Furiously fast and red

Sub-second optical flaring in V404 Cyg during the 2015 outburst peak

P. Gandhi, S. P. Littlefair, L. K. Hardy, V. S. Dhillon, T. R. Marsh, A. W. Shaw, D. Altamirano, M. D. Caballero-Garcia, J. Casares, P. Casella, A. J. Castro-Tirado, P. A. Charles, Y. Dallilar, S. Eikenberry, R. P. Fender, R. I. Hynes, C. Knigge, E. Kuulkers, K. Mooley, T. Muñoz-Darias & 8 others M. Pahari, F. Rahoui, Dave Russell, J. V. Hernández Santisteban, T. Shahbaz, D. M. Terndrup, J. Tomsick, D. J. Walton

    Research output: Contribution to journalArticle

    Abstract

    We present observations of rapid (sub-second) optical flux variability in V404 Cyg during its 2015 June outburst. Simultaneous three-band observations with the ULTRACAM fast imager on four nights show steep power spectra dominated by slow variations on ~100-1000 s time-scales. Near the peak of the outburst on June 26, a dramatic change occurs and additional, persistent sub-second optical flaring appears close in time to giant radio and X-ray flaring. The flares reach peak optical luminosities of ~ few × 1036 erg s-1. Some are unresolved down to a time resolution of 24 ms. Whereas the fast flares are stronger in the red, the slow variations are bluer when brighter. The redder slopes, emitted power and characteristic time-scales of the fast flares can be explained as optically thin synchrotron emission from a compact jet arising on size scales ~140-500 Gravitational radii (with a possible additional contribution by a thermal particle distribution). The origin of the slower variations is unclear. The optical continuum spectral slopes are strongly affected by dereddening uncertainties and contamination by strong Hα emission, but the variations of these slopes follow relatively stable loci as a function of flux. Cross-correlating the slow variations between the different bands shows asymmetries on all nights consistent with a small red skew (i.e. red lag). X-ray reprocessing and non-thermal emission could both contribute to these. These data reveal a complex mix of components over five decades in time-scale during the outburst.

    Original languageEnglish (US)
    Pages (from-to)554-572
    Number of pages19
    JournalMonthly Notices of the Royal Astronomical Society
    Volume459
    Issue number1
    DOIs
    StatePublished - Jun 11 2016

    Fingerprint

    outburst
    timescale
    flares
    slopes
    night
    erg
    asymmetry
    nonthermal radiation
    radio
    loci
    power spectra
    synchrotrons
    contamination
    x rays
    time lag
    luminosity
    flaring
    continuums
    radii
    distribution

    Keywords

    • Accretion, accretion discs
    • Stars: black holes
    • Stars: individual: V404 Cyg
    • Stars: jets
    • X-rays: binaries

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science

    Cite this

    Gandhi, P., Littlefair, S. P., Hardy, L. K., Dhillon, V. S., Marsh, T. R., Shaw, A. W., ... Walton, D. J. (2016). Furiously fast and red: Sub-second optical flaring in V404 Cyg during the 2015 outburst peak. Monthly Notices of the Royal Astronomical Society, 459(1), 554-572. https://doi.org/10.1093/mnras/stw571

    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.; Fender, R. P.; Hynes, R. I.; Knigge, C.; Kuulkers, E.; Mooley, K.; Muñoz-Darias, T.; Pahari, M.; Rahoui, F.; Russell, Dave; Hernández Santisteban, J. V.; Shahbaz, T.; Terndrup, D. M.; Tomsick, J.; Walton, D. J.

    In: Monthly Notices of the Royal Astronomical Society, Vol. 459, No. 1, 11.06.2016, p. 554-572.

    Research output: Contribution to journalArticle

    Gandhi, P, Littlefair, SP, Hardy, LK, Dhillon, VS, Marsh, TR, Shaw, AW, Altamirano, D, Caballero-Garcia, MD, Casares, J, Casella, P, Castro-Tirado, AJ, Charles, PA, Dallilar, Y, Eikenberry, S, Fender, RP, Hynes, RI, Knigge, C, Kuulkers, E, Mooley, K, Muñoz-Darias, T, Pahari, M, Rahoui, F, Russell, D, Hernández Santisteban, JV, Shahbaz, T, Terndrup, DM, Tomsick, J & Walton, DJ 2016, 'Furiously fast and red: Sub-second optical flaring in V404 Cyg during the 2015 outburst peak', Monthly Notices of the Royal Astronomical Society, vol. 459, no. 1, pp. 554-572. https://doi.org/10.1093/mnras/stw571
    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. ; Fender, R. P. ; Hynes, R. I. ; Knigge, C. ; Kuulkers, E. ; Mooley, K. ; Muñoz-Darias, T. ; Pahari, M. ; Rahoui, F. ; Russell, Dave ; Hernández Santisteban, J. V. ; Shahbaz, T. ; Terndrup, D. M. ; Tomsick, J. ; Walton, D. J. / Furiously fast and red : Sub-second optical flaring in V404 Cyg during the 2015 outburst peak. In: Monthly Notices of the Royal Astronomical Society. 2016 ; Vol. 459, No. 1. pp. 554-572.
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    T2 - Sub-second optical flaring in V404 Cyg during the 2015 outburst peak

    AU - Gandhi, P.

    AU - Littlefair, S. P.

    AU - Hardy, L. K.

    AU - Dhillon, V. S.

    AU - Marsh, T. R.

    AU - Shaw, A. W.

    AU - Altamirano, D.

    AU - Caballero-Garcia, M. D.

    AU - Casares, J.

    AU - Casella, P.

    AU - Castro-Tirado, A. J.

    AU - Charles, P. A.

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    AU - Eikenberry, S.

    AU - Fender, R. P.

    AU - Hynes, R. I.

    AU - Knigge, C.

    AU - Kuulkers, E.

    AU - Mooley, K.

    AU - Muñoz-Darias, T.

    AU - Pahari, M.

    AU - Rahoui, F.

    AU - Russell, Dave

    AU - Hernández Santisteban, J. V.

    AU - Shahbaz, T.

    AU - Terndrup, D. M.

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    N2 - We present observations of rapid (sub-second) optical flux variability in V404 Cyg during its 2015 June outburst. Simultaneous three-band observations with the ULTRACAM fast imager on four nights show steep power spectra dominated by slow variations on ~100-1000 s time-scales. Near the peak of the outburst on June 26, a dramatic change occurs and additional, persistent sub-second optical flaring appears close in time to giant radio and X-ray flaring. The flares reach peak optical luminosities of ~ few × 1036 erg s-1. Some are unresolved down to a time resolution of 24 ms. Whereas the fast flares are stronger in the red, the slow variations are bluer when brighter. The redder slopes, emitted power and characteristic time-scales of the fast flares can be explained as optically thin synchrotron emission from a compact jet arising on size scales ~140-500 Gravitational radii (with a possible additional contribution by a thermal particle distribution). The origin of the slower variations is unclear. The optical continuum spectral slopes are strongly affected by dereddening uncertainties and contamination by strong Hα emission, but the variations of these slopes follow relatively stable loci as a function of flux. Cross-correlating the slow variations between the different bands shows asymmetries on all nights consistent with a small red skew (i.e. red lag). X-ray reprocessing and non-thermal emission could both contribute to these. These data reveal a complex mix of components over five decades in time-scale during the outburst.

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    KW - Stars: jets

    KW - X-rays: binaries

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