Please Note: the e-mail address(es) and any
external links in this paper were correct when it was written in 1995,
but may no longer be valid.
Go to: Front page, Participants, Group Photograph, Preface, Contributions, Acknowledgements
Structure of Active Galactic Nuclei in a model
with the radial magnetic field
Yuri Y.
Kovalev
e-mail:
yyk@dpc.asc.rssi.ru
Astro Space Center of the Lebedev Physical Institute, Moscow State
University, Moscow, RUSSIA
Sternberg Astronomical Institute, Universitetskiy pr. 13, 119899 Moscow,
RUSSIA
Abstract:
Synchrotron emission of a jet in the strong radial magnetic field of an AGN is
analyzed. Variable radio spectra of some quasars and galaxies had been earlier
explained by this model. A next interesting step is a simulation of a
theoretical structure for such objects in the model and comparison of it with
the VLBI observations. First results for such model maps are presented here. It
is shown that jet maps depend strongly on the observed angle of the jet axis,
opening angle of a cone jet in the picture plane, frequency, as well as on the
time variability of geometrical and physical parameters of a relativistic
plasma flow across the jet entrance. Preliminary qualitative comparing shows
that the obtained model maps may be similar with the typical observed VLBI
structure of AGNs. Often observed multi-component structures can be explained
by the single jet in the model, if the flow of relativistic particles from the
active nucleus varies in time.
Observed radio images of extragalactic sources (see, for example,
Mutel & Phillips (1987),Unwin et al. (1989),Gabuzda et al. (1989),Gabuzda et al. (1992),Bååth et al. (1992)) - such as QSO, BL Lac's and radio
galaxies - stimulated great scientific interest always. Carrying a lot of new
information, radio images are very important for interpretation, but are very
difficult material to be successfully compared with the model images. Only a
few works to this moment are known with more or less successful comparison of
the observed and simulated maps for variable sources.
One of the first studies on this topic is one by Hughes et al. (1989a) and Hughes et al. (1991), where the
interpretations of the variable flux densities of BL Lac (at 6 frequencies from
0.4 to 14.5 GHz), 3C 279 and OT 081 (at 3 frequencies - 4.8, 8.0 and 14.5
GHz) had been conducted using the shock relativistic beaming model
(Hughes et al. (1989b)). In the same work the model images for BL Lac (Hughes et al. (1989a))
and 3C 279 (Hughes et al. (1991)) had been obtained. The model structures of BL Lac and
3C 279 are compared with the results of observations Mutel & Phillips (1987) and
Unwin et al. (1989) respectively.
Calculations of the structure in some particular cases had been done by
Gomez et al. (1993a) for a shock relativistic curve jet model with the synchrotron
emission. On the base of numerical simulations by Gomez et al. (1993a), the maps of
the angular distribution for the total and polarized flux density of the
quasars 3C 273B by Gomez et al. (1993b) and 3C 295 by Lara et al. (1993) was studied.
The common characteristics of the variable spectra of radio emission for
extragalactic objects had been explained earlier by synchrotron emission in the
model of a narrow jet of relativistic particles in a strong radial magnetic
field of a source nucleus (the Hedgehog model, see
Kardashev (1969),Kurilchik (1972),Ozernoy & Ulanovskiy (1974),Kovalev & Mikhailutsa (1980)) In particular, a comparison of this model with
combined spectra of the sources 3C 345, 3C 454.3, 
, 
and 4C
39.25 as well as with 3-years evolution of the instantaneous 1-22 GHz spectra
of BL Lac at 7 frequencies had been carried out by Kovalev (1984). 0.3-15 GHz
spectra during 1.5 years set of bursts in a quasar 0235+16 are explained by the
Hedgehog model in Kovalev & Larionov (1994). A study of `quiet' spectra and spectra of
bursts for extragalactic objects with the strong millimeter emission were
conducted in Nesterov et al. (1994), and the emission of 11 sources were interpreted by
the same model.
Thus, this success in explanation of spectra observations by this model leads
to a wish to study the jet's structure in the same model. The main aim of this
work is a numerical analysis of the model images of the angular distribution of
the radio brightness in dependence on some physical and spatial parameters of a
jet, as well as a qualitative comparison of them with the typical VLBI-images.
The task can be in short formulated as follows (see Kovalev & Mikhailutsa (1980) for more
details). Let us consider a radial magnetic field of an active nucleus and a
narrow jet of relativistic particles. The particles move from the nucleus
because of the longitudinal components of their velocities and emit by the
synchrotron mechanism in quasi vacuum conditions. It is assumed that the
density of the magnetic field energy is much more than the density of the jet
particles energy. The adiabatic invariant and the energy of the emitted
particles are conserved. Other emissions, excluding by the synchrotron
emission, are neglected. It is required to make numerical simulations of the
angular distribution of the observed intensity in typical VLBI-measurements.
I calculate maps of the observed (visible) intensity
on the formulae:
where 
is the emitted intensity (brightness) in the model
(see Kovalev & Mikhailutsa (1980),Kovalev & Larionov (1994) for more details), 
is the synthesized
pattern diagram of a radio interferometer, which is approximated by the
Gaussian function here, 
is the spatial angle of the jet and
is the VLBI-beam.
Calculations have been done by setting the pattern diagram of the VLBI, the
geometrical and physical parameters of the jet to obtain 
and
in the model.
Figure 1: The structure of the fully evaluated stationary jet in the radial
magnetic field: a dependence of visible intensity on the relative frequency
with fixed values of the exponent 
in the energy
distribution of emitted particles, the angle to an observer
, and the cone angle of the jet in the picture plan
. The contours are at 1, 3, 5, 10, 20, 30, 50, 70, and 90 %
of the maximum value.
Figure 2: The structure of the fully evaluated stationary jet emitting in the
radial magnetic field in a particular case, defined by the following
parameters: 
, 
, 
,
, 
milli arcseconds. The relative right ascension
and declination are shown on the horizontal and vertical axes, respectively, to
compare with Figure 1a in Gabuzda et al. (1989). Contours are at 2, 2.8, 4, 5.6,
8, 11, 16, 23, 32, 45, 64, and 90 % of the maximum value.
All maps of the fully evaluated stationary jet in the radial magnetic field -
see Figures 1 and 2 - can be considered as a result of a
superposition of two main components (relatively compact and extent), but their
relative contribution to a total emission depend in a complicated way on the
exponent 
in the energy distribution of emitted relativistic particles,
normalized frequency 
, the angle 
between the jet axis
and the direction to an observer, the cone angle of the jet in the picture plan
and the angle dimension of the jet 
. A weak third component
at the end of the jet appears at some narrow frequency interval
(
). It should be pointed out that a mean
value of 
was earlier estimated as 
by
Kovalev (1994).
The dependence of structure on the frequency 
and on the angle of
observation 
is the following. The changes of the observational
brightness distribution with increasing the angle 
at the fixed
frequency 
are qualitative similar to the changes of one with
decreasing the frequency at the fixed angle 
- the importance of
an extended component is increasing (see Figure 1 for the dependence
on the frequency). Moreover, a weak third component of the images (at the level
of several percent of the maximum) at frequencies of approximately the average
of the flattened part of the flux density spectra appears (see
Figure 1) as a result of increasing the emitted intensity at the end
of the jet. This increase in the emission is explained by increasing the number
of particles, emitted along to the direction of observation because of the
pitch-angles evolution in the model (see Kovalev & Mikhailutsa (1980) for more details).
The maximum of the compact component shifts to the middle from the start of the
jet, and width of it increases with increasing the frequency or the angle of
the observation. This tendency is weak or absent at higher frequencies
(
) and is most observed at lower frequency (
) - see the visible brightness distribution on Figure 1.
It should be emphasized that the quantitative comparison of the model maps with
results of observations is not the aim of the presented work. This is planned
to be done in the future, but here some main results are pointed out
only.
Even the particular case of a fully evaluated stationary emitting jet in the
model is in qualitative agreement with the typical VLBI-images by
Gabuzda et al. (1989) and Gabuzda et al. (1992). Unfortunately, the real diagram patterns of the VLBI
measurement influence the resultant maps, and often distort of them essentially
(it concerns particular to the diagram patterns with the elliptical
cross-sections).
Let us consider in more detail a map of the source 0454+84 (see Figure 1a in
Gabuzda et al. (1989)). A qualitative result of the fitting the structure of the fully
evaluated stationary jet in the model to this image is shown on
Figure 2. The diagram pattern has been used as in Gabuzda et al. (1989). The
values obtained for the model parameters are shown in the caption of
Figure 2. Contours are the same as in Gabuzda et al. (1989) in Figure 1a. One
can see a good agreement between the observed and model maps by visual
comparison.
Figure 3: The structure of the fully evaluated non-stationary jet in the radial
magnetic field for the fixed values of the relative frequency
, the angle to an observer 
, the exponent
in the energy distribution of emitted particles and the angle
dimension of the jet 
milli arcseconds. Values of the angle 
are varied in time. Contours are at 1, 2, 3, 4, 5, 7, 10, 20, 30, 40, 50, 60,
70, 80, and 90 % of the maximum value.
Let us also consider also the image of the fully evolved model jets, if the
flux of the number of emitted particles at the base of the jet is varied in
time. One of the infinite possible cases is presented on Figure 3.
Some important parameters are shown in the caption. The angle between the jet's
axis and the right ascension axis is equal to 
.
The radio image on Figure 3 is a typical image of a multi-component
structure, and has an objective to show to skeptics that a multi-component
source must not always have several jets or clouds. An influence of the
non circle cross-section of the pattern diagram can be also seen on
Figure 3. If the angle dimension 
of the jet would be equal to
or less than the dimension of the main lobe of the pattern diagram, then the
contours of the model map in the form of ellipses for an any jet would be
obtained, in accordance with observations (see, an example, Figure 1 in
Gabuzda et al. (1992)).
It is obtained that the visible structure of the jet in the discussed model
strongly depends on some physical and geometrical parameters of the jet and on
the used synthesized pattern diagram of the VLBI. Calculated maps are in
qualitative agreement with the typical VLBI-images. The multi-component sources
can be explained by this model in a case of a nonstationary flow of the jet. It
is concluded that a narrow jet of relativistic particles emitted by the
synchrotron mechanism in the strong radial magnetic field of an active galactic
nucleus can be a real model for variable extragalactic radio sources.
I wish to thank the Cavendish Laboratory and the Russian Foundation for
Fundamental Researches for the support of my travel to YERAC'94, as well as
Priya Shah for the help in preparation of this paper.
- Bååth L.B., Rogers A.E.E., Inoue M. et al., 1992,
A&A, 257, 31.
- Gabuzda D.C., Cawthorne T.V., Roberts D.H. & Wardle J.F.C., 1989,
ApJ, 347, 701.
- Gabuzda D.C., Cawthorne T.V., Roberts D.H. & Wardle J.F.C., 1992,
ApJ, 388, 40.
- Gomez J.L., Alberdi A. & Marcaide J.M., 1993a,
A&A, 274, 55.
- Gomez J.L., Alberdi A. & Marcaide J.M., 1993b,
in Astrophysical Jets, STScI Symposium Series,
eds Burgarella D., Livio M. & O'Dea C., p.6, (Cambridge University Press).
- Hughes P.A., Aller H.D. & Aller M.F., 1989a,
ApJ, 341, 68.
- Hughes P.A., Aller H.D. & Aller M.F., 1989b,
ApJ, 341, 54.
- Hughes P.A., Aller H.D. & Aller M.F., 1991,
ApJ, 374, 57.
- Kardashev N.S., 1969,
epilogue to Russian edition of Quasars,
Burbidge G.R. & Burbidge E.M., (Freeman, 1967), (Mir, Moscow).
- Kovalev, Yu.A., 1984,
preprint.
- Kovalev Y.Y., 1994,
SovA, in press.
- Kovalev Y.Y. & Larionov G.M., 1994,
ALet, 20, 3.
- Kovalev, Yu.A., Mikhailutsa V.P., 1980,
SovA, 24, 400.
- Kurilchik V.N., 1972,
ALet, 10, 115.
- Lara L., Alberdi A., Gomez J.L. et al., 1993,
in Sub-arcsecond Radio Astronomy, eds Davis R.J. & Booth R.S.,
p.241, (Cambridge University Press).
- Mutel R.L. & Phillips R.B., 1987,
in Superluminal Radio Sources, eds Zensus J.A. & Pearson T.J.,
p.60, (Cambridge University Press).
- Nesterov N.S., Kovalev Y.Y., Babak S.V. & Larionov G.M., 1994,
SovA, in press.
- Ozernoy L.M. & Ulanovskiy L.E., 1974,
AZh, 51, 8.
- Unwin S.C., Cohen M.H., Biretta J.A. et al., 1989,
ApJ, 340, 117.
YERAC 94 Account
Wed Feb 22 19:33:06 GMT 1995
