A&A 431, 831-846 (2005)
DOI: 10.1051/0004-6361:20041831
A. P. Lobanov1 - J. Roland2
1 - Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, Bonn 53121, Germany
2 -
Institut d'Astrophysique, 98 bis bd. Arago,
75014 Paris, France
Received 11 August 2004 / Accepted 16 October 2004
Abstract
Radio loud active galactic nuclei present a remarkable
variety of signs indicating the presence of periodical processes
possibly originating in binary systems of supermassive black holes,
in which orbital motion and precession are ultimately responsible
for the observed broad-band emission variations, as well as for the
morphological and kinematic properties of the radio emission on
parsec scales. This scenario, applied to the quasar 3C 345, explains the observed variations of radio and optical
emission from the quasar, and reproduces the structural variations
observed in the parsec-scale jet of this object. The binary system
in 3C 345 is described by two equal-mass black holes with masses of
7.1
separated by
0.33 pc and orbiting with a period
480 yr. The orbital motion induces a precession of the accretion disk around the primary black hole, with a period of
2570 yr. The jet plasma is
described by a magnetized, relativistic electron-positron beam
propagating inside a wider and slower electron-proton jet. The
combination of Alfvén wave perturbations of the beam, the orbital
motion of the binary system and the precession of the accretion disk
reproduces the variability of the optical flux and evolution of the
radio structure in 3C 345. The timescale of quasi-periodic flaring
activity in 3C 345 is consistent with typical disk instability
timescales. The present model cannot rule out a small-mass orbiter
crossing the accretion disk and causing quasi-periodic flares.
Key words: galaxies: individual: 3C 345 - galaxies: nuclei - galaxies: jets - radio continuum: galaxies
Most of the radio loud active galactic nuclei (AGN) exhibit emission
and structural variability over the entire
electromagnetic spectrum, on timescales ranging from several hours to
several years and on linear scales of from several astronomical units to
several hundreds of parsecs. In AGN with prominent relativistic jets,
this variability appears to be related to the observed morphology and
kinematics of the jet plasma on parsec scales (e.g., Zensus et al.
2002; Jorstad et al. 2001). In a number of radio-loud
AGN, enhanced emission regions (components) embedded in the jet
move at superluminal speeds along helical trajectories (Zensus et al. 1995, 1996; Zensus 1997), and the ejection of such components is often associated with optical and/or
-ray outbursts.
In a growing number of cases, explanation of the observed nuclear variability and structural changes in parsec-scale jets is linked to the presence of supermassive binary black hole (BBH) systems in the centers of radio loud AGN. The BBH model was originally formulated by Begelman et al. (1980), and it was studied subsequently in a number of works (e.g., Polnarev & Rees 1994; Makino & Ebisuzaki 1996; Makino 1997; Ivanov et al. 1999). The BBH model postulates that an accretion disk (AD) exists around at least one of the two black holes (typically, around the more massive one), and the resulting variability and structural changes are determined by the dynamic properties of the disk itself and the BBH-AD system (this includes the disk and black hole precession, orbital motion, passages of the secondary component through the AD).
The most celebrated examples of successful application of the BBH model include OJ 287 (Sillanpää et al. 1988; Valtaoja et al. 2000), 3C 273 and M 87 (Kaastra & Roos 1992), 1928+738 (Roos et al. 1993), Mrk 501 (Rieger & Mannheim 2000) and PKS 0420-014 (Britzen et al. 2001). The epochs and structure of major outbursts in OJ 287 have been suggested to result from passages of the secondary black hole of a binary pair in OJ 287 through the accretion disk around the primary during the orbit (Lehto & Valtonen 1996). The component ejection epochs are correlated with the optical outbursts, and the component trajectories show evidence for a helical morphology of the jet in OJ 287 (Vicente et al. 1996). Several jet components observed in 3C 345 also move along distinct helical paths (Steffen et al. 1995; Lobanov 1996, hereafter L96). Lobanov & Zensus (1999, hereafter LZ99) have shown that shocks are not likely to play a significant role in the dynamics and emission of parsec-scale regions in 3C 345. Recently, the presence of binary black hole systems (albeit with extremely short precession periods and small orbital separations) has been suggested for 3C 279 (Abraham & Carrara 1998), 3C 273 (Abraham & Romero 1999) and 3C 345 (Caproni & Abraham 2004), from the observed periodic variations of the component ejection angle.
The observed helical trajectories of jet components are a strong
indication of precession or perturbation of the jet flow. The
dynamics and emission of such perturbed outflows have been explained
in the framework of the two-fluid model (Sol et al. 1989; Hanasz & Sol 1996) describing the structure and emission of the jet in terms of an ultra-relativistic
electron-positron (
)
beam with
surrounded by a slower, electron-proton (
)
jet moving at a
speed
.
Observational evidence for the two-fluid model is reviewed in Roland
& Hetem (1996). The presence of two fluids in extragalactic
outflows was first inferred in the large-scale jet of Cygnus A
(Roland et al. 1988). The two-fluid model is
supported by -ray observations of MeV sources (Roland &
Hermsen 1995; Skibo et al. 1997) and
polarized structures in the compact radio jet of 1055+018
(Attridge et al. 1999). The X-ray and
-ray observed in Cen A may be formed by the
beams (Marcowith et al. 1995, 1998). Direct detections of fast and slow jet speeds
have been reported for Cen A (Tingay et al. 1998)
and M 87 (Biretta et al. 1999).
Subluminal speeds, possibly related to the
plasma, have been
observed in the double-sided jets in 3C 338 (Feretti et al.
1993). The two-fluid scenario has been used to explain the
morphology and velocity field in the large-scale jet in
3C 31 (Laing & Bridle 2002, 2004).
Orbital motion and disk precession in the BBH-AD system perturb (but
not disrupt) the
beam, which leads to the complex
variability and kinematic patterns observed in the compact jets. On
scales of up to several hundreds of parsecs, the
beam is not
disrupted by Langmuir turbulence (Sol et al. 1989) and Alfvén and whistler waves (Pelletier & Sol 1992; Achatz & Schlickeiser 1993). No strong
Kelvin-Helmholtz instability should form on these scales, despite the
significant transverse stratification of the outflow (Hanasz & Sol
1996). Kinematic and emission properties of perturbed beams
have been calculated (Roland et al. 1994; Desringe
& Fraix-Burnet 1997), and it has been shown that the observed
variability and non-linear trajectories of jet components may indeed
be caused by a perturbed
plasma. In a more general
approach, solutions for the restricted three-body problem (Laskar
1990; Laskar & Robutel 1995) have been applied for
deriving the parameters of the perturbed beam and precessing accretion
disk in the BBH system in the quasar PKS 0420-014 (Britzen
et al. 2001).
In this paper, the dynamics of the BBH-AD system and the two-fluid model are combined together in order to construct a single framework for explaining optical flaring activity, kinematic properties and internal structure of the compact parsec-scale jets. The combined model is developed to describe the quasar 3C 345 . It is presented in Sect. 2. Section 3 provides an overview of the basic properties of the optical and radio emission from 3C 345. In Sects. 4-5, the model is applied to describe the evolution of the optical emission and radio jet in 3C 345 after a strong flare in 1992 that resulted in the appearance of a new superluminal jet component C7.
Throughout the paper, the Hubble constant
H0=70 h km s-1 Mpc-1, deceleration parameter q0=0.5, and negative definition of the spectral index,
are used. For 3C 345 (z=0.595, Hewitt & Burbidge 1993), the adopted
cosmological parameters correspond to a luminosity distance
Gpc. The corresponding linear scale is
3.79 h-1 pc mas-1. A proper motion of 1 mas/year
translates into an apparent speed of
19.7 h-1c. Subscripts "b'',
"j'', "c'', and "m'' are introduced to identify quantities related
to the beam, jet, moving plasma condensation, and magnetic field,
respectively.
The binary black hole model provides a general formalism for calculating the emission and kinematics of an outflow originating from a BBH-AD system. The two-fluid description of the outflow is adopted (Fig. 1), with the following assumptions:
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Figure 1:
Two-fluid model for relativistic outflows. A fast,
relativistic
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The basic geometry of the model is illustrated in Fig. 2.
Two black holes M1 and M2 orbit each other in the plane (O
). The center of the mass of the binary system is in O. An accretion disk around the black hole M1 is inclined at an angle
with respect to the orbital plane. The angle
is
the opening half-angle of the precession cone. The disk precesses
around the
axis, which is parallel to the
-axis. The
momentary direction of the beam axis is then given by the vector
.
The line of sight makes an angle
with the precession axis.
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Figure 2:
Geometry of the BBH model. M1 and M2 denote the locations of
two black holes orbiting around each other in the plane (![]() ![]() ![]() ![]() ![]() ![]() |
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The beam exists inside the jet for as long as its density
![]() |
(1) |
![]() |
(2) |
![]() |
(3) |
The relativistic plasma injected into the beam in the vicinity of the black
hole M1 moves at a speed characterized by the bulk Lorentz factor
.
The condensation follows the magnetic field lines. The perturbation of the magnetic field propagates at the Alfvén speed
.
For highly energetic beams, the classical density
is substituted with the enthalpy
,
where P is the pressure,
is the relativistic
density and
is the specific internal energy. Taking into
account relativistic corrections, and recalling that the enthalpy becomes
in a plasma moving at a bulk speed
,
the resulting phase
speed of the Alfvén perturbation with respect to the beam plasma is
.
Thus, for a typical
beam propagating at
inside a jet with
,
the relativistic Alfvén speed is
0.1 c.
The observed trajectory of the beam is affected primarily by the
orbital motion in the BBH system and the precession of the AD around
the primary BH. The jet structure on parsec scales is
mainly determined by the precession. The orbital motion introduces a
small (0.1 mas) oscillation of the footpoint of the jet, and it
is not immediately detectable in VLBI images with a typical resolution
of
1 mas. Consider first the effect of the precession of the accretion disk. The coordinates of the component moving in the perturbed beam are given by
The form of the function
depends on the evolution of
the speed
of the plasma condensation injected into
the perturbed beam. From Eq. (4), the components of
are:
The trajectory of a superluminal feature travelling in the perturbed
beam is modified also by the orbital motion of the two black holes in
the binary system. This motion can be taken into account by solving
the restricted three-body problem for the BBH-AD system (Laskar
1990; Laskar & Robutel 1995). The orbital plane is
,
and the coordinate system is centered at the mass
center of the system. The orbit is given by
,
where
is the true anomaly, pis the semi-latus rectum and e is the numerical eccentricity of the
orbit. The resulting coordinates (
,
)
of the black
hole M1 are given by
The strongest effect on the form of the beam trajectory is produced by
three model parameters: the beam speed
,
the Alfvén
speed
,
and the precession period
.
The orbital
period in the BBH system affects only the position of the footpoint of
the jet. Since VLBI measurements are made with respect to the
footpoint position,
would not have an effect on fitting
the observed trajectories of the jet components (the orbital period is
constrained by the variability of the optical emission). It is
easy to see that the remaining parameters (R0,
,
and
)
describe only the orientation and shape of the overall
envelope within which the perturbed beam is evolving.
The
solutions described by Eqs. (10) and (16) can degenerate or become non-unique with respect
to
,
,
,
if these parameters are
correlated. The combinations of
,
and
describing the same beam trajectory must not depend on the
rest frame time t or beam location
.
However, the solution given by Eq. (10)
implies that the beam trajectory would remain the same if
The model introduces two new properties in the description of the
BBH-AD system: 1) the Lorentz factor of the relativistic beam is time
variable; 2) the initial perturbation of the magnetic lines in the
beam dissipates on a timescale
.
These modifications
are introduced in order to accommodate for the kinematic properties of
the jet in 3C 345 in which the curvature of component trajectories
decreases at larger distances, and accelerated motions are strongly implied
by the observed trajectories and apparent speeds of several jet
components (LZ99). The beam accelerates from
to
over a rest frame time interval
,
and
Corrections for the relativistic motion of the beam must be made, in
order to enable calculations of physical quantities in the observer's
frame. This requires a knowledge of the angle
between the
instantaneous velocity vector of the component and the line of sight (LOS). Since the observed angular size of the BBH orbit is small (
1 milliarcsecond), the LOS can be assumed to lie in the plane parallel to
,
making an angle
with the
axis. In this case,
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Figure 3: Optical and radio variability of 3C 345. Shown are the epochs of spectral flares (LZ99) that have resulted in appearances of new superluminal components in the jet. The radio data are from the Michigan monitoring program (Aller et al. 2003). The optical data are from Kinman et al. (1968); Smyth & Wolstencroft (1970); Lü (1972); McGimsey et al. (1975); Pollock et al. (1979); Angione et al. (1981); Kidger (1988) Webb et al. (1988); Kidger & de Diego (1990); Vio et al. (1991); Schramm et al. (1993); Babadzhanyants et al. (1995); Belokon' & Babadzhanyants (1999). All optical data have been converted into the B magnitude scale. |
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The 16m quasar 3C 345 exhibits remarkable structural and
emission variability on parsec scales around a compact unresolved
radio core. The total radio flux density of 3C 345 has been
monitored at 5, 8, and 15 GHz (Aller et al. 2003), and at 22 and 37 GHz (Teräsranta et al. 1998). The source has also been monitored with the Green Bank
Interferometer at 2.7 and 8.1 GHz (Waltman et al. 1991).
The observed variability of the optical emission is possibly
quasi-periodic with a period of 1560 days (Babadzhanyants &
Belokon' 1984; Kidger 1989), although it has been suggested that the
light curve may originate from a non-linear and non-stationary
stochastic process (Vio et al. 1991). A strong optical flare
observed in 3C 345 in 1991-92 (Schramm et al. 1993; Babadzhanyants et al. 1995) was connected with the appearance of a new
superluminal jet component, C7 (L96). LZ99 report possible 3.5-4 year
quasi-periodicity manifested by spectral flares of the radio emission
and appearances of new jet components. Historical optical and 8 GHz radio lightcurves are shown in Fig. 3 together with the epochs of the spectral flares and component ejections.
The evolution of the parsec-scale jet of 3C 345 has been studied extensively with the VLBI: see Unwin et al. (1983), Biretta et al. (1986), Zensus et al. (1995, hereafter ZCU95), L96, Ros et al. (2000, hereafter R00), Klare (2003). The relativistic jet model (Blandford & Rees 1978) has been applied to explain the emission and kinematic changes observed in the parsec-scale structures. Using the X-ray data to constrain the jet kinematics, ZCU95 and Unwin et al. (1994) derive physical conditions in the jet from a model that combines the inhomogeneous jet model of Königl (1981) for the core with homogeneous synchrotron spheres for the moving jet components (Cohen 1985). The evolution of the core flux density is well represented by a sequence of flare-type events developing in a partially opaque, quasi-steady jet (LZ99). Spectral properties of the parsec-scale emission have been studied in several works (L96; Lobanov et al. 1997; Lobanov 1998b). Unwin et al. (1997) have analyzed a correlation between the X-ray variability and parsec-scale radio structure of 3C 345.
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Figure 4:
Parsec-scale jet in 3C 345 at 5 and 15 GHz. Contours are
drawn at -1, 1, ![]() |
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Figure 4 shows VLBI images of the jet in 3C 345 at 5
and 15 GHz, with the trajectories of eight superluminal components
identified and monitored in the jet in 1979-1999. The morphology of
3C 345 is typical for a core dominated source: a bright and compact
core responsible for 50-70% of the total emission, and a compact
jet that contains several enhanced emission regions (jet components)
moving at apparent speeds of up to
.
The compact jet in 3C 345 extends over 25 mas, which
corresponds to a projected distance of
95
pc
(
800 h-1 pc, assuming the most likely viewing angle of
the jet
). The jet has two twists at separations of
1 and
5 mas. The twists appear to
retain their positions in the jet over the entire duration of the VLBI
monitoring. Such stability of the twists suggests that they are
likely to reflect the three-dimensional structure of the jet. The
unchanging positions of the twists are also consistent with the
similarities of the apparent trajectories of the jet components at separations
1 mas (Fig. 4). The two-dimensional
trajectories observed in several jet components follow similar,
overlapping tracks, and outline a continuous picture of a curved jet.
The separation of 20 mas from the core can mark a transition point
from compact jet to large-scale jet. At this separation, the jet
finally turns to the direction
observed in the
large-scale jet of 3C 345 (Kolgaard et al. 1989).
The measured positions of the component C7 (Fig. 4)
provide an accurate account of the jet kinematics within 2 mas
distance from the core. The large amount and good quality of the data
at 22 GHz are sufficient for determining the component trajectory
without using measurements made at lower frequencies. The total of 18
VLBI observations of 3C 345 at 22 GHz are combined for the
epochs between 1989 and 1998 (L96; R00; Klare
2003). For several of these observations, new model fits have
been made for the purpose of obtaining consistent error estimates for
the positions and flux densities of C7. Table 1 presents
the resulting database of the positions and flux densities of C7 at 22 GHz.
C7 was first reliably detected in 1991.5 at 22 GHz
at a core separation of
as (L96)
Reports of earlier detections in 1989.2 at 100 GHz (Bååth et al. 1992) and 22 GHz (L96) are inconclusive because of the likely confusion and blending with the nearby component C6 and the core itself.
Table 1: Positions and flux densities of C7 at 22 GHz.
Following the procedure established in ZCU95 and L96,
polynomial functions are fitted to the component coordinate offsets x(t) and y(t) from the core position which is fixed at the point (0, 0). From the polynomial fits, the proper motion
,
apparent speed
and apparent distance
travelled by the component along its
two-dimensional path can be recovered. For the purpose of comparison
to the previous work of L96 and Lobanov & Zensus (1996), the
detection reported for the epoch 1989.25 is included in the polynomial
fitting, but all results for the epochs before 1991 should be viewed as only tentative.
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Figure 5:
Kinematic properties of the jet determined from the observed
trajectory of C7. In all panels, the solid line represents a model
with
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Kinematic properties of C7 are presented in Fig. 5. The
apparent speed
of C7 increased from
2 c in 1989 to
in 1997 (the decrease of
after 1997 is an artifact of the
polynomial fit). The component reached the minimum apparent speed of
in 1990.5. Such large changes
of
indicate that C7 has moved along a
substantially curved three-dimensional path, with the bulk Lorentz factor most likely changing with time. If the Lorentz factor is constant, then
(to satisfy the observed
). If
varies, the lower boundary of the variations is given by the minimum Lorentz factor
,
which describes the
motion with the least kinetic power. In Fig. 5,
is set, and the kinematic parameters of C7
are described for all allowed variations of
(
). As seen in Fig. 5, neither of
the two boundaries provides a satisfactory description of the
component kinematics. The case
results in uncomfortably small
and disproportionally large travelled distance
.
With
,
the resulting
variations of
exceed
,
which is difficult to explain. A plausible evolution of the rest
frame speed of C7 can be described by a linear increase of the Lorentz
factor from
in 1989 to
in 1998. In this case, none of the kinematic parameters change
drastically in the course of the component evolution:
varies within
,
and
pc. This indicates that the motion of C7 is likely to be accelerated during the initial stages of its evolution, at distances of
2 mas from the core. At such distances, acceleration of the
component rest frame speeds has also been reported in several other
jet components in 3C 345 (Lobanov & Zensus 1996,
LZ99). Consequently, this must be a general property of the jet
plasma on these spatial scales, and it should be taken into account in
the modelling of the jet kinematics.
The observed consistency of the two-dimensional component tracks
plotted in Fig. 4 suggests that all jet
components in 3C 345 travel along similar trajectories.
There is however a substantial amount of evidence for global evolution
of the entire jet. In Fig. 6, the apparent
accelerations,
,
of the components are
plotted against the respective component origin epochs taken from
L96. The apparent accelerations show a steady increase,
following the order of component succession, which is a strong
indication for long-term changes in the jet. While it is conceivable
that the younger components are intrinsically faster, this would
require the presence of a non-stationary process affecting
significantly the particle acceleration in the nuclear regions of
3C 345. The more likely explanation of the observed
long-term trend of the component accelerations is precession of the
jet axis. In this case, variations of the component ejection angle
alone are sufficient to explain the observed trend: the components
ejected at angles closer to
would have higher
.
In the course of the precession, the ejection angle would
change steadily, and the measured
should show sinusoidal variations. The rather large errors in the measured
in Fig. 6 allow only a very rough estimate of the precession period
,
which should be in the range of 850-3670 years, to be made.
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Figure 6:
Apparent acceleration
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Periodic changes are also evident in the variations of the jet
position angle measured at 22 GHz for successive jet components as
they reach a 0.5 mas separation from the core. The resulting
position angles are plotted in Fig. 7. The position
angle varies with a period of
9.5 years and
an amplitude of
20
.
A longer term
trend is suggested by the slope in the linear regression of the
position angle data, which implies a rate of change in the position
angle of
0.4
/year. The time coverage of the data is
too short to provide an accurate estimate of the period of the long
term variations. A formal fit constrains the period to within a range
of 300-3000 years, with a mean value of
years and an amplitude of
.
The
shaded area in Fig. 7 represents a combined fit to
the data by the two periods. The estimate of
obtained from the change in position angle falls within the range of
periods implied by
,
and it can therefore be associated with the precession. The nature of
the short-term changes of the jet position is unclear, but they could
be related to the rotation of the accretion disc or the jet itself or
to the orbital motion in the BBH system.
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Figure 7:
Position angle of different jet components measured at
22 GHz at 0.5 mas separation from the core. Short-term variations, with a
period of ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The BBH model applies a single model framework to explain the emission and kinematic properties of the jet in the optical and radio regimes. The evolution of superluminal features observed in the radio jet is explained by a precession of the accretion disk around the primary black hole M1, and the characteristic shape of the optical light curve is produced by the orbital motion of the black hole M1.
The different characteristic timescales and amplitudes of changes in
the radio and optical regimes imply a complex composition of the
emitting material. The optical emission of 3C 345 during the strong
flare in 1990-93 varied by 2.5 mag, with a typical
timescale of
0.6 yr for individual sub-flare events (see
Fig. 15). The radio emission of the jet component C7
associated with that flare showed only a single rise and decay
event, changing by a factor of
15 on a timescale of
6 years. This difference in the behavior of the optical and radio variability can be reconciled by assuming that the
relativistic
plasma responsible for the optical emission is injected into
the beam during a time
which is much
shorter than the injection timescale
for the
plasma responsible for the radio emission. For C7, the plasma
responsible for the optical emission is modelled by a point-like
component and the plasma responsible for the radio
emission is modelled by a temporally extended
component, so that
.
In the BBH frame, the
radio component emits over a period of
1.9 yr, with
component ejections occurring approximately every 4 years. A general
sketch of the composition of a flare is shown in Fig. 8.
The application of the model consists of two parts. In the first part, the observed trajectory and flux density of the jet component C7 are reconstructed, using the precession of the accretion disk around the black hole M1, and solving formally the problem defined by Eq. (8). In the second part, the optical light curve of 3C 345 is calculated from the orbital properties of the BBH system provided by the solution of Eq. (16).
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Figure 8:
Composition of a nuclear flare. The flare begins with an
injection of highly energetic particles emitting optical synchrotron
radiation. The timescale of the injection
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Basic geometrical parameters of the system are recovered by fitting the precession model to the observed two-dimensional path of C7. These include
The velocity of the jet component is constrained by fitting the coordinate offsets, x(t) and y(t), of the component from the position of the radio core. In 3C 345, the apparent motion of all jet components is non-linear and shows accelerations and decelerations (ZCU95; L96; LZ99). To account for this, a variable Lorentz factor must be introduced, using Eq. (18). With a variable Lorentz factor, the x(t) and y(t) offsets are fitted simultaneously, and a three-dimensional trajectory of the component is recovered. This procedure yields the following parameters:
In addition to the geometrical and kinematic factors, the flux density
evolution in the optical and radio regimes is characterized by the
synchrotron energy loss timescales
and
and by the duration
during which the relativistic plasma
responsible for the flare remains optically thick in the radio
regime. The radio lightcurve of C7 constrains uniquely the viewing
angle of the jet. The information contained in the radio lightcurve is
fundamental for determining the duration of the radio emitting stage,
,
of the flare and ensuring the consistency
between the precession fit and the BBH fit.
The main effect of the BBH system is the orbital motion of both black holes around the center of mass of the system. This motion explains the observed optical light curve, the fine structure of the parsec-scale jet, and peculiarities of component motions around the general trajectory determined by the precession. The fit by the BBH system yields the following parameters:
The BBH model, which is obtained by fitting a point-like component to reproduce the observed optical lightcurve, must be consistent with the results derived from the precession fit. In other words, the evolution of the extended, radio emitting, component determined from fitting the BBH model must reproduce the trajectory and kinematic properties of the radio emitting plasma obtained from the precession fit.
Consistency between the fits by the precession and orbital motion is ensured under the following five conditions which connect the formal fitting with the specific properties of the two-fluid model:
The best fit parameters of the precessing jet and the BBH system in 3C 345 determined using the model developed in Sect. 2 and the fitting method described in Sect. 4 are presented in Table 2. The procedural steps through which the fit was obtained are summarized below.
Table 2: Precession, orbital and flare properties of the BBH system in 3C 345.
The position angle of the jet axis in the plane of the sky is determined directly from the VLBI images of 3C 345, which yields
The jet viewing angle, ,
opening half-angle of the precession cone
,
and initial precession phase,
,
are obtained by fitting the observed two-dimensional path of C7. This fit constrains uniquely the initial precession phase at
Table 3:
Precession solutions for -
.
The fit for
depends on the Alfvén speed
in
the jet. For each value
an appropriate
can be
found from the precession fit that would reproduce exactly the same
two-dimensional trajectory. This family of solutions is given in
Table 4. The value of
is constrained
after a similar family of solutions is obtained for the BBH fit to the
optical light curve. A unique solution for
is then found
by reconciling the values of
and
obtained from
the BBH and precession fits.
Table 4:
Precession solutions for
-0.15 c.
After the two-dimensional path of C7 has been fitted by the geometrical
and precession parameters, the precession model is applied to fit the
trajectory and flux density evolution of C7, which yields
,
and
yr. These parameters are independent from the choice of
.
It should also be noted that the Lorentz factors derived above are
related to the radio emitting plasma. The optically emitting plasma
may have a different range of Lorentz factors.
In order to ensure consistency between the precession and BBH fits,
and to bootstrap together the fit of the trajectory and kinematics
of C7 and the fit of the optical light curve, two general model classes
are considered for describing the properties of the radio and
optically emitting plasmas, in terms of the minimum (or initial)
Lorentz factors of the two plasmas (these scenarios are based on the
consistency conditions described in Sect. 4.4):
Model 1:
,
Model 2:
.
In both cases, the radio emitting plasma is injected immediately after the optically
emitting plasma (as illustrated in Fig. 8).
For model 1,
and
are chosen from the precession fit (note that this
corresponds to injection speeds of
and
). For model 2,
is assumed.
Both scenarios fit well the optical light curve, but model 1
reproduces better the kinematics of C7, particularly at the beginning
of the component evolution.
Figure 9 compares the rest frame time obtained from the precession fit and the BBH fit by the two models. The injection epoch
obtained
from the precession fit is recovered by model 1, but cannot be
reproduced by model 2.
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Figure 9:
Rest frame time recovered from the precession fit (describing
the trajectory of the VLBI component) and the BBH models 1 and 2
(describing the optical variability) under the consistency
constraints discussed in Sect. 4.4. The injection
epoch
![]() ![]() |
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For the earliest epochs of the evolution of C7, model 2 does not reproduce satisfactorily the separations of C7 from the core (Fig. 10) and the flux density changes (Fig. 11). The discrepancy between the fits by the precession model and model 2 is particularly visible in the speed evolution obtained from the precession and BBH fits (Fig. 12). Model 1 is therefore a better counterpart to the precession fit described in Sect. 5.1.
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Figure 10: Evolution of the separation of the jet component C7 from the core. The fit by the precession model is consistent with the fit by BBH model 1. BBH model 2 fails to reproduce the earliest position measurements of C7. |
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Figure 11: Radio flux density of C7 at 22 GHz. The fits by the precession model and BBH model 1 are consistent. BBH model 2 cannot reproduce the onset of the radio emission. |
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Figure 12: Apparent speed evolution of the jet component C7 recovered from the precession and BBH fits. The fit by BBH model 1 is consistent with the fit by the precession, while BBH model 2 cannot be reconciled with the precession fit at all. |
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Figure 13 compares the precession and the BBH model 1 fits to the observed two-dimensional path of C7. The agreement between the fits is excellent. The two fits differ by less than 0.02 mas (Fig. 14). This deviation is caused by the orbital motion of the black hole ejecting the jet, which is not accounted for in the precession fit.
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Figure 13: Two-dimensional path of C7. The precession fit and the fit by BBH model 1 are presented. |
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Figure 14: Two-dimensional path of C7 within 1 mas of the nucleus. The difference between the precession and BBH model 1 fit is due to the orbital motion of the black hole ejecting the jet. |
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Fitting the optical light curve constrains the orbital period ,
orbital eccentricity and initial orbital phase
in the BBH system, under the conditions of consistency described in Sect. 5.3. The fit to the optical curve requires
a nearly circular orbit (e < 0.1), constrains uniquely the initial orbital phase
Table 5:
Orbital period solutions for
-0.15 c.
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Figure 15: Optical variability in 3C 345 in 1990-93. Solid lines show the fits by the BBH model. Individual peaks result from the orbital motion in the BBH system. |
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In order to reconcile the two families of -dependent
solutions (the precession solutions in Table 4 and
the orbital period solutions in Table 5), the value of
the Alfvén speed must be constrained. The periodic changes of the
position angle of component ejection plotted in
Fig. 7 can be used for this purpose. The Alfvén wave perturbations, propagating at
cannot account for such a short period. The observed period
yr must therefore reflect perturbations of the
magnetic field caused by the orbital motion in the BBH system. In this
case,
,
where
is the Doppler factor of the beam plasma measured at the same location (0.5 mas from the nucleus) at
which the oscillations of the ejection angle are registered. The
kinematic fit yields
,
which results in the orbital period
yr. The corresponding value of the Alfvén speed is then
With the Alfvén speed determined, the masses of the black holes can
now be estimated from
and
.
Since the orbital
separation
,
a family of
solutions can be produced that satisfy
yr,
yr and reproduce the observed optical lightcurve of
3C 345 and radio properties of C7 (the shapes of the optical and
radio lightcurves depend on the mass ratio
). The
resulting solutions are presented in Table 6 for five
selected values of M1.
In order to identify an acceptable solution for the black hole masses,
stability conditions of the accretion disk must be considered. The
rotational timescale of a precessing accretion disk in a binary system is
given by a characteristic period (Laskar & Robutel 1995)
Table 6:
Black hole mass solutions for
yr.
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Figure 16:
Range of acceptable solutions for the mass M1 of the primary
black hole in 3C 345. The acceptable solutions exist within the range
7.1 ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The BBH model applied to the quasar 3C 345 ties together, within a single framework, the variability of the optical flux and the evolution of the superluminal feature C7 resulted from a powerful flare that occurred in 1991. The model reconstructs the physical conditions in the plasma ejected into the jet during the flare, and the dynamic properties of the binary system of supermassive black holes.
The binary black hole scenario was previously applied to 3C 345 by
Caproni & Abraham (2004) who attributed variations of the ejection
angle (similar to those plotted in Fig. 7) to
precession of the accretion disk. This approach yielded an
uncomfortably short precession period of
yr, very high black hole masses (M1 = 4-5
,
M2 = 3-4
), and extremely small orbital separation
0.017-0.024 pc, which is
of the primary black hole. In such a
tight binary, the effective accretion radius of the secondary,
-0.050 pc, is larger than the orbital separation, and
it is very difficult to maintain the accretion disk intact. Indeed, it
can be shown (Ivanov et al. 1998) that the mass loss
rate
due to a shock that is formed after each passage of
the secondary through the accretion disk may be very large, and for
the BBH parameters of Caproni & Abraham (2004), it can reach
,
where
is the rate of accretion on the primary black hole
and
is the disc viscosity.
For weak accretion rates or small disk viscosity,
can even
become larger than
,
which would most likely lead to a
subsequent destruction of the accretion disk.
Another argument against associating the ejection angle variability
with the disk precession is presented by the observed long-term
kinematic changes in the jet. The apparent accelerations plotted in
Fig. 6 and the long-term trend present in the
ejection angle changes plotted in Fig. 7 indicate
the presence of a periodic process on timescales of 2000 yr. This observed periodicity, though very difficult to establish, agrees well with the precession period recovered by fitting
the BBH model to the trajectory and flux evolution of C7.
The trajectory of C7 (and indeed the trajectories of several other superluminal features monitored closely in the jet of 3C 345; Lobanov & Zensus 1996; L96; LZ99) is clearly non-ballistic on timescales spanning almost 10 years in the observer's frame (and more than 100 years in the comoving frame). This is a clear observational argument against a very short precession period in 3C 345. The model developed in this paper attributes non-ballistic trajectories of the jet features to Alfvén perturbations of the magnetic field, possibly modulated by the orbital motion in the system.
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Figure 17:
Characteristic timescales of disk activity in 3C 345,
compared to the quasi-period of the nuclear flares and to the
orbital and characteristic disk rotation periods). The
disk thermal instability operates at ![]() ![]() ![]() ![]() ![]() |
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The sum of the black hole masses (
M1,2 = 1.4
)
estimated in our model agrees well with the mass
obtained
from measurements of nuclear opacity and magnetic field strength in
3C 345 (Lobanov 1998a). Gu et al. (2001)
report a higher value of
for 3C 345, from the properties of the broad line region. This estimate, however, depend strongly on the monochromatic luminosity (
)
for
which the errors can be as large as 30% even for the nearby objects
(Kaspi et al. 2000). This error could be even larger for weaker
objects (3C 345 is about 100 times weaker in the optical than an
average object from the sample of Kaspi et al. 2000), and
overestimating
by only 50% would be bring
the
down to a value similar to the one derived in
this paper.
Finally, let us consider the periodicity of the flares and component
ejections in 3C 345. A 4 yr period of the flaring activity
is inferred from the observed variability of the radio nucleus
(LZ99), and it is related with the ejections of superluminal
features (see Fig. 3). Since this period is much
shorter than
yr,
yr and
yr obtained in our model, there should be some
other periodic process affecting the accretion disk on timescales of
4 yr. Figure 17 shows four different
characteristic disk timescales in 3C 345, for the black hole and
disk parameters determined in Sect. 4. These
timescales can be compared to the periodicity of the nuclear
flares. It appears that several processes occurring in the disk at
radial distances of 20-200
may have similar
characteristic timescales. Thermal instability at
and mechanical (sound wave) instability at
will develop at timescales of
4 yr. In addition to
this, the orbital and drift (accretion rate variation) timescales
approach
4 yr at
.
A combination of
these effects can cause the observed flare activity.
Another possible explanation for the observed flares is an
intermediate or stellar mass black hole or even a neutron star
orbiting the primary black hole at
at a large angle with respect to the
accretion disk plane. Close approaches to, or even passages
through, the accretion disk should lead to formation of shock waves
in the disk (Ivanov et al. 1998; Subr
& Karas 1999), which in turn would cause an increase in the
accretion rate resulting in a nuclear flare and injection of a dense
plasma cloud into the jet.
The periodicity of the flares may also be caused by a turbulence in the jet. Formation of a turbulent layer close to the beam-jet boundary can enhance the pair creation rate in the beam, which would be observed as the appearance, and propagation, of a bright feature embedded in the flow.
The model developed in this paper explains the variability of the
optical flux and the kinematic and emission properties of the feature C7 observed in the relativistic jet of 3C 345 after a strong
nuclear flare. The dynamic properties of the system are described in
the framework of the orbital motion in a binary system of supermassive
black holes and the precession of an accretion disk around the primary
black hole. The emission and physical properties of relativistic
plasma produced during the flare are treated within the two-fluid
approach postulating a highly relativistic
beam propagating
inside an
jet. The
jet transports a major
fraction of the kinetic power associated with the outflow from the
nucleus of 3C 345, stabilizes
the
beam against plasma instabilities and shields it from
interaction with the external medium.
Determination of the properties of the jet and the binary system in
3C 345 is a difficult process involving two major steps. First, the
precession characteristics of the accretion disk are determined by
fitting the kinematic and flux density evolution of C7. This yields a
precession period
yr, which is in a good
agreement with the periodicities inferred from the changes of
kinematic properties of individual jet components and variations of
the ejecting position angle in 3C 345.
In the second step, the orbital and physical properties of the binary system are recovered
by fitting the observed optical variability. This yields an orbital
period of
.
The consistency between
the two fits is ensured by requiring the fit to the optical light
curve to reproduce also the kinematic and flux density evolution of C7. The consistency argument constrains the properties of radio and optically emitting plasma, with
and
.
After ensuring consistency between the two fits, they are combined in
order to constrain the Alfvén speed
,
and
make it popsible to recover the physical properties of the system. In
the last step, the conditions of disk stability are employed to
determine the individual masses of the binary companions. This
procedure yields
M1 = M2 = 7.1
.
Finally, the model offers an explanation for the observed 4-year quasi
periodicity of nuclear flares in 3C 345. This
quasi-periodicity arises naturally from the characteristic timescales
of the accretion disk surrounding the primary black hole. The overall
success that the model has in explaining a variety of the
observational properties in 3C 345 argues in favor of this
explanation.