The Astrophysical Jets – After 30 Years of Deliberation

THE ASTROPHYSICAL JETS – AFTER 30 YEARS OF DELIBERATION

Wolfgang Kundt

Argelander Institute of Bonn University Auf dem Hugel¨ 71, D-53121 Bonn, Germany

Abstract

An update is presented of my 2004 semi-analytical model with Gopal Krishna for E x B-drifting jets, which may well describe sat- isfactorily all the astrophysical jet sources, or bipolar ﬂows, both on galactic and on stellar scales.

1 Introduction

Jet sources have been discovered in the sky, and mapped since more than 30 years, beginning at radio frequencies, closely followed by optical frequencies, then X-rays and γ-rays up to GeV energies, and recently even up to TeV energies, or even . PeV energies! Whilst more and more morphological and temporal details have been elaborated, the modes of their generation, propagation, termination, and evolution have found deploringly diverging treatments in the extended literature. This decades-long stagnation of insight, it appears to me, is predominantly due to the black-hole dictatorship in present-day astrophysics: Black Holes (BHs) lack persistent, time-varying magnetic ﬁelds (for pair formation, and for their post-acceleration via an outgoing strong wave), and lack quasi-static (high-density) deLaval nozzles for the formation of a pair of supersonic antipodal jets – as opposed to fast- rotating magnetized stars inside shearing accretion disks, or diﬀerentially rotating magnetized coronas of central galactic disks which have been the preferred jet sources in Kundt & Gopal-Krishna [1980, 2004] as well as in

1 Blome & Kundt [1988], and in Kundt [1979, 1989, 1996, 2001a,b, 2002, 2005, 2009a,b], to be revisited in section 4. In my mother language: Schwarze L¨ocher schlucken, sie spucken nicht. This compact review of jet formation will begin with a listing of the wide range of observed jet sources, in section 2, grouping them into a 4-headed family according to their diﬀerent central engines: (a) newborn stars, (b) forming white dwarfs, (c) middle-aged binary neutron stars, and (d) nuclear- burning centers of galactic disks. Note that these four source types are expected to anchor a rapidly corotating transverse magnetic moment, whilst the often proposed (stellar-mass) ‘BH-Candidates’ may have been mistaken for neutron stars inside of massive accretion disks, and the ‘supermassive BHs’ at the centers of galaxies mistaken for nuclear-burning central galactic disks (BDs). Section 3 will collect a number of plausible constraints on a jet source to function, and sort between viable and non-viable mechanisms for their real- isation. The quasi-unique analytical class of jet solutions found in [Kundt & Krishna 2004] will then be reviewed in section 4, and its key features summarized in section 5. Quite similar approaches have been taken by Phil Morrison [1981], and by Peter Scheuer [1996], who are both, unfortunately, no longer with us.

2 The Bipolar-Flow Family

Even though the astrophysical jet sources range through huge factors in {size, age, power, mass(CE)}, CE := central engine, viz. range through respective factors .{108,107,109,109}, they appear astonishingly similar in their essential properties such as the jets’ opening angle Θ, the jet/core power ratio, maximal speed, morphology, stability, spectrum, and variability. They tend to be divided into ‘micro-quasars’ and ‘quasars’ – the prefix “micro” alluding to a typical discriminating factor of 106 in their listed properties – a classification which essentially agrees with ‘stellar’ and ‘galactic’ CEs. Here is a table of the complete family of Jet Sources, or Bipolar Flows (BFs): Newborn Stars (YSOs) Forming White Dwarfs (inside PNe) Elderly Binary Neutron Stars Centers of Galactic Disks (AGN) For an easier comparison of the subsequent modeling with the best- studied astronomical jet sources, let us look at a few outstanding represen- tatives of the four classes: (a) Newborn stars, or young stellar objects (YSOs), tend to emit predominantly thermal spectra, hence look often rather different from their more powerful and larger relatives of the BF family. Yet as already stressed in earlier publications, a few exceptions exist which look exactly like minia- ture copies of their bigger relatives, e.g. like fast expanding radio triples, of age . 103yr, see Blome & Kundt [1988], or Kundt [1996, 2001, 2002, 2005]. New is last year’s discovery of half a dozen brown dwarfs with jets [Whelan et al 2009]: Even such YSOs, of subsolar mass, have strong enough magnetospheres in fast enough corotation to blow (feeble) jets. (b) Forming white dwarfs, inside planetary nebulae (PNe), have various morphologies, but at least some of them show clear evidences of feeble jets near their symmetry axes, see [Kundt 1996], and indications of two antipodal elongated lobes, and their various morphologies cannot be understood without multiple plasma components of different densities in extended in- teraction. Also, an increasing number of cataclysmic variables is joining the jet set. (c) This class, of BFs powered by elderly neutron stars, is likewise known since more than 30 years, with SS 433 as its most powerful, and most exotic representative: SS 433 is particularly famous for its multiple, transrelativis- tic and almost periodic optical and X-ray emission lines, which I understand as multiply excited recombination lines from dragged-along channel-wall material of the innermost parts of its precessing jets [Kundt 1996, 1998, 2001, 2005]. The channel-wall material may well stem from the wind of its (or- dinary) stellar companion. Its controversial distance – of 3 kpc – has been recently challenged by Lockman et al [2007], a paper with whose con- clusions I disagree: the 42 km/s association with W50 proposed by Dubner et al [1998] corresponds to a minimum of 21cm-line column depth in both emission and absorption, hence to a hole in HI, plausibly created by the SN. Just recently, Pakull et al [2010] have discovered a 20 times older brother of its kind, ‘nebula’ S 26 in the outskirts of the Sculptor galaxy NGC 7793, which is similarly impressive even if somewhat fainter than SS 433. The authors may have overestimated its mechanical power by assuming that its lobes were filled homogeneously with (optically) radiating matter, rather than with small-filling-factor filamentary inclusions, at about 103times higher densities, by 103times less matter, and therefore with 103times less mechanical power than stated. By taking care of a filling factor f ≈10−3 of the optical source in its lobes, the properties of S 26 conform nicely with those of all the other class-(c) sources. With the probable exception of our Galactic center, neutron stars are the only jet class with central engines which have shown (eight cases of) emission of the 511 keV e-pair annihilation line, redshifted by some 7% [Kaiser & Hannikainen 2002], an indication of significant local pair excess. There is also narrow, and very broad, and partially redshifted emission of the 511 keV line from a vicinity of Felix Mirabel’s ‘great annihilator’ [1998]. Interesting, and still poorly understood are the frequently reported time- periodic motions of X-ray spectra of class-(c)-sources in the intensity-vs- hardness plane, and of their correlations with radio outbursts, and with flaring jet formation, on varying time scales between weeks and years, which take the shape of a repeatedly traversed turkey head, [Körding et al 2006]. They should eventually allow us to get a deeper insight into the quasi- periodic physical processes taking place near the jets’ CE. Similar quasi- periodicities take place in the AGN sources, though on distinctly longer time scales, and detected in different frequency ranges [Körding et al 2008]. (d) Many new discoveries have been made relating to those luminous ‘un- resolved massive objects’ (UMOs) at the centers of massive galaxies often called ‘supermassive black holes’, or simply ‘active galactic nuclei’ (AGN), which may instead be the dense, nuclear-burning centers of their massive disks. Their best known representative is the source Sgr A* at our own Galactic center, whose supposed BH nature has received another eight fierce- ful blows during the past four years. Serious doubts on its BH interpretation were already expressed in [Kundt 1990, 2001b], among others because of its strong wind, mapped in the Brackett α and γ lines, of mass rate −2.5 3 10 M /yr, speed .10 km/s, which is seen to blow off tails from wind- zones of &8 stars within 1 lyr from Sgr A*. These doubts were enhanced when Aharonian et al [2004] detected Sgr A* in emission at TeV energies, because the temperature of an accreting BH (of mass M) is predicted to 1/4 be below 1 keV(M /M) , controlled by the Eddington limit on its lumi- nosity. Then came the news from Frank Eisenhauer, in his 16 Nov. 2007 Bonn colloquium, that the 16 yr-Kepler ellipse of the star S2 (around Sgr A*) did not close, by some 3 degrees, some 102.2 times larger than the general-relativistic periastron precession rate, cf. [Gualandris et al 2010] : Did the non-pointlike gravity of a massive central disk make itself felt? Such a high-density, flat central mass array had already indicated its presence by a monotonicly increasing estimated mass of Sgr A*, between 2003 and 2007, 6.41 6.58 6.63 during increasing approach, from 10 to 10 M , or even to 10 M , as well as by an increasing estimated distance d of Sgr A*, towards 8.33 kpc, in mild conflict with other, independent distance estimates, of d .8.1 kpc; (d allows the conversion of angular velocities into linear transverse velocities of a test star). A fourth, and fifth orbital anomaly of star S2 were reported by Gillessen et al [2009]: When approaching its peri-astron in 2002, S2 flared by half a magnitude, and six successive position measurements were shifted by .10 mas towards NE, reminiscent of an IR fata morgana caused by a 12 −3 discal medium of (high) number density ne ≈ 10 cm . Finally, there is the sub-event-horizon VLBI morphology of Sgr A* at 1.3 cm wavelength, 12.6 with structure down to the scale of 4 RS = 10 cm, reported in Nature of 4 September 2008, plus the concern of Hagai B. Perets and Alessia Gua- landris that star formation between . 0.01 pc and 0.5 pc separation from Sgr A* would have been prevented by tidal forces if the mass of Sgr A* was distributed only of BH extent. All these nine facts are inconsistent with a BH interpretation of Sgr A*. Proceeding towards more distant AGN, there is the recent news by Che- ung et al [2010] that Cen A is a ‘γ-ray galaxy’ rather than a ‘radio galaxy’, its lobes emitting more than ten times its radio power near MeV energies. Being the nearest of its kind, we may speculate that the lobes of most radio galaxies are dominant emitters at VHE energies, often reaching TeV, or even PeV energies. Consistent with this expectation is the highest-resolution message on the nearest cD galaxy M87 by Kovalev et al [2007], further the broadband variability data on the quasar PKS 1510-089 by Marscher et al [2010], and on quasar 3C 454.3 by Jorstad et al [2010], and also the news on γ-ray flaring of the blazar 3C 279, with strong, monotonic variation of its linear optical polarization reported in [Abdo et al 2010]. Note that this last-mentioned, rapidly varying source with its (large) redshift z = 0.536 has a marginally harder spectrum than expected, due to inverse-Compton propagation losses on the cosmic radiation background. And there is the un- forgotten optical quasar 3C 273 of extreme sidedness, of redshift z = 0.158, with a softening spectrum towards its approaching head [Kundt & Krishna 1986]. According to the long list of arguments collected in [Kundt 2009a,b], none of the above UMOs qualifies as a candidate for a BH. If so, the rare, huge, very-soft X-ray flares detected by ROSAT in the centers of ≥seven galaxies – by factors between 102 and 104, reaching transient powers of .1044erg/s, with short rise times, and longer decline times (as t−5/3) between months and years – which have been reported by Stefanie Ko- mossa [2005], cannot be explained as tidal disruptions of stars by a central SBH. They should rather be re-interpreted as rare plunges of stars deflected into strongly inclined orbits, into the dense innermost disk of that galaxy. 3 Constraints on the Astrophysical Jet Sources, and their Satisfaction

In line with common wisdom, let us assume that all the jets are blown by some material, leptonic, hadronic, or of multiple composition, whose motion is supersonic at least in some neighbourhood of the central engine where the jets’ opening angles are narrow (.1%). Moreover, jets have never been seen to split, or branch, or disappear without flaring (like in ‘knots’, or ‘heads’). We then have the following more restrictive further constraints: (1) For the longest known BFs (of length several Mpc), the CE must supply the (abundant!) jet material continually for at least 107yr, equal 10.5 9 to 10 (10 M /MBH ) infall times into a supermassive BH (whose mass 9 enters in units of 10 M )! As one consequence, the CE cannot be a BH. Note that many further reasons against BH CEs have been given in [Kundt 2001, 2002, 2009a,b]. (2) The spectra of BFs range from low radio frequencies all the way up into the hard γ-ray regime, to TeV, or even PeV energies (corresponding to frequencies of .1030Hz). Individual charges must therefore have kinetic energies reaching TeV, or even PeV energies. For electrons, this constraint implies Lorentz factors γ reaching 106, or even 109. If instead, the emission came from relativistic protons, the protons would have to be &1013 times more numerous in the emission region than their accompanying electrons of at least the same energy, because the radiated electromagnetic power scales 2 2 2 −4 as (e γ /m) ∼ m0 with the particles’ charge e, Lorentz factor γ, and (relativistic) mass m = γ m0. This rules against hadronic radiation. (3) The often extreme sidedness of the mapped jets, and the frequent superluminal propagation of radiating knots in the jets argue in favour of 2 extremely relativistic bulk Lorentz factors γbulk (> 10 ) of the jet substance. (4) The narrowness of the beams (.1%), with occasional refocussing, no splitting ever, and only mild bending (for their large power) speak in favour of low inertia, i.e. prefer leptonic jets over hadronic ones. (5) The extremely low synchrotron losses of the beams, on supersonic propagation scales reaching Mpc distances, require E x B-drifting pair plasma: relativistic electrons and positrons coasting at equal speeds. Their only losses are inverse-Compton radiation on the background radiations. Of- ten quoted in situ post-accelerations have been shown to be in conflict with the Second Law of thermodynamics [Kundt 1984]. They would, moreover, ask for unrealistically large magnetic field strengths of their booster, most notably at the high-energy end of the beam-particle spectrum. (6) The high observed jet-formation efficiency, a lobe/core power ratio of 10−22, requires a suitable (universal) functioning of the CE. I like to think of reconnecting magnetic fields of the central rotating magnet, followed by buoyancy in its deep gravitational potential well plus post-acceleration by its outgoing strong low-frequency waves. (7) The comparatively slow propagation speeds of the heads of the flows (for their power), determined by ram pressure balance, argue against 2 1/2 −2.7 1/2 hadronic beam loading: βhead = (2kT/mH c ) = 10 T7 holds for 7 pure pair plasma running into hydrogen, where T7 := T/10 K. From these seven constraints I conclude that every jet engine requires a heavy, rotating magnet at its center which provides the allimportant (highly relativistic) pair plasma, via shearing friction on a surrounding disk, leading to permanent magnetic reconnections. We see this process at work at the surface of our Sun. All sources discussed in section 2 are of this type, whilst BHs cannot possibly do it. An estimate of the Lorentz factors γ of the newly formed pairs can be gleaned from the energy integral ∆W, via γ = 106(∆W/TeV):

∆W = e (E + β × B) · dx ≈ TeV β− B ∆x , (1) Z 3 6 6.5 in which ∆W has been coarsely evaluated in the comoving system with vanishing electric field E, for β = v/c = 10−3, B =106G, and ∆x = 106.5cm. The newly formed pair plasma is practically weightless compared with thermal matter (.10−8), hence escapes by buoyancy, at right angles to the central disk. It thereby blows the broad-line region (BLR), in which it is post-accelerated by the outgoing strong, low-frequency waves of the central magnetized rotator, but at the same time is decelerated by inverse-Compton losses on the ambient photon bath. Part of the dense, warm ambient plasma is pushed aside by the buoyant pair plasma, into the shape of Blandford & Rees’s deLaval nozzle, beyond which it turns supersonic, i.e. escapes faster than at (2/3)c, with frozen-in transverse toroidal magnetic fields, and self- generated radial electric Hall fields, in the form of an almost charge-neutral, highly ordered, monoenergetic beam of E x B-drifting charges, as will be detailed in the next section. Small deviations from charge-neutrality, of order 10−10, are automatically imposed by Maxwell’s equations, through the constraints of equipartition electromagnetic fields and their anchoring charge and current densities, which accompany the flow. Such a highly structured charged pair-plasma flow forms automatically, so I claim, under the prescribed conditions, and satisfies all the preceding seven constraints, including a comoving voltage Φ of strength ≈ 1/2 19.5 1/2 eΦ e(πL/c) = 10 eV L44 (2) for a beam power L in units of 1044erg/s. No stochastic post acceleration in the beam is required to guarantee radiating electrons at the termination hotspot up to PeV energies, where the comoving voltage redistributes the energies of the transrelelativistically streaming leptonic charges from 2 −ε monoenergetic into a broad, almost white power law: E dNE ∼ E with ε & 0.

4 Our Semi-Analytical Model, reviewed

Here comes a short review of our preferred semi-analytical jet model [Kundt & Krishna 2004, Kundt 2005], whose main characteristics have already been sketched in the preceding section. It treats a bipolar flow as a composite of three successive stages: (i) the formation of relativistic jets around a magnetized rotator, (ii) the electromagnetic structure of a leptonic beam, approximated cylindrically, and (iii) the discharging of a jet, on running into an extended, heavy conductor. Whilst stages (i) and (iii) can only be coarsely described, by pointing out analogies to better known, simpler scenarios, stage (ii) allows the presentation of a complete set of solutions whose simplified cylindrical geometry can be easily reshaped into a more realistic conical geometry. (i) In order to blow a relativistic twin jet of power L, one needs a heavy · 2 45.7 −1 generator of N = L / 2 γ mec = 10 s L44/γ4 electron-positron pairs per second, of mean Lorentz factor γ of order 104, which we conceive of as a rotating magnetosphere rubbing (shearing) against the inner edge of a surrounding gaseous accretion disk, but the precise mode of magnetic re- connection should not matter, cf. [Kundt 1996, 2001, 2002]. In the case of our present Sun, pair formation at its surface may rather be due to reconnections of excess magnetic flux dragged out of its convection zone by the · escaping solar wind, but only at a low rate N, insufficient for jet formation. At the same time, we have learned from pulsars that a rotating magnetosphere of angular velocity ω emits strong, low-frequency spherical waves of (large) strength parameter f : 14.2 f := e B / me c ω = 10 B3/ω−4 (3) 3 for a typical coronal field of strength B measured in kG, with B3 := B/10 G, −4 −1 ω−4 := ω/10 s , which post-accelerates charges of either sign to an asymp- Figure 1: Sketch of a plausible Central Engine: Coronal magnetic reconnections create e-pairs, the warm central (star and/or) disk emits high-frequency (HF) photons, low-frequency waves (LFW) post-accelerate the escaping e, and the latter boost the HF photons to HE γ-rays. An ambient thermal bulge serves as the deLaval nozzles from which a twin jet emerges, along the spin axis of the central rotator. In galactic-center sources, this region is observed as the BLR.

2/3 9.5 2/3 totic Lorentz factor γ of order f = 10 (B3/ω−4) , in the absence of damping [Kulsrud et al 1972]. Some such damping is expected to occur via scattering on the ambient photon bath, whose main eﬀect will be to narrow the energy distribution of the escaping charges, as is known from laboratory experiments where atomic beams are routinely cooled via scattering on laser light. On the other hand, only 10% of all AGN have low enough inverse- Compton losses to blow jets, the rest of them is radio quiet, or even radio silent, through exactly such collisional losses on the photon bath of the BLR [Jiang et al 2007]. Taken together, the newly created pair plasma is post accelerated both by buoyancy and by the outgoing strong low-frequency (LF) waves. Both the outgoing LF waves and the more isotropic high-frequency (HF) background radiation tend to narrow the energy distribution of the escaping relativistic electrons (of the radio-loud subpopulation) towards a relativistic Maxwellian, whereupon their subsequent funneling into two antipodal jets – by the heavy thermal deLaval nozzles – has been shown [2004] to further narrow it into a mono-energetic (delta) ﬂow: E x B-drifting rear- ranges the speeds of the charges until they are uniform across the beam. (Remember that the radio spectrum of Sgr A* is consistent with monoenergetic synchrotron radiation, with γ = 104). In this way, their propagation is loss-free, apart from inverse-Compton losses on the background radiations.

Figure 2: Cross section through (the ground mode of) a cylindrical beam segment, showing the radial dependences (∼s/R) of ρ, j, Es, Bϕ, R := beam radius.

(ii) Beyond a deLaval nozzle, the relativistic charges – of bulk Lorentz factor γ . 104 – rearrange their velocities orderly as E x B-drifters, β = E x B / B2, whereby in a cylindical section of a beam, the toroidal B-ﬁelds and radial electric Hall ﬁelds imply unique charge- and current-densities {ρ, j}, smoothly distributed throughout the beam, whose lowest-order Fourier components read:

Es = Bϕ = C sin(πs/R)/s , Bz = const (4)

ρ = jz/c = (πC/R) cos(πs/R)/s in cylindrical coordinates {s, ϕ, z}, with R := cylinder radius; βϕ turns out to be small, of order γ−1, unrelated to observed spiral patterns. Note that such beams have vanishing net charges and currents, and that their charge amplitudes are of order 10−10 of those of the convected charges. Fields and charges are in equipartition, and determined by the power L of a beam and its cross section A via:

2 2 2 2 ne γ me c ≈ L/A c ≈ (E + B )/8π ≈ B /4π (5) for a pair-plasma number density ne. Note that these ﬁeld strengths cor- respond to (huge) convected electric potentials Φ given in (2) above, which guarantee electron energies up into the PeV range in the ultimate power-law distribution of the charges, once they have been stalled by a heavy obstacle. No stochastic (in situ) acceleration is required anywhere in the beams. Note also that the E x B-drifting leptons described by equns. (4) and • by c(γβ) = (e/me) (E + β x B) have their dominating losses through inverse Compton collisions on the 2.73 K background radiation, with a (large) degradation e-folding length ldeg given by

0 2 4 ldeg := γ/γ = 3 me c /4 σT u3K γ = Mpc / γ6 (1 + z) , (6) which puts only marginal restrictions on the longest observed jet sources. (iii) What happens when a pair-plasma beam is stalled by an extended, heavy conductor? Such conductive obstacles form naturally, as (part of the) swept-up and heated circumsource matter. Already before any mechanical contact, the approaching, polarized beam plasma will induce mirror charges, and mirror currents in the obstructing plasma, which mediate the transfer of its impact momentum to the ‘head material’, and more or less reflect the beam particles into the (expanding) ‘lobe’, or ‘cocoon’, which is thereby inflated. This momentum transfer, of course, implies a small energy transfer to the (compressed) head material. At the same time, a large fraction of the comoving electromagnetic energy is transferred to the pair plasma, converting its delta-type energy distribution into a broad power-law distribution, and energizing it such that it squeezes the ambient (thermal) circumsource plasma into high-pressure, small-filling-factor filaments. Such explosive compression is known to be Rayleigh-Taylor unstable during its switch-on phase, like water being blown apart by pressurized air. Only later, during the switch-off phase of rapid relaxation, does the lobe develop a smooth, Rayleigh-Taylor stable outer envelope (of expanded cocoon matter), observed as its bounding, shocked outer layer. Blowing the cocoon consumes one third of the stalled beam-plasma’s energy: The pressure p of a relativistic gas equals one third of its energy density u, p = u/3. Consequently, in order to compress pre-existing plasma near the distal end of the cocoon into a tiny subvolume, the relativistic gas has to perform work given by p times its volume V, which amounts to 1/3 of its original energy uV. This means that the cocoon of a BF is still a powerful container of relativistic pair plasma, storing two thirds of its energy at injection. No wonder that Cen A, one of the nearest radio galaxies, has been recently mapped at MeV energies, glowing with more than ten times its radio power; Teddy Cheung [2010] has called it a ‘gamma-ray galaxy’. And Pakull et al [2010] have concluded that S 26 in the Sculptor galaxy NGC 7793 was a (105.3yr old) microquasar of (excessive mechanical) power L = 1040.7erg/s, from the optical emission of its radio lobes, whereby they omitted a likely filling factor f of order 10−3 of the radiating lobe material, corresponding to an f-times lower radiating mass, and an f-times lower inferred mechanical power f L = 1037.7erg/s of the microquasar. With such a filling factor included, their source smoothly joins the class (c) of BFs from binary neutron stars, as an older brother of SS 433.

Figure 3: Simplified cross section through a beam head, sketching the distributions of relativistic electrons (e), electric and magnetic fields, mirror charges and fields (on the side of the obstructing ambient plasma), and particle orbits. Omitted are the motions of the stalled charges escaping from the impact center, whereby they are post-accelerated by the huge convected potential.

And what about occasional recent reports of TeV sources, or even PeV sources among the members of the jet family? No need for stochastic (in situ) acceleration: When the huge convected potential Φ of section 3 is discharged, at the termination shock of a jet, a comparison with Michel’s relativistic Child’s Law for a space-charge limited discharge, borrowed from the polar-cap discharges of pulsars, yields an expected upper-end (of the power law) Lorentz factor γ∞ of order:

1/2 7.2 1/4 γ∞ ≈ (8γΦ) ≈ 10 (L44) , (7) large enough to explain all the recent reports of TeV sources among the BFs as inverse-Compton radiation from the relativistic pair plasma of their jet engines. The heads of young sources move fast enough to penetrate supersonically into their surroundings, whilst those of older versions tend to fall short of this critical speed, and expand more calmly. As is well known, the morphologies of these two source classes differ strongly. Jean Eilek [2002] has termed them of type A and type B respectively, in slight (though important) revision of the earlier Fanaroff & Riley classification of double radio sources.

5 Summarizing the preferred Model

In short, the key properties of my updated jet model are as follows: No jets, or bipolar flows, without a rotating magnet, whose magnetic reconnections supply the necessary (relativistic) pair plasma, and whose outgoing low-frequency waves provide the necessary post-acceleration of the leptonic charges. In this process, inverse-Compton losses on the ambient photon bath must not be excessive, otherwise we deal with radio-silent QSOs (without jets). Supersonic E x B-drifting jets emerge at two opposite outlets of the (central) BLR, via naturally forming deLaval nozzles, whereby equipartition electromagnetic fields convect half of the escaping power through their self- rammed jet channels. They serve as comoving batteries wherever the beams get tapped by obstacles, most notably at their downstream shocks (heads). At their heads, the jets blow the lobes, which are observed as low-density relativistic balloons with thermal inclusions of small filling factor, and with spectra which extend up in energy to the VHE γ-ray regime.

6 Acknowledgements

My thanks go to Ole Marggraf for both the manuscript and for its electronic data handling.

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DISCUSSION GIORA SHAVIV: The impression from observations of jets is that they show inhomogeneities, at least in brightness. Do your jets show such a phenomenon? Are they homogeneous? WOLFGANG KUNDT: Your comment is well taken. The complete set of solutions for cylindrical jets which I presented was assumed homogeneous in time, but realistic engines are thought to inject their relativistic pair plasma in a non-stationary way – a bit like a snake swallowing a frog. However, such unsteady injections are believed not to cause any beam in- stabilities.