© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Department of Physics and Astronomy, University of Southampton, Southampton SO17 3RT, UK.
INAF-Osservatorio Astronomico di Cagliari, via della
Scienza 5, I-09047 Selargius, Italy.
Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK.
Instituto de Astrofisica de
Canarias, 38205 La Laguna, Santa Cruz de Tenerife, Spain.
Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK.
Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA.
IRAP, Université de toulouse,
CNRS, UPS, CNES, Toulouse, France.
Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1098 XH Amsterdam, The Netherlands.
Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
Jet Propulsion Laboratory, California Institute of Technology,
4800 Oak Grove Drive, Mail Stop 169-221, Pasadena, CA 91109, USA.
Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley,
CA 94720-7450, USA.
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK.
di Roma, Via Frascati 33, I-00040 Monteporzio Catone, Italy.
DiSAT, Universitá degli Studi dell’Insubria, Via Valleggio 11, I–22100 Como, Italy.
INAF—Osservatorio Astronomico di Brera Merate, via E. Bianchi 46, I-23807 Merate, Italy.
Departmento de Astrofísica Universidad de La Laguna
(ULL), E-38206 La Laguna, Tenerife, Spain.
ISDC, Department of Astronomy, University of Geneva, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland.
Department of Physics and Astronomy, Louisiana State University, 202 Nicholson Hall, Tower Drive, Baton Rouge, LA 70803, USA.
Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands.
Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind,
Pune 411007, India.
European Southern Observatory, K. Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany.
New York University
Abu Dhabi, PO Box 129188 Abu Dhabi, United Arab Emirates.
Department of Physics, University of Alberta, CCIS 4-183, Edmonton AB T6G 2E1, Canada.
Relativistic plasma jets are observed in many systems that
host accreting black holes. According to theory, coiled mag-
netic fields close to the black hole accelerate and collimate the
plasma, leading to a jet being launched
. 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
easily be disentangled from other accreting components. Here,
we show that rapid optical flux variations from an accreting
Galactic black-hole binary are delayed with respect to X-rays
radiated from close to the black hole by about 0.1 seconds, and
that this delayed signal appears together with a brightening
radio jet. The origin of these subsecond optical variations
has hitherto been controversial
. Not only does our work
strongly support a jet origin for the optical variations but it
also sets a characteristic elevation of ≲ 10
radii for the main inner optical emission zone above the black
, constraining both internal shock
models. Similarities with blazars
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
, so this
size scale may be a defining feature of such systems.
In June 2015, the Galactic X-ray binary V404 Cygni under-
went the brightest outburst of an X-ray binary so far this century.
We coordinated simultaneous optical observations from the
William Herschel Telescope (WHT) with X-ray observations from
the NuSTAR space observatory on the morning of 25 June. 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
(Fig. 1). The Arcminute
Microkelvin Imager (AMI) telescope provided conti guous radio
coverage throughout this period. Details of the observations may be
found in Methods.
These coordinated observations occurred on 25 June, the day
preceding the peak of the 2015 outburst. When the optical obser
vations began, the X-ray intensity was two orders of magnitude
below peak, and the spectrum was dominated by low-energy X-rays
(that is, it was in a state characterized as being relatively ‘soft’).
Steady, compact jet activity is not expected in such a state, and con
sistent with this, the radio spectral index is negative, as is typical of
emission from discrete optically thin ejecta. NuSTAR observations
were interrupted about 2,000 seconds later by 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 striking 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 10 keV than below (that is, the state was
significantly ‘harder’). In addition, there was a sharp rise in radio
flux as well as spectral index, all of which signal the strengthening
of compact radio jet emission in an X-ray hard state. The fractional
root-mean-square amplitude characterizing the strength of the
variations also increased in both optical and X-rays. Together, these
facts indicate that we serendipitously caught a rapid state transition,
with a sharp divide in observed properties between the first and
An elevation of 0.1 light-seconds for the optical jet
base in an accreting Galactic black hole system
*, M. Bachetti
, V. S. Dhillon
, R. P. Fender
, L. K. Hardy
, F. A. Harrison
S. P. Littlefair
, J. Malzac
, S. Markoff
, T. R. Marsh
, K. Mooley
, D. Stern
, J. A. Tomsick
D. J. Walton
, P. Casella
, F. Vincentelli
, D. Altamirano
, J. Casares
, C. Ceccobello
P. A. Charles
, C. Ferrigno
, R. I. Hynes
, C. Knigge
, E. Kuulkers
, M. Pahari
, F. Rahoui
D. M. Russell
and A. W. Shaw
NATURE ASTRONOMY | VOL 1 | DECEMBER 2017 | 859–864 | www.nature.com/natureastronomy