The scientific collaboration running the High Energy Stereoscopic System (H.E.S.S.) telescope, using observations with the Cherenkov Telescope Array located in Namibia, has detected very high-energy gamma-rays above several tera-electronvolts coming from the SS 433 microquasar’s "western" and "eastern" jets in our Galaxy. This confirms the first ever detection of a microquasar in the tera-electronvolt range that the  High Altitude Water Cherenkov Gamma-ray (HAWC) collaboration had purported in 2018. 

Now, with H.E.S.S observations, scientists from the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg and DESY in Zeuthen led a dedicated study on the SS 433 system. They found that the emission from both jets depends on the location and varies as a function of energy. Therefore relativistic particles, here assumed electrons and positrons with velocities lower than but comparable to the speed of light, are accelerated and transported along the re-collimated plasma beams. This means that particles somehow receive an energy boost in the jets. The results of this study, published in Science, locate the highest-energy particles more precisely and get a step closer to understanding the mechanism behind this highly efficient acceleration.

The SS 433 microquasar consists of one of the most puzzling systems in our Galaxy and has for decades sparked the curiosity of the scientific community with its intriguing observational characteristics that have been difficult to reconcile with theoretical expectations. In other words, scientists have been trying to match what they observe with what they would have expected to observe.

The binary system consists of a central object, a black hole accreting material from its companion star. A pair of oppositely-directed collimated plasma beams, two jets, precess perpendicularly from the accretion disc’s surface at about 26% the speed of light, and are referred to as the “eastern” and “western” jets, based on their position coordinates in the celestial sphere.

The H.E.S.S. experiment in Namibia has now detected very high-energy gamma-rays from the jets of SS 433, shedding light onto the sites of efficient extreme acceleration of the particles within the outflows. Through comparison of the gamma-ray morphology at different energies, scientists from MPIK, DESY and the H.E.S.S. collaboration revealed the dynamics of a relativistic jet system in the Milky Way, offering an unprecedented view of such astrophysical phenomena.

So what does the SS 433 system consist of? The binary system is at the centre of the Manatee nebula (W50) a large cloud, seen only in the radio band, that formed when a giant star exploded several tenths of thousands of years ago. The objects at the centre of SS 433 are a black hole with a mass approximately ten times that of the Sun, and a nearby companion star, with a similar mass but occupying a much larger volume, orbiting each other with a period of about 13 days. The black hole, through the action of its gravitational field, accretes material from the surface of the star, which accumulates in a disc of hot gas. As matter is pulled in towards the black hole, two collimated jets of charged particles are launched, perpendicular to the plane of the disc, 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 a fraction of a parsec at either side of the central binary star, before their emission becomes too faint to be detected. Astonishingly, at more than 75 times the distance from their launch site (about 25 parsecs), the jets suddenly reappear with a bright X-ray emission along the continuation of the inner jet axis, and illuminate the outer region of the jets  (Figure 1). The reasons for this reappearance have long been poorly understood.

Similar relativistic jets are also observed emanating from the centres of active galaxies (like quasars), though these jets scale much larger in size up to several kilo-parsecs than the galactic jets of SS 433. Due to this analogy, objects like SS 433 are classified as microquasars.

Gamma-ray emission had never been detected from a microquasar up until the ground-breaking discovery in 2018, when scientists in HAWC, detected very-high-energy gamma rays from two lobes of emission coinciding with the sides of the SS 433 jets. This was the first strong hint that particles are accelerated to extreme energy regimes, somewhere close or in the jets. 

Understanding particle acceleration within astrophysical jets still remains an elusive topic that astrophysicists are hypothesising about and attempting to explain. The study of very high-energy 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, thus 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 to study are in reach with the current generation of gamma-ray telescopes.

Prompted by the HAWC detection, H.E.S.S. initiated a deep dedicated observation campaign of the SS 433 system. This resulted in around 200 hours of data, that led to a significant detection of gamma-ray emission from the jets of SS 433 with ground-based imaging atmospheric Cherenkov telescopes. The angular resolution of the H.E.S.S. experiment in comparison to earlier measurements allowed the researchers to study the morphology of the emission and discern the sites of the origin of very high-energy gamma-ray emission within the jets for the first time, yielding surprising results: while no significant gamma-ray emission is detected from the central binary region and only upper limits can be derived, emission abruptly appears in the outer jets at a distance of about 25 parsecs either side of the central object, coinciding with the site of emergence of the X-ray-observed jets. Furthermore, scientists found that a spatial shift in the position of the associated gamma-ray emission, following a trend when probed at different energies, from 800 giga-electronvolts to above 10 tera-electronvolts.

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 farther along each outer jet.

“This is the first-ever observation of energy-dependent morphology in the gamma-ray emission of an astrophysical jet,” says Laura Olivera-Nieto, from MPIK in Heidelberg, who led the H.E.S.S. study of SS 433 as part of her doctoral thesis. 

“Such a spatially and energy-resolved detection of the SS 433 jets provides paramount information on the internal dynamics of such systems and opens up the landscape of gamma-ray astronomy to a possible new class of very high-energy sources: galactic microquasars,” adds co-author Michelle Tsirou from DESY in Zeuthen.

Olivera-Nieto and collaborators performed a one-dimensional simulation to explain the observed energy dependence of the very high-energy gamma-ray emission seen along the jet axis. This enabled them to estimate the velocity of the outer jets under those assumptions. The difference between this velocity and the one with which the jets are launched suggests that the mechanism which accelerated the particles further out is a strong shock, indicating a medium discontinuity. The presence of a shock would then also provide an explanation for the X-ray reappearance of the jets, as accelerated leptons also produce non-thermal X-ray emission under the presence of strong magnetic fields. 

“When these fast particles then collide with a particle of light (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 co-author Brian Reville (MPIK). “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 allows us to probe the motion of the large-scale jets launched by the black hole,” points-out coauthor Jim Hinton (MPIK).

Despite this discovery, the origin of the shocks at the sites where the jet reappears is still unknown. “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 notes that the relative proximity of SS 433 to Earth offers a unique opportunity to study the occurrence of particle acceleration in relativistic jets. 

Understanding particle acceleration in extreme jetted environments in our own local backyard, the Milky Way, could help shed some light on the perplexing origin of cosmic rays.

Original publication

Acceleration and transport of relativistic electrons in the jets of the microquasar SS 433
H.E.S.S. Collaboration
Science, Volume 383, issue 6681; DOI: 10.1126/science.adi2048

Press release by MPIK 

 

Astrophysical Jet Caught in a “Speed Trap”

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 towards 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 1835m, officially went into operation in 2002 (figure 3). 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.