VERITAS observations explore a unique microquasar
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
associate editor
Using the Very Energetic Radiation Imaging Telescope Array System (VERITAS), German astronomers have observed a unique microquasar known as SS 433. Results of the observational campaign, Sept. 25 on the arXiv pre-print server, could help us better understand the nature and behavior of this peculiar object.
Quasars, or quasi-stellar objects (QSOs), are active galactic nuclei (AGN) in the centers of active galaxies, exhibiting very high luminosity. Microquasars are their smaller versions—binary systems in which a compact object (such as a black hole or neutron star) draws matter from its companion star. Studies suggest that both quasars and microquasars are powered by spinning supermassive black holes (SMBHs).
SS 433 was discovered in 1978 in the center of the supernova remnant W50 as the first microquasar. It is a massive X-ray binary system at an advanced stage of evolution, located some 18,000 light years away.
Previous observations have found that SS 433 consists of a stellar-mass black hole accreting matter from an A-type companion star. The orbital period of the system was measured to be approximately 13.08 days.
SS 433 is unique among other microquasars as it has precessing relativistic jets embedded within the supernova remnant. These jets propagate over tens of light years, interacting with the surrounding medium and producing emissions across the electromagnetic spectrum.
A team of astronomers led by Tobias Kleiner of the DESY Research Center in Zeuthen, Germany, employed the VERITAS four-telescope imaging atmospheric Cherenkov telescope (IACT) array, located at the Fred Lawrence Whipple Observatory in Arizona, in order to take a closer look at the jets of SS 433, hoping to shed more light on the nature of the microquasar.
IACT observations carried out between 2009 and 2023 found that the jets of SS 433 accelerate particles to TeV energies. The emission from the microquasar appears to have elongated Gaussian morphologies aligned with the jet axis. This indicates that the very-high-energy (VHE) emission originates from extended structures likely associated with jet-interstellar medium interaction regions.
The observations have not detected any significant gamma-ray emission from the system and found no evidence of variability with the orbital or precessional periods. These results seem to suggest that the VHE emission in SS 433 arises from steady, large-scale acceleration processes at the jet termination regions, where the jets interact with the ambient medium.
According to the study, the observed spatial extent and alignment of SS 433, with the jets, point to a shock acceleration at jet termination sites interacting with the surrounding interstellar medium or from jet self-interaction.
"These results firmly establish SS 433 as a benchmark system for VHE particle acceleration in microquasar jets and offer a key reference for jet-powered sources within the galaxy," the authors of the paper conclude.
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More information: Tobias Kleiner et al, VERITAS Observations of the Microquasar SS 433, arXiv (2025).
Journal information: arXiv
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