Free Access
Volume 528, April 2011
Article Number L2
Number of page(s) 4
Section Letters
Published online 16 February 2011

© ESO, 2011

1. Introduction

The study of the possible high-energy radiation from QSOs and MQs is uncertain in one central aspect, the particle composition of the jets. Is the radiation emitted produced mainly via inverse Compton (self-synchrotron and/or with external fields) or via proton-proton/proton-photon interactions leading to subsequent meson decay? In the latter case, hadronic jet models have been used to assess this possibility. Most of these models represent beams of particles (e.g. protons) linearly propagating in a direction normal to an accretion disk, interacting with the matter in the stellar wind of the companion star in the case of MQs or with matter in clouds in the case of quasars. The protons in the beams are usually assumed to be distributed with a power law that may be cut at high energies, as is the case when they were accelerated in Fermi processes within the jet and subject to losses. It is then crucial to have a correct description of the proton distribution as seen by the observer, since the emissivity of γ-rays and neutrinos produced by charged and neutral pions make use of the cross sections known in that frame. In this paper we present the derivation of a beam particle distribution as seen by the observer and compare it with the expression used earlier, finding significant differences. Consequences are discussed.

2. Derivation of the particle distribution

In what follows, primed quantities refer to quantities that reside in the jet frame, whereas unprimed quantities are used for those in the observer frame. We consider the Lorentz transformations for energies and momenta (1)where p ∥  = pcos(θ) and p ⊥  = psin(θ) are the parallel and perpendicular momentum, respectively. For the relativistic case, mc ≪ p and mc2 ≪ E, the transformation of the energy simplifies to E = DE′, with D = 1/ [ Γ(1 − βcos(θ)) ] .

The elementary invariants under Lorentz transformations are (see, e.g., Dermer & Menon 2010) the invariant four-volume, dxdt = dVdt; the invariant phase-space element, dp/E = p2dpdΩ/E → ϵdϵdΩ, with the final expression applying to photons and extremely relativistic particles; and the invariant phase volume, dν = dpdx, where the boldface stands for three-dimensional space magnitudes. Because the number N of particles or photons is invariant, one finds that (2)is also invariant. For photons and extremely relativistic particles, the last invariant becomes (3)Using the equality E2 = c2p2 + m2c4 allows writing p2dp = (pE/c2)dE. Using the latter in Eq. (2) implies that the expression (4)is invariant under Lorentz transformations. Again, for photons and extremely relativistic particles, pc = E, ( = ϵ, in the notation above for this case), and the last invariant becomes expression (3). We define the differential number density of particles as n(E,Ω) ≡ dN/ [ dVdEdΩ ] , i.e., the differential number of photons or relativistic particles with dimensionless energy between E and E + dE that are directed into differential solid angle interval dΩ in the direction Ω of some physical volume dV. Because of invariant (4) and the definition of n(E,Ω), we can write (5)which is valid in general, and (6)which is valid for photons and extremely relativistic particles. Equation (5) just says that the expression for the distribution function f(r,p) appearing in kinetic theory of gases, such that the product f(r,p)dpdV is the number of particles lying in a given volume element dV and having momenta in definite intervals dp, is Lorentz-invariant (see e.g., Eq. (10.5) of Landau & Lifshitz 1987). We note that (7)and the invariance of f(r,p) is the one expressed by Eq. (4).

2.1. The extremely relativistic case

In this case, the particle density in the jet frame transforms into the observer frame, if it is a power law defined as (8)by virtue of the previous formulae ((6) and (8)), as (9)

thumbnail Fig. 1

A power-law spectrum of protons in a beam as seen by the observer for different values of viewing angles, and an example of Lorentz factor and slope (solid lines). The gray dashed lines stand for the earlier derivation results for the same parameters (see Sect. 2.4).

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2.2. The general case

Using Eqs. (1) and the one for E2, it is possible to find that (10)In addition, using the Lorentz transformation for momenta (Eq. (1) and that ), one has (11)Using the latter expression to get p/p′ and inserting it into Eq. (5), together with Eqs. (8) and (10) we get, (12)In the limit when mc2 ≪ E, the last expression reduces to (A/4π)E − αD2 + α. To prove this we use which can be established by squaring both sides, and then by summing and subtracting Γ2sin2(θ) + Γ2β2cos2(θ) on the lefthand side, with additional use of the definition of Γ2 = 1/ [ 1 − β2 ] . Also we do not assume a specific jet composition, so it is a general result for any kind of particle distributions. Figure 1 shows the resulting expression (12) for a power-law spectrum of protons in a beam as seen by the observer, for different values of viewing angles, and an example of Lorentz factor and slope (solid lines). For high energies, when E > Γmc2, n(E,Ω) is a power-law distribution, as the original primed one, albeit it presents a different behavior for lower values of E.

2.3. A power law with an exponential cutoff

We also briefly consider the case in which the intrinsic particle distribution in the beam is a power law with an exponential cutoff. This would be a natural consequence of particles being subject to losses on the same site as where they are accelerated. As such, the cutoff appears in the primed referenced frame, where (13)Here, Ecut should be fixed in the beam frame, where acceleration and losses are supposed to occur, but once fixed it is not subject to Lorentz transformations. However, the energy variable in the exponential function is. The result is (14)When mc2 ≪ E, the exponential cutoff exp( − E′/Ecut) is transformed into exp( − E/DEcut), with D the bulk Doppler factor.

2.4. Comparison with Purmohammad & Samimi (2001)

thumbnail Fig. 2

Comparison of the derived expression for the spectrum of protons in beams, following Eq. (22). Left: a function of the viewing angle (θ) for Γ = 10. Right: a function of the bulk Lorentz factor (Γ), for θ = 20°.

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Purmohammad & Samimi (2001, hereafter PS01) state that the isotropic particle distribution transformed from the jet into the observer frame results in (in this section and to avoid confusion we label their expressions with   ): (15)Purmohammad &Samimi’s (2001) paper does not provide a derivation of their Eq. (3), quoted above as (15), but earlier private communication with one of us was sufficient to reconstruct it, as follows. They start by assuming conservation of particles in the form (16)thus one has (17)From the Lorentz-invariance of the phase-space element dp/E, the invariance of pdΩdE can also be proved. Then, one can write (18)Using the expressions derived above for p/p′ and the equality dV′ = ΓdV into (17), one gets (19)From here, after replacing in the numerator , expression (15) follows immediately.

Formally, one can say that there is a problem hidden in Eq. (16), from which the rest of the derivation follows. The invariant phase volume is not dVdΩdE but rather p2dVdΩdp. To avoid inconsistencies, invariants (from which to derive transformation laws) should be written as operations over individually invariant factors, e.g., dN over dν, rather than from identities (which is what one obtains by replacing the definition of n and n′ in Eq. (16)). However, the latter can also yield the correct result, too, if care is exercised. In practice, then, the problem with the prior derivation is in the use of the equality dV′ = ΓdV. This equation implies that dV′ is the proper volume dV0, which is clearly incorrect, given that the momentum, p, of the particles in that volume is not zero. Only for the proper volume is the referred equality true (see, e.g., Eq. (4.6) of Landau & Lifshitz 1987). To clarify this issue, let us introduce, in addition of the two reference systems (observer and beam), another frame K0 in which the particles with the given momentum are at rest. The proper volume dV0 of the element occupied by the particles is defined relative to this system. The velocities of the primed and unprimed systems relative to the system K0 coincide by definition with the velocities v and v′, which these particles have in the systems of the observer and the beam, respectively. Thus, we can write (20)from which one can write (21)If we use Eqs. (18) and (21) in Eq. (16), the last transforms back into the expression of the invariant quantity 1/(pE) × dN/(dVdEdΩ) and Eq. (5) follows.

The difference between expression (15) and our Eq. (12) is important. Dividing one by another, we obtain (22)which is a non-trivially dependent function of Γ, E, and θ. However, it is not a function of α. In the extremely relativistic case mc2 ≪ E, one notices a factor D/Γ of difference. Put otherwise, in the stated asymptotic regime, the ratio expressed by Eq. (22) reduces to , which is only a function of Γ and θ. One can see that, in the case of θ = 0°, n(E,Ω)/n  (E,Ω) → 2 for growing Γ. For other values of θ, this 2D function quickly falls off. We plot the comparison between our derivation and the one by PS01 in Fig. 2, both as a function of viewing angle θ (for fixed Γ = 10) and bulk Lorentz factor Γ (for θ = 20°). We see that the differences range from a factor of  ~2 in the case of directly-pointing beams of blazars and microblazars (of very low θ, in our example, 0.6°), to one order of magnitude at least for the more common cases of moderate inclinations (e.g., for 20°; with the difference being significantly larger for larger inclinations). As soon as θ is larger than  ~5°, the proton spectrum in the beam increasingly undershoots PS01’s calculation. This leads to an analogous correction of previously calculated γ-ray and neutrino emission (typically by more than one order of magnitude) in all cases in which expression (15) was used. In the low energy decline, PS01’s calculation always overestimates (by about two orders of magnitude) the correct yield of the proton spectrum (see Fig. 1 at proton energies between 1 and 10 GeV).

3. Discussion

The proton-beam model presented by PS01 gives an expression for a power-law proton spectrum in a beam, as seen by the observer. We have demonstrated that this expression is incorrect, since it does not comply with relativistic invariants. We derived the correct transformation from the jet frame particle density to the observer’s, which strictly obeys these Lorentz invariance constraints. By comparing our result with PS01, we noticed that differences depend strongly on the viewing angle. For many cases, PS01 calculation entails a significant overprediction (order of magnitude) of the proton spectra for E > Γmc2, whereas it always significantly overpredicts (two orders of magnitude) the proton spectrum at lower energies, for all viewing angles. The γ-ray luminosity is related to the particle density through the corresponding emissivity of the processes by which the particles interact. For instance, the charged and neutral pion decay channels are the most effective yield of photons and neutrinos in many situations, and its emissivity is directly proportional to the particle density n(E,Ω). Many publications that rely on the calculation by PS01 for estimating γ-ray and neutrino emission yields in hadronic beam scenarios are thus affected in this way.

One particular example can be seen in the hadronic MQ case (Romero et al. 2003)1 and in their applications (e.g., Romero et al. 2005; Orellana et al. 2007; Reynoso et al. 2008). These are all affected by our results. MQs can have Γ ~ 2, but θ-values above 10° are typical. MQs can precess, as for SS433, where θ can even change up to 20° around an already large mean viewing angle.

Only a small part of the γ-ray AGNs are not blazars. The majority among those associated with radio sources possess relatively large core dominance (Abdo et al. 2010), indicating that their viewing angle is probably not large. Assuming θ < 20°, if Γ < 10, PS01 deviation is no more than a factor of  ~6 (at E > mc2). For the average blazar with Γ = 10 and θ < 10° the difference is a factor of  ~2. However, it has been argued that during the extreme TeV flaring events of some blazars (e.g. Mkn 501, PKS 2155-304), Lorentz factors greater than Γ ~ 50 are required (e.g. Begelman et al. 2008). If the viewing angle remains at θ ~ 1/Γquiet ~ 1/10 ~ 5.7° during such events, PS01 formulation error can grow to more than an order of magnitude. Furthermore, a few radio galaxies with large viewing angles are also detected at γ-ray energies: M87 with viewing angle θ <  ~ 40°, and Cen A where estimates for the viewing angle range from 15° to 80°. These radio galaxies have been already modeled with the PS01 derivation (e.g., Reynoso et al. 2010). Here, the difference between the PS01 formula and ours can reach a factor 20 for Γ not larger than 5.

Finally, we note that models designed to predict future detections of large-viewing angle sources with higher sensitivity instruments than currently available will strongly overpredict the number of sources if the PS01 formula is used. Similarly, a model for an AGN that overestimates the γ-ray flux could make wrong associations of unidentified γ-ray sources. Given that γ-ray fluxes will be diminished in many cases, and that the time of observations in Cherenkov facilities grows with the square of the flux-reduction factor, the likelihood of observing hadronic beams is undermined.


Note that the Eq. (2) in the latter paper has a missing square in the denominator factor . This error has been adopted in several of the papers quoting Romero et al. (2003).


D.F.T. acknowledges support from grants AYA2009-07391, SGR2009-811, and TW2010005. A.R. acknowledges support by the Marie Curie IRG grant 248037 within the FP7 Program. We acknowledge O. Reimer for discussions.


All Figures

thumbnail Fig. 1

A power-law spectrum of protons in a beam as seen by the observer for different values of viewing angles, and an example of Lorentz factor and slope (solid lines). The gray dashed lines stand for the earlier derivation results for the same parameters (see Sect. 2.4).

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In the text
thumbnail Fig. 2

Comparison of the derived expression for the spectrum of protons in beams, following Eq. (22). Left: a function of the viewing angle (θ) for Γ = 10. Right: a function of the bulk Lorentz factor (Γ), for θ = 20°.

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In the text

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