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The Forefront of Space Science

Understanding Relativistic Jets
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Astrophysical jets are collimated streams of magnetized plasma produced by compact accreting objects such as neutron stars or black holes. Particularly prominent jets are associated with supermassive black holes (SMBHs) hosted by active galaxies, and these are the main focus of this article. In general, we differentiate between stellar-mass black holes, which form by a direct collapse of dying stars, and more massive objects, including SMBHs, formed by a gradual growth of smaller-mass systems via accretion of the surrounding matter and/or via series of black hole mergers. There are strong observational indications for the presence of SMBHs in the centers of many galaxies, including our own Milky Way, and the mass estimates for such, roughly in the range M ≃ (106 - 1010) Mʘ where Mʘ ≃ 2 1033 g is the mass of the Sun, are now accurate to within 10% in a few best cases. The current understanding is that every galaxy hosts a SMBH in its center, and that "active galaxies" are the ones in which central SMBHs accrete at high rates. The accretion process involves an inflow of a large amount of matter toward the galactic center, leading to a formation of an "accretion disk" around a black hole. Above and below the disk, hot and magnetized "corona" is present, constituting a low-density disk magnetosphere. The other portion of the accreting matter forms a net of clouds of gas and dust orbiting the center. The radiative outputs of all these components contribute together to the observed phenomenon of Active Galactic Nuclei (AGN).

Figure 1
Figure 1. The optical image of the jet emanating from the center of galaxy M 87, taken by the Hubble Space Telescope (J.A. Biretta et al.). The jet was first discovered in 1918 by H. Curtis, who described it as a "curious straight ray" extending from the galactic nucleus.

Jets are associated with black holes with very different masses. In particular, stellar- mass black holes in binary systems, as well as those freshly produced in hypernova-type explosions, do produce short-lived collimated outflows characterized by relativistic bulk velocities. But the connection between SMBHs and relativistic jets is particularly clear, since hundreds of steady jets are directly resolved to originate in different types of AGN. The first structure like this was discovered at optical frequencies in a nearby galaxy M 87 located in the center of the Virgo cluster (see Figure 1). What is common for all the jetted black holes is therefore not the mass range, but a supply of the accreting matter. In the case of the binary systems it is the companion star which provides the accretion fuel. For hypernovae it is the collapsing stellar interior which falls onto a newly formed black hole. In the case of SMBHs it is the galaxy merger process depositing large amounts of a matter around galactic centers which triggers the onset of the accretion and so the AGN activity.

Yet the other crucial observational finding is that jets discriminated neither between different masses of black holes nor between different accretion rates, and also that only some vigorously accreting black holes are characterized by a prominent jet activity. It is important to emphasize that by a "prominent jet activity" we mean well collimated (almost cylindrical) beams of plasma emitting non-thermal radiation from radio to γ-ray frequencies, and moving with relativistic bulk velocities vc, where c ≃ 3 1010 cm s-1 is the speed of light. In fact, un-collimated and non-relativistic (v < c) outflows emanating from black hole/accretion disk systems seem to be a common property of all AGN, and possibly even of all the accreting black holes. Yet relativistic jets are relatively rare phenomena. So, why do only some accreting black holes produce relativistic jets, while others don't? It is not easy to answer this question, because it is not well understood how exactly jets associated with black holes are formed. The presence of an accretion disk is crucial, as noted above. Are jets therefore simply launched from the inner parts of accretion disks, as some particular form of a disk wind carrying excess angular momentum and matter from the surface of a disk? If this is the case indeed, then a black hole itself is not directly involved in the jet launching, as it serves only as a compact massive object gravitationally attracting the accreting gas.

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