The Mystery of Astrophysical Jets
The study of astrophysical jets (or relativistic jets) has long captured the curiosity of astronomers, revealing insights into the nature of black holes and challenging our understanding of the universe.
Astrophysical jets are linear structures that originate from a common source, the largest ones being around 100 times the size of our galaxy (De Young). Their discovery dates back to 1918 when H. Curtis observed ‘a curious straight ray’ (Japan Aerospace Exploration Agency) extending from the nucleus of nearby galaxy M87 (Gourgouliatos), marking the first documented sighting of these enigmatic phenomena.
Jets usually originate from accretion processes that contain compact central objects. Highly collimated jets are associated with various astrophysical objects, including active galactic nuclei, young stellar objects, and X-ray binaries. The largest jets are created by supermassive black holes in active galactic nuclei such as quasars and radio galaxies (Livio).
The exact way they accrete and collimate is unknown, but the most probable model is “an accretion disk threaded by a reasonably ordered, perpendicular, large-scale magnetic field.” This model assumes that the mechanism for acceleration and collimation is the same for all the classes of objects with jets and doesn’t account for objects with bipolar weakly collimated outflows. However, the fact that jets originate from the center of accretion disks remains true for all the classes, as the jet velocity is equal to the escape velocity from the central object (Livio).
The composition of astrophysical jets varies depending on the size of the object. Jets from stars contain ionized gas moving at a few hundred kilometers per second while extragalactic jets can carry energy fluxes of over 10⁴⁶ ergs per second (De Young) and could have initial outflow velocities up to 99.9% of the speed of light (Gourgouliatos). These jets are composed of relativistic particles, magnetic fields, and possibly cooler ionized plasma ejected in the jet or absorbed from the surrounding gaseous medium (De Young). The magnetic fields play a crucial role in collimating and accelerating the plasma along the jet's trajectory (Borkowski et al.).
Jets usually emit radiation spanning the entire range of the electromagnetic spectrum, peaking in gamma-ray wavelengths and a range of lower-energy wavelengths. Their immense power and acceleration enable scientists to explore questions regarding supermassive black holes throughout the universe (Lea).
Some jets from these supermassive black holes are aimed directly at Earth, giving rise to active galactic nuclei known as blazars (Lea). The relativistic speeds of these jets, with average outflow speeds exceeding thousands of kilometers per second, provide an opportunity to study theories of relativity and the behavior of particles at high energies (NuSTAR).
The exact processes of the formation and evolution of astrophysical jets are still shrouded in mystery. However, continued research promises to push the boundaries of our understanding of physics to new limits. As we continue to demystify astrophysical jets, we gain deeper insights into the fundamental principles that govern our cosmos.
References:
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Borkowski, Kazimierz J., et al. “Collimation of Astrophysical Jets: The Proto–Planetary Nebula He 3-1475.” The Astrophysical Journal, 1997.
De Young, David S. “Astrophysical jets.” Science (New York, N.Y.), vol. 252, no. 5004, 1991, pp. 389-396.
Gourgouliatos, Konstantinos N. “How we discovered the strange physics of jets from supermassive black holes.” The Conversation, 2 March 2018, https://theconversation.com/how-we-discovered-the-strange-physics-of-jets-from-supermassive-black-holes-92390.
Japan Aerospace Exploration Agency. ISAS | Understanding Relativistic Jets / The Forefront of Space Science, 2011, https://www.isas.jaxa.jp/e/forefront/2011/lukasz/index.shtml?utm_source=feedburner&utm_medium=twitter&utm_campaign=Feed%253A+jaxa%252Fnew_e+(JAXA+Web+What%2527s+New).
Lea, Robert. “100 black hole jets aimed at Earth unleash controversial physics theory.” Space.com, 18 May 2023, https://www.space.com/black-hole-discovery-100-blazars-physics.
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“Physicists Identify the Engine Powering Black Hole Energy Beams.” Quanta Magazine, https://www.quantamagazine.org/physicists-identify-the-engine-powering-black-hole-energy-beams-20210520/.
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