It is proposed that the important connection between 3C 273 and 3C 279, the first two extragalactic sources detected at greater than 100 MeV energies, is their superluminal nature. In support of this conjecture, we propose a radiation mechanism that focuses gamma rays in the superluminal direction, due to Compton scattering of accretion-disk photons by relativistic nonthermal electrons in the jet

Dermer, Charles D.

Mastichiadis, Apostolos

Schlickeiser, Reinhard

English

NASA Technical Reports Server

N92-21 917, 
GAMMA RAYS FROM EXTRAGALACTIC RADIO SOURCES
Charles D. Dermer
Department of Space Physics and Astronomy, Rice University, Houston, TX 77251, USA
Reinhard Schlickeiser
Max-Planck-Institut fiir Radioastronomie, Auf dem Hfigel 69, D-5300 Bonn 1, Germany
Apostolos Mastichiadis
Max-Planck-Institut ffir Kernphysik, Postfach 10 39 80, Heidelberg, Germany
ABSTRACT
We propose that the important connection between 3C 273 and 3C 279, the first two extra-
galactic sources detected at > 100 MeV energies, is their superluminal nature. In support
of this conjecture, we propose a radiation mechanism that focuses gamma rays in the su-
perluminal direction, due to Compton scattering of accretion-disk photons by relativistic
nonthermal electrons in the jet.
INTRODUCTION
A black-hole accretion disk with a collimated perpendicular jet currently represents
the baseline model for explaining the origin of the luminous, broadband radiation emitted
from the central engines of active galactic nuclei (AGN). In the unified scenario _'2, line-of-
sight determines family character: thus, Seyfert 2 galaxies are accretion disks seen through
obscuring equatorial clouds; Sy ls are the disks seen directly; and superluminal (SL) radio
sources are seen when looking at small angles to the jet (polar) axis. > 100 MeV 7 rays
have now been detected from two extragalactic radio sources 3C 273 a and 3C 2794. both
of which display strong SL behavior. We show that strong enhancements of gamma-ray
emission near the SL direction occur when relativistic electrons in the outflowing radio-
emitting blobs Comptonize UV and X-ray photons emitted by the accretion disk. An
important consequence of this model is that BL Lac Objects should be rather feeble sources
of high-energy radiation, because here we are looking nearly straight down the jet axis.
Correlated radio and 7-ray observations will provide a strong test of this model and the
AGN unification scenario.
328
https://ntrs.nasa.gov/search.jsp?R=19920012674 2020-03-17T13:15:42+00:00Z
MODEL
Radio observations of AGN are reviewed in Refs. 5 and 6, and theory in Ref. 7.
In the relativistic jet model of Blandford & Rees s, blobs of magnetized plasma with bulk
velocity fire and I,orentz factor F = (1 - fl_.)-1/2 erupt from the central nucleus at nearly
right angles to the plane of the accretion disk. Relativistic electrons with a quasi-isotropic
distribution in the rest frame of the blob ( BF) produce incoherent synchrotron radiation
which, because of the blob's motion, is focused into a cone with half-angle _ F -1 about the
jet axis in the observer's frame (OF). The maximum apparent transverse velocity 13"a*e =
_app
/_rFc is observed when the angle 0: between the blob's velocity vector and the direction to
the observer is given by 0_ = cos -1 #: = cos -_/3r (see Fig. 1). The possible existence of
sources showing apparent transverse velocities exceeding c was predicted by Rees 9. The first-
discovered and brightest known SLs, 3C 279 and 3C 273, respectively, each have exhibited
SL components with/_avp > 8 (Ref. 10).
In the standard model for jets from AGN, sketched in Figure 1, a plasma blob moves out-
ward from the central nucleus with speed _rc. We depart from this modeP _ only by assuming
that the relativistic radio-emitting electrons are homogeneously distributed throughout the
blob, and that the randomly-oriented magnetic field has uniform strength everywhere in
the blob. This simplification does not affect the central result, and can be relaxed in more
detailed treatments. We also assume that (1) the energetic electrons have an isotropic en-
ergy distribution in tile BF; (2) the accretion-disk source emits photons isotropically with
spectrum Nph(C) [photons s -_ (.-1] in the OF, where (" = hu'/rn_c _ is the dimensionless
photon energy (asterisks refer to quantities measured in the OF); and (3) photons passing
through the blob follow trajectories parallel to the jet axis. This last is true if the blob is
sumciently far from the central source. We also require that the blob be optically thin
to Thomson scattering along the jet axis, which is necessary to produce highly polarized
radio emission s .
Neglecting redshift corrections, the flux from the central source seen by the observer is
given by
Nph(_') (1)(I)(_') _ 4rd2 ,
where d is the distance between the observer and the AGN. In the BF, photons have energy
= F(1 - flr)C < C, due to the Doppler effect. From the invariance 12 of nph(_, f_)/_2, we
find that the differential photon density in the BF is given by
d 2
nph(e, f_) - 2rrr'c ¢[(1 +/3r)Fe] 3(# - 1), (2)
where r is the distance of the blob from the central source and # is the dircction cosine
329
Active Galactic Npcleus
Source of Quasi-lsotropic
UV and X-Rays
0
E* ....... t1_
Redlo-Emlttlno Blob Observer
Lorentz Factor F
Velocity 13r c
Fig. 1-Cartoon illustrating the proposed model for high-energy radiation from extra-
galactic radio sources. A plasma blob moving with speed 3pc focuses its radio emission
into a forward cone in the observer's frame. Relativistic electrons in the blob also scatter
radiation from the central source. The most energetic of these jai alai photons (see final
paragraph) are scattered preferentially in the direction cos 0j = 2 -/3r 1 _ 3r.
of the photons in the BF. We describe the electron distribution in the BF by the function
n,(3', fl) [electrons cm -a 3`-' sr-1], where 3' is the electron Lorentz factor. For the isotropic
case, n_(7, f_) = ne(3`)/4_r. The angle-dependent scattered photon spectrum in the BF is
given by la'14
fiph(e,, 9t,)=C fo °°de Jdf_ f°°d"/ fdf_e (1 -//cos_,k) nph(e,f_) n_(3`,f_) dGdf_,'d2cr (3)
where the subscript s refers to scattered quantities, cos _ --_ #_ when # = 1, and d2cr/de,dfl,
is the differential scattering cross section.
The electrons that produce the nonthermal radio emission are highly relativistic, so that
3` >> 1. If the accretion-disk photons are in the UV to X-ray range, e* _ 10 -4 - 10 -_, and
thus e << 1. We treat the Thomson limit of Compton scattering, where 3`e(1 - fife) << 1.
The average energy e, of scattered photons in the BF is _ 3'2e(1 -/3#e), and these photons
are beamed into a cone with half-angle angle O, _ 3"-1 << 1 about the original direction of
the electron's motion. We therefore approximate the differential Compton cross section by
_~ _ - - (4)
de,df_, 27r
330
Substituting equations(4) and (2) into equation (3), and transforming back to the OF
usingthe invariance12of hph(e,, #0)/e,, we find that
ar d_ f_ (1 + flr)r_e:(1 -/3rU:) _
_ph(_:,p:):2r(l_/3rt,;)r_ d_n-_n_(3')¢{_[_-_--_-;;_-/_T_] }, (5)
giving the photon emissivity per unit emission time. To determine the observed flux (I); _
per unit reception time 15, note that the two time intervals are related through t,_ = t_m(1 -
/3r#:). For a blob with volume Vb* = V_/F, (I):_(e:,/t:) = Vbhph(_*,,#*,)/2rd_F(1 -/3r#:).
Thus the scattered photon flux, in terms of the flux observed directly from the central
source, is given by
c_ 2* *2
/, (_1(I):"_(c:'#:) = 4_rr 2 F_ --/3r#;) 2 d3' n_(3') t3'2[1 +/3/3r - #;(/3 +/3r)l }" (6)
We now consider the _-function properties of equation (6). Let n_('7) = n06(3' - 9) and
(I)(e') = (I)o3(e* - _*). The energy of a scattered photon emitted into a given direction is
related to the original photon energy _* through the expression
eZ = 3'211 4-/3/3r - #:(/3 4-/3r)] (7)
g" (1 4-/3r)r2(1 -/3r#;) e
Equation (7) can also be obtained by making a series of transformations of a photon's energy
and angle from the OF to the BF and then to the electron rest frame, and then retracing
the steps after scattering. Figure 2 shows values of the ratio (7) for a blob moving with
F = 5, and for isotropically-distributed electrons with 3, = 100 in the BF. As can be seen,
the scattered photon energy is increased relative to the original photon energy at all angles
except in the extreme forward direction, where/,_ _ 1 and e:/_* _ 1/2. The weak forward
scattering is a consequence of the factor (1 -/3_u_), which reduces both the scattering rate
(eq. 3) and the photon energy (eq. 4) in the electron's rest frame. The energy increase is
greatest at shallow angles with respect to the forward direction. The angle #:.,_ of the
peak scattered energy is determined by letting O(e*,/g*)/cOp_ = 0, giving
/3_+ 2/3_ -/3 (s)m
_o- &(/_ + &)
If 3' >> F, then /3 _ 1 and #:,,,_ ---* 2 -/31: _. If the b!ob is also moving at relativistic
speeds such that F >> 1, which indeed is required for SL effects, then/3r _ 1 -(2F:) -1, and
#*,,m,, "_ fir. This is equal to the observer's direction at which the apparent SL velocity is
greatest. The maximum value of the scattered photon energy is given, from equations (7)
331
' ' I ' ' I ' ' I ' ' 1 ' ' 1 ' '
1 0 4 " '
/
%/f
1 0 2 //
J
- f .
r i .i -3
0 o - .,- I i Scattered -
1 :'- .- .--t Photon Energy -'
I
f
10 2 :_,, , I , 1 I I , I I A I , ; I , ,
180 150 120 90 60 30 0
t
0
s
Fig. 2-Angle-dependences of the ratio of the photon energy before and after scattering
as measured by an observer (solid curve), and of the relative amount of energy in scattered
photons per unit reception time (dashed curve). In this calculation, the blob's Lorentz
factor F = 5 and the electrons' Lorentz factor 7 = 100. Tile most energetic photons are
scattered in the direction 0_ _ cos-'(2 - fr z) _ 11.7 °, and the largest fraction of photon
energy is scattered in the direction 0: _ cos-1[(3flr - 1)/2fr] _ 8.2 °. The maximum
apparent superluminal velocity is observed at the angle cos -1 _r _ 11-6 °. If F = 10, the
most energetic photons are scattered at the superluminal angle 5.7 °, with the bulk of the
energy scattered in the direction 0: _ 4.1%
and (8), by (c:/_'),_,, = 3'2(8 + fv)2/4ffr(1 + fir) _ _/2, where the limiting expression
holds when 3' >> F >> 1.
We can also determine the directional dependence of the total energy in scattered pho-
tons by evaluating the quantity fo dc:e:¢_'_(e:,#:) • The principal angular dependence of
this quantity is cx [1 + fflv - #:(fl + flr)]2/(1 - fir#:) 6, and is also plotted in Fig. 2. In
the limit 3' >> F, the greatest amount of energy in scattered photons per interval of #: is
produced in the direction It: -_ (3/3r - 1)/2fir. This is close to the direction into which the
highest energy photons are scattered.
Recall that the maximum SL velocity occurs when/t: = fr. Our results show that SL
radio sources should scatter the most energetic photons in this direction, with the largest
fraction of scattered photon energy scattered at a slightly shallower angle. However, the
amount and energy of photon emission scattered directly forward is weak. Thus SL radio
sources should be strong 3'-ray emitters, but if observations are made directly down the
symmetry axis of the jet, we should see very little X-ray and ")'-ray emission. According to
332
the unified AGN scenario, BL Lac objects are sources where we happen to be looking very
nearly directly down the jet axis. Thus we conclude that SL radio sources should be strong
"7-ray sources, and BL Lac objects should be weak 7-ray sources.
Attempts to observe 7-ray emission from BL Lac objects have so far been unsuccess-
ful 16, and the X-ray emission that has been observed is soft, with energy spectral index
_1.6 in the 1-3 keV range It. The X-rays are probably the result of Comptonization of
the synchrotron radiation emitted in the outflowing blob TM. According to our model, the
intensity of gamma-ray emission should be positively correlated with the appearance and
apparent transverse velocity of SL components. By contrast, BL Lac objects that display
no SL character should not be "y-ray sources. We strongly encourage correlated VLBI radio
observations and GRO EGRET observations to test these predictions. Superluminal sources
such as 3C 120 or 3C 3455'1° are obvious candidate sources of extragalactic gamma radiation.
Comparatively weak X-ray and 7-ray emission is expected from subluminal BL Lac objects
such as 1803+7819'5 .
SUMMARY
We have proposed a model in which the relativistic electrons in an outflowing blob
Comptonize photons from a central source. This process has much in common with the game
jai alai, where the combined motions of the player and his basket shoot the ball forward with
a much greater energy than would be possible were either the player stationary or his basket
immobile. This model avoids 7-3' pair attenuation of > 100 MeV photons 2° from a compact
highly variable source such as 3C 273 by upscattering UV and X-ray photons far from the
compact nucleus. Correlated multiwavelength variability could be associated with either
the central photon source or the outflowing blob: each implies different relationships. The
speed of the blobs may be explained by a Compton-rocket effect 21 produced by the Compton-
scattering impulse. Such an interpretation avoids the difficulties in Rees' suggestion 2= that
Compton drag of a parallel ultrarelativistic jet by disk photons regulates the jet's speed (for
example, such jets should be highly unstable2a). As discussed earlier, our model is in accord
with the AGN unification scenario, and it could be argued that evidence in favour of it
strengthens the unifying scheme. Spectral implications, which imply additional predictions
that can be used to test this model, will be reported elsewhere.
Acknowledgements. C. D. acknowledges the kind hospitality of the Max Plan& Institutes in
Bonn and Heidelberg, discussions with Steven J. Sturner, and the support of the Gamma-
Ray Observatory Guest Investigator Program through NASA grant NAG5-1547.
333
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Gamma rays from extragalactic radio sources

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