Artist's impression of the SS 433 system, depicting the large-scale jets (blue) and the surrounding Manatee Nebula (red). The jets are initially observable only for a short dis-tance from the microquasar after launch—too small to be visible in this picture. The jets then travel undetected for a distance of approximately 75 light-years (25 parsecs) before un-dergoing a transformation, abruptly reappearing as bright sources of non-thermal emission (X-ray and gamma-ray). Particles are efficiently accelerated at this location, likely indicating the presence of a strong shock: a discontinuity in the medium capable of accelerating particles. Credit: Science Communication Lab for MPIK/H.E.S.S.
The
science fiction author Arthur C. Clarke selected his own seven wonders of the
world in a BBC television series in 1997. The only astronomical object he
included was SS 433. It had attracted attention already in the late 1970s due
to its X-ray emission and was later discovered to be at the center of a gas
nebula that is dubbed the manatee nebula due to its unique shape resembling
these aquatic mammals.
SS 433 is a binary star system in which a black hole, with a mass approximately
ten times that of the sun, and a star, with a similar mass but occupying a much
larger volume, orbit each other with a period of 13 days.
The intense gravitational field of the
black hole rips material from the surface of the star, which accumulates in a
hot gas disk that feeds the black hole. As matter falls in toward the black
hole, two collimated jets of charged particles (plasma) are launched,
perpendicular to the plane of the disk, at a quarter of the speed of light.
The jets of SS433 can be detected in the
radio to X-ray ranges out to a distance of less than one light year on either
side of the central binary star, before they become too dim to be seen. Yet
surprisingly, at around 75 light years distance from their launch site, the
jets are seen to abruptly reappear as bright X-ray sources. The reasons for
this reappearance have long been poorly understood.
Similar relativistic jets are also
observed emanating from the centers of active galaxies (for example quasars),
though these jets are much larger in size than the galactic jets of SS 433. Due
to this analogy, objects like SS 433 are classified as microquasars.
Artist's impression video visualization of the SS
433 system and summary of the main results of the paper. Credit: Science
Communication Lab for MPIK/H.E.S.S.
Until recently, no gamma ray
emission has ever been detected from a microquasar. But this changed in 2018,
when the High Altitude Water Cherenkov Gamma-ray Observatory (HAWC), for the
first time, succeeded in detecting very-high-energy gamma rays from the jets of
SS 433. This means that somewhere in the jets particles are accelerated to
extreme energies.
Despite decades of research, it is
still unclear how or where particles are accelerated within astrophysical jets.
The study of gamma-ray emission from microquasars provides one crucial advantage: while the jets of SS 433 are 50 times smaller than those of the closest active galaxy (Centaurus A), SS 433 is located inside the Milky Way a thousand times closer to Earth. As a consequence, the apparent size of the jets of SS 433 in the sky is much larger and thus their properties are easier to study with the current generation of gamma-ray telescopes.
Composite
images of SS 433 showing three different gamma-ray energy ranges. In green,
radio observations display the Manatee Nebula with the microquasar visible as a
bright dot near the center of the image. Solid lines show the outline of the
X-ray emis-sion from the central regions and the large scale jets after their
reappearance. Red colors represent the gamma-ray emission detected by H.E.S.S.
at a) low (0.8–2.5 TeV, left), b) in-termediate (2.5–10 TeV, middle) and c)
high (>10 TeV, right) energies. The position of the gamma-ray emission
shifts further from the central launching site as the energy decreases. Credit:
Background: NRAO/AUI/NSF, K. Golap, M. Goss; NASA's Wide Field Survey Ex-plorer
(WISE); X-Ray (green contours): ROSAT/M. Brinkmann; TeV (red colors): H.E.S.S.
collaboration.
Prompted
by the HAWC detection, the H.E.S.S. Observatory initiated an observation
campaign of the SS 433 system. This campaign resulted in around 200 hours of
data and a clear detection of gamma-ray emission from the jets of SS 433.
The superior angular resolution of the
H.E.S.S. telescopes in comparison to earlier measurements allowed the
researchers to pinpoint the origin of the gamma-ray emission within the jets
for the first time, yielding intriguing results:
While no gamma-ray emission is detected
from the central binary region, emission abruptly appears in the outer jets at
a distance of about 75 light years on either side of the binary star, in
accordance with previous X-ray observations.
However, what surprised the astronomers
most, was a shift in the position of the gamma-ray emission when viewed at
different energies.
The gamma-ray photons with the highest energies of more than 10 teraelectron-volts, are only detected at the point where the jets abruptly reappear. By contrast, the regions emitting gamma rays with lower energies appear further along each jet.
The
H.E.S.S. observatory, located in the Khomas Highlands of Namibia at an altitude
of 1835m below the southern sky. Credit: Sabine Gloaguen
"This is the first-ever
observation of energy-dependent morphology in the gamma-ray emission of an astrophysical jet," said Laura Olivera-Nieto, from the
Max-Planck-Institut für Kernphysik in Heidelberg, who was leading the H.E.S.S.
study of SS 433 as part of her doctoral thesis.
"We were initially puzzled by
these findings. The concentration of such high energy photons at the sites of
the X-ray jets' reappearance means efficient particle acceleration must be
taking place there, which was not expected." The findings have been published in Science.
The scientists did a simulation of
the observed energy dependence of the gammy-ray emission and were able to
achieve the first-ever estimate of the velocity of the outer jets. The
difference between this velocity and the one with which the jets are launched
suggests that the mechanism that accelerated the particles further out is a
strong shock- a sharp transition in the properties of the medium.
The presence of a shock would then
also provide a natural explanation for the X-ray reappearance of the jets, as
accelerated electrons also produce X-ray radiation.
"When these fast particles
then collide with a light particle (photon), they transfer part of their
energy—which is how they produce the high-energy gamma photons observed with
H.E.S.S. This process is called the inverse Compton effect," explains
Brian Reville, group leader of the Astrophysical Plasma Theory group at the Max
Planck Institute for Nuclear Physics in Heidelberg.
"There has been a great deal
of speculation about the occurrence of particle acceleration in this unique
system—not anymore: the H.E.S.S. result really pins down the site of
acceleration, the nature of the accelerated particles, and allows us to probe
the motion of the large-scale jets launched by the black hole," points out
Jim Hinton, Director of the Max Planck Institute for Nuclear Physics in
Heidelberg and Head of the Non-thermal Astrophysics Department.
"Just a few years ago, it was
unthinkable that ground-based gamma-ray measurements could provide information
about the internal dynamics of such a system," adds co-author Michelle
Tsirou, a postdoctoral researcher at DESY Zeuthen.
However, nothing is known about the
origin of the shocks at the sites where the jet reappears. "We still don't
have a model that can uniformly explain all the properties of the jet, as no
model has yet predicted this feature," explains Olivera-Nieto.
She wants to devote herself to this
task next—a worthwhile goal, as the relative proximity of SS 433 to Earth
offers a unique opportunity to study the occurrence of particle acceleration in
relativistic jets. It is hoped that the results can be transferred to the
thousand-times larger jets of active galaxies and quasars, which would help
solve the many puzzles concerning the origin of the most energetic cosmic rays.
The H.E.S.S. observatory
High-energy gamma rays can only be
observed from the ground with a trick. When a gamma ray enters the atmosphere,
it collides with atoms and molecules and generates new particles that race on
toward the ground like an avalanche. These particles emit flashes lasting only
a few billionths of a second (Cherenkov radiation), which can be observed with
specially equipped large telescopes on the ground.
High-energy gamma astronomy
therefore uses the atmosphere like a giant fluorescent screen. The H.E.S.S.
observatory, located in the Khomas Highlands of Namibia at an altitude of
1,835m, officially went into operation in 2002. It consists of an array of five
telescopes.
Four telescopes with mirror
diameters of 12 m are located at the corners of a square, with a further 28 m
telescope in the center. This makes it possible to detect cosmic gamma
radiation in the range of a few tens of gigaelectronvolts (GeV, 109 electronvolts) to a few tens of teraelectronvolts (TeV, 1012 electronvolts).
For comparison: visible light particles have energies of two to three electron volts. H.E.S.S. is currently the only instrument that observes the southern sky in high-energy gamma light and is also the largest and most sensitive telescope system of its kind.
No comments:
Post a Comment