Astronomy Object of the Month: September 2024

< previous Archive next >

Strange galactic microquasar SS 433

The H.E.S.S. observatory in Namibia has detected high-energy gamma rays coming from the plasma jets of the SS 433 microquasar, and pinpointed the exact location within them of one of the galaxy's most efficient particle accelerators. By comparing gamma-ray images at different energies, it was possible for the first time to estimate the speed of the jet away from its emission site. This enabled to identify the mechanism that so effectively accelerates the particles.

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. SS 433 is also one of the most intriguing objects in the Milky Way. The H.E.S.S. observatory has succeeded in detecting very high-energy gamma rays from the jets of SS 433, and in pinpointing the exact location within the jets of one of the galaxy's most efficient particle accelerators. By comparing gamma-ray images at different energies, scientists from the H.E.S.S. collaboration have revealed the motion and dynamics of relativistic jets in our own galaxy, offering valuable insight into these extraordinary astrophysical phenomena. The results are published in Science.

The jets of SS433 can be detected in the radio to x-ray ranges out to a distance of less than one light year 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 centres of active galaxies (for example quasars), though these jets are much larger than the galactic jets of SS 433. Due to this analogy, objects like SS 433 are classified as microquasars.

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.

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. 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 either side of the binary star, in accordance with previous X-ray observations.

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 (see fig 2c). By contrast, the regions emitting gamma rays with lower energies appear further along each jet. The emission position varies according to the energy observed.

The team did a simulation of the observed energy-dependence of the gamma-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 which accelerated the particles further out is a strong shock, a sharp transition in the properties of the medium. Presence of a shock would also provide a natural explanation for the x-ray reappearance of the jets, as accelerated electrons also produce x-ray radiation. And when these fast particles then collide with photons, they transfer part of their energy – which is how they produce the high-energy gamma photons observed with H.E.S.S. However, nothing is known about the origin of the shocks at the sites where the jet reappears, and there is no model uniformly explaining all the properties of the jet. According to the team, the 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 their origin.



Original publication: H.E.S.S. Collaboration, Acceleration and transport of relativistic electrons in the jets of the microquasar SS 433, Science 383, 6 (2024).

The findings described are part of a study conducted in the Department of High Energy Astrophysics of the Jagiellonian University Astronomical Observatory in Kraków. The participation of Polish scientists in the H.E.S.S. project was co-financed from the program of the Minister of Education and Science "Support for participation of Polish scientific teams in international research infrastructure projects" under agreement no. 2021/WK/06.


Contact:

Łukasz Stawarz Astronomical Observatory Jagiellonian University L.Stawarz [@] oa.uj.edu.pl