Artist's impression of the GX 339-4 black-hole binary system.
Credit: ESA/ATG medialab
Herschel image of the GX 339-4 black-hole binary system.
Credit: ESA/Herschel/PACS/S.Corbel et al.
Astronomers using ESA's Herschel space observatory have detected
emission from the base of black-hole jets for the first time. While
studying the black-hole binary system GX 339-4 in a multi-wavelength
observation campaign, they noticed changes in the source's X-ray and
radio emissions signalling the onset of powerful jets being released
from the black hole's vicinity. This prompted the astronomers to observe
the source at far-infrared wavelengths with Herschel. As the first
observation of emission from jets in a black-hole binary system at these
wavelengths, the data have allowed the astronomers to probe the jets
down to their base, where the far-infrared emission originates.
Herschel's contribution to the multi-wavelength observations has proved a
crucial addition to the understanding of black-hole jets and of the
physical processes that take place very close to a black hole.
When black holes – the densest objects in the Universe – accrete matter
from their surroundings, they also trigger the release of powerful jets
of highly-energetic particles that stem from the accretion disc into
outer space. This phenomenon happens both at the stellar-mass black
holes that derive from the death of massive stars and at the
supermassive black holes lurking at the centre of massive galaxies. The
physical mechanisms underlying the outburst of jets and their connection
with the accretion process, however, are still unclear although
astronomers have been studying them for decades, first via observations
at radio wavelengths and, more recently, across the entire
electromagnetic spectrum
Stellar-mass black holes that are accreting mass from a companion star
in a binary system are of great help to astronomers interested in the
dynamics of jets. Since they are much smaller than their supermassive
counterparts, stellar-mass black holes give rise to jets whose
properties change on relatively short time scales – of the order of a
few hours or days – providing astronomers with a great opportunity to
study their evolution and, possibly, the ignition mechanisms that
trigger the appearance of jets.
"One of the best-studied stellar-mass black holes is the one hosted
in the binary system GX 339-4: we can monitor its evolution quite
closely because the source gives rise to bright outbursts every couple
of years," explains Stéphane Corbel from Laboratoire AIM, France.
Corbel led a new study of this system based on far-infrared (FIR) data
from ESA's Herschel Space Observatory, as well as on observations
performed at X-ray, optical, near-infrared (NIR) and radio wavelengths.
"The multi-wavelength approach is essential for us to explore the
vicinity of black holes, as different regions radiate at different
wavelengths. Broadly speaking, the accretion disc shines most brightly
in X-rays, whereas the jets emit mainly radio waves. But there is more:
the base of the jets – closer to the black hole – emit light at shorter
wavelengths than radio waves, up to the infrared: this is where
Herschel's contribution proved crucial," Corbel adds.
While GX 339-4 has been studied extensively at radio, NIR, optical and
X-ray wavelengths, astronomers had rarely observed it in the vast
portion of the spectrum between radio and NIR wavelengths. In fact,
until now hardly any data from any stellar-mass black hole had been
collected in this broad wavelength range.
The astronomers requested to observe GX 339-4 with Herschel after they
detected changes to its X-ray emission signalling that the outburst
phase of this source, which had been going on for several months, was
about to cease. Since timing was essential, the observations were
performed under Director's Discretionary Time.
"We believe that black-hole binaries give rise to outbursts when
enough material has piled up in the accretion disc: then, just like a
dam that bursts because it can no longer hold any more water, the
material is accreted onto the black hole, giving rise to an enormous
increase of the source's emission at soft X-ray wavelengths," explains Corbel.
The outburst phase, also known as 'soft' state, is accompanied by the
release of 'ballistic jets' – jets that are very bright at radio
wavelengths, consist of multiple ejections and extend up to 10 000
Astronomical Units (AU). When the outburst is over and the source
evolves to the so-called 'hard' state, the appearance of the jets
changes: with weaker radio emission and an extent up to only about 10
AU, these are known as 'compact jets'.
"We had been monitoring GX 339-4's outburst across the
electromagnetic spectrum for several months. When we saw that it was
transitioning to a more quiescent state, we were extremely curious to
see what would happen to the jets," says Corbel.
"It is the first time that we could witness the onset of compact jets and follow their evolution," he adds. "By
combining radio observations with Herschel's FIR data, we could probe
the jet emission down to the base, very close to the black hole."
The Herschel data confirmed the current view, based on radio
observations, which explains the emission from jets as synchrotron
radiation released by highly-energetic electrons. In particular, the
most energetic electrons, present at the base of the jets, radiate at
FIR wavelengths, whilst the lower-energy ones, which are more abundant
at larger distances from the black hole, give rise to radio emission.
The new data, however, raise questions about what causes the emission
detected at NIR and optical wavelengths; this emission is also
associated with the jets but does not seem to have the same origin as
the radio and FIR emission. Since the optical and NIR emission follows
that at radio and FIR wavelengths, one of the possible explanations is
that radio and FIR photons emitted in the jets are then reflected off
the disc, gaining energy in the process and thus radiating at shorter
wavelengths.
"With this result, Herschel has filled a long-standing gap in the
monitoring of stellar-mass black-hole jets across the electromagnetic
spectrum, bridging observations performed at radio and near-infrared
wavelengths," comments Göran Pilbratt, Herschel Project Scientist at ESA. "This
new view complements our current picture of these fascinating objects,
while highlighting, at the same time, how their emission is even more
complex than previously thought," he concludes.
Notes for editors
The study presented here is based on observations of the black-hole
binary system GX 339-4 performed at 70 and 160 microns with the
Photodetector Array Camera and Spectrometer (PACS) on board ESA's
Herschel Space Observatory. The data were gathered under Director's
Discretionary Time on 25 February and 6 March 2011.
GX 339-4 is a binary system hosting a 7-solar-mass black hole that
accretes mass from its companion, a low-mass star. The system is located
at a distance of about 26 000 light years.
In this study, the Herschel observations were combined with data from a
multi-wavelength campaign aimed at monitoring GX 339-4 across the
electromagnetic spectrum; the team of astronomers used data from NASA's
Rossi X-Ray Timing Explorer (RXTE) and Swift satellites to study the
source's X-ray emission, from the SMARTS 1.3-m telescope at Cerro Tololo
Inter-American Observatory, located in Chile, to observe the source at
optical and near-infrared wavelengths, and the Australia Telescope
Compact Array to study its radio emission.
Herschel is an ESA space observatory with science instruments provided
by European-led Principal Investigator consortia and with important
participation from NASA. The PACS instrument contains an imaging
photometer (camera) and an imaging spectrometer. The camera operates in
three bands centred on 70, 100, and 160 μm, respectively, and the
spectrometer covers the wavelength range between 51 and 220 μm. PACS has
been developed by a consortium of institutes led by MPE (Germany) and
including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM
(France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy);
IAC (Spain). This development has been supported by the funding agencies
BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR
(Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain).
Related publications
S. Corbel, et al., "Formation of the compact jets in the black hole GX 339-4", 2013, Monthly Notices of the Royal Astronomical Society, in press
Contacts
Stéphane CorbelLaboratorie AIM (CEA/IRFU, CNRS/INSU, and Université Paris Diderot)
Gif-sur-Yvette, France
Email: stephane.corbel@cea.fr
Phone: +33-1-6908-4562
Göran Pilbratt
Herschel Project Scientist
Research and Scientific Support Department
Science and Robotic Exploration Directorate
ESA, The Netherlands
Email: gpilbratt@rssd.esa.int
Phone: +31 71 565 3621
