cited by 68 publications

References 70 publications

An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni

Future Exoplanet Research: Science Questions and How to Address Them

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