Extreme jet ejections from the black hole X-ray binary V404 Cygni

V404 Cygni is a binary star system in which a sun-like
star is orbited by a black hole. X-rays are produced when the
black hole’s gravity pulls matter in from the normal star. In
this X-ray image, V404 Cyg is the bright point source at the
center and the bright rings are X-ray “echoes” produced by
walls of dust. Astronomers have modeled the recent flare as a
combination of eight discrete ejections. Credit: Andrew
Beardmore (Univ. of Leicester) and NASA/Swift

A black hole X-ray binary (BHXB) is a black hole orbiting a
normal star. When matter from the normal star accretes onto
the black hole, a jet of charged particles is ejected at
relativistic (near-light) speeds, and these particles emit
strong X-ray radiation. The processes involved are thought to
be similar to ones active under the more dramatic conditions
in active galactic nuclei. Most known BHXBs are located in
our galaxy, and being much closer to us they can be studied
in more detail than their more distant cousins.

Black hole X-ray binaries occasionally flare in outbursts that
can last days to weeks, offering an opportunity to probe how
their jets evolve. Two different types of relativistic jets are
known, depending on the accretion rate of mass in the system.
At low mass accretion rates, the magnetic fields bend the
compact jet, prompting it to emit radiation. At high accretion
rates, discrete jet ejecta are launched that can interfere with
this process in several ways, resulting in more complex
characteristics. (A very rare third type
of emission displays quasi-periodic oscillations.) There are
usually a few bright BHXB events each year, but the more
powerful kind occurs only about once a decade.

On June 15, 2015, the BHXB V404 Cygni underwent just such a
rare, active outburst, and CfA astronomers Glen Petitpas and
Mark Gurwell were members of a team that obtained simultaneous
radio through submillimeter observations of the emission using
the Submillimeter Array along with the Very Large Array and the
James Clerk Maxwell Telescope (SCUBA-2). They tracked the
activity over four hours, during which time they saw multiple,
rapidly changing flares that were bright at all the frequencies
they observed. The scientists best-fitting model worked well
with eight discrete, bipolar, jet ejection events. The model
also estimated the speed, structural properties, geometry, and
energetics of the jets. These unprecedented coordinated
observations of a BHXB highlight the importance of multi-band
observations in studying BHXB jet emission.

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More information: A. J. Tetarenko et al. Extreme jet
ejections from the black hole X-ray binary V404 Cygni,
Monthly Notices of the Royal Astronomical Society
(2017). DOI:
10.1093/mnras/stx1048

Journal reference:
Monthly Notices of the Royal Astronomical Society

Provided by:
Harvard-Smithsonian Center for Astrophysics