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The Forefront of Space Science

Understanding Relativistic Jets
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R.D. Blandford & R.L. Znajek were the first ones who realized the potential of the above mechanism for launching of astrophysical jets. In their pioneering work back in 1977, these authors showed how exactly electromagnetic currents are driven, and the energy is released in the expense of the black hole rotational energy. Once the energy is released from the ergosphere, it moves outward in the polar directions in the form of a Poynting flux carrying electron-positron pairs spontaneously created within the vacuum of the black hole magne- tosphere. The jet formed in this way is therefore magnetic-dominated, relativistic from the start, and purely leptonic in its matter content. Importantly, the energy available for such an extraction is exactly in the range needed for the most luminous AGN. Moreover, the maximum released power of the outflow is similarly in the correct range inferred observa- tionally for AGN jets, namely Pj ~ (M/108Mʘ) × (J /Jmax)2 × 1045 erg s-1. Note that in the framework of the discussed model the jet power scales with the square of the black hole spin. Hence the emergence of a "spin paradigm", which offers a qualitative explanation for why only some accreting black holes are jetted, and the others are not: "black hole systems producing relativistic jets are the ones with large value of the spin parameter".

The general picture drafted here regarding the launching of relativistic jets from the ergospheres of rotating SMBHs is recently being studied in quite detail by means of pi- oneering general-relativistic magnetohydrodynamical simulations carried out by different groups of researchers using different numerical codes (see Figure2). But the particularly novel and exciting possibility is that the Blandford-Znajek mechanism may be also tested observationally. That is because only now we can attempt the measurements of the spin parameter for astrophysical black holes by means of X-ray and γ-ray observations using modern telescopes such as Suzaku or Fermi-LAT. Once different methods proposed for such measurements are established, the observationally deduced values of black hole spins can be compared with the estimates for the jet power in a number of different systems, and in this way the Pj ∝ a2 dependence can be eventually verified confirming the spin paradigm. The positive verification would exemplify how extreme relativistic jets associated with AGN are. Note that in the Blandford-Znajek mechanism extremely powerful outflows Ewhich typically reach terminal sizes from 1022 cm up to even 1025 cm Eare launched on scales as small as rG ~ (1012 - 1015) cm, and that the invoked source of the enormous jet power is ultimately the spacetime, against which the electromagnetic field in the ergosphere of a supermassive black hole is doing all the work. As such, relativistic jets in active galaxies are truly remarkable objects.



Figure 2
Figure 2. One of the very first general-relativistic magnetohydrodynamical simulations of the Blandford-Znajek process performed by S. Koide et al. (Science 2002, vol. 295, p. 1688). The figure shows the three-dimensional graphics of magnetic field lines around a maximally rotating black hole (the yellow surface around the hole depicts the ergosphere).


The physics of relativistic jets is the major field of my research here in ISAS. Utilizing the most recent X-ray and γ-ray observations, together with several of my colleagues from Japan and abroad we are investigating particle acceleration and generation of high-energy emission in jets, constraining jet internal structure and content, and studying cosmological evolution of jetted AGN. We all hope that with the new instruments in hand (e.g., Suzaku, Chandra, or Fermi/LAT), or in operation in a near future (ASTRO-H, CTA), many open questions regarding these enigmatic objects will be finally answered.

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