134 research outputs found
On invisible plasma content in radio-loud AGNs: The case of TeV blazar Markarian 421
Invisible plasma content in blazar jets such as protons and/or thermal
electron-positron () pairs is explored through combined arguments of
dynamical and radiative processes. By comparing physical quantities required by
the internal shock model with those obtained through the observed broadband
spectra for Mrk 421, we obtain that the ratio of the Lorentz factors of a pair
of cold shells resides in about , which implies that the shocks are
at most mildly relativistic. Using the obtained Lorentz factors, the total mass
density in the shocked shells is investigated. The upper limit of
is obtained from the condition that thermal bremsstrahlung emission should not
exceed the observed -ray luminosity, whilst the lower limit is
constrained from the condition that the energy density of non-thermal electrons
is smaller than that of the total plasma. Then we find is -
times heavier than that of non-thermal electrons for pure pairs,
while - times heavier for pure electron-proton () content,
implying the existence of a large amount of invisible plasma. The origin of the
continuous blazar sequence is shortly discussed and we speculate that the total
mass density and/or the blending ratio of pairs and plasma
could be new key quantities for the origin of the sequence.Comment: Accepted MNRAS; 8 pages, 3 figures, 1 tabl
On the origin of X-shaped radio-sources: new insights from the properties of their host galaxies
A significant fraction of extended radio sources presents a peculiar X-shaped
radio morphology: in addition to the classical double lobed structure, radio
emission is also observed along a second axis of symmetry in the form of
diffuse wings or tails. We re-examine the origin of these extensions relating
the radio morphology to the properties of their host galaxies. The orientation
of the wings shows a striking connection with the structure of the host galaxy
as they are preferentially aligned with its minor axis. Furthermore, wings are
only observed in galaxies of high projected ellipticity. Hydrodynamical
simulations of the radio-source evolution show that X-shaped radio-sources
naturally form in this geometrical situation: as a jet propagates in a
non-spherical gas distribution, the cocoon surrounding the radio-jets expands
laterally at a high rate producing wings of radio emission, in a way that is
reminiscent of the twin-exhaust model for radio-sources.Comment: 7 pages, 6 figures, 1 table, accepted for publication in A&
Cosmological Studies with Radio Galaxies and Supernovae
Physical sizes of extended radio galaxies can be employed as a cosmological
"standard ruler", using a previously developed method. Eleven new radio
galaxies are added to our previous sample of nineteen sources, forming a sample
of thirty objects with redshifts between 0 and 1.8. This sample of radio
galaxies are used to obtain the best fit cosmological parameters in a
quintessence model in a spatially flat universe, a cosmological constant model
that allows for non-zero space curvature, and a rolling scalar field model in a
spatially flat universe. Results obtained with radio galaxies are compared with
those obtained with different supernova samples, and with combined radio galaxy
and supernova samples. Results obtained with different samples are consistent,
suggesting that neither method is seriously affected by systematic errors. Best
fit radio galaxy and supernovae model parameters determined in the different
cosmological models are nearly identical, and are used to determine
dimensionless coordinate distances to supernovae and radio galaxies, and
distance moduli to the radio galaxies. The distance moduli to the radio
galaxies can be combined with supernovae samples to increase the number of
sources, particularly high-redshift sources, in the samples. The constraints
obtained here with the combined radio galaxy plus supernovae data set in the
rolling scalar field model are quite strong. The best fit parameter values
suggest a value of omega is less than about 0.35, and the model parameter alpha
is close to zero; that is, a cosmological constant provides a good description
of the data. We also obtain new constraints on the physics of engines that
power the large-scale radio emission.Comment: 32 pages. Accepted for publication in the Astrophysical Journa
Was the Cosmic Web of Protogalactic Material Permeated by Lobes of Radio Galaxies During the Quasar Era?
Evidence for extended active lifetimes (> 10^8 yr) for radio galaxies implies
that many large radio lobes were produced during the `quasar era', 1.5 < z < 3,
when the comoving density of radio sources was 2 -- 3 dex higher than the
present level. However, inverse Compton losses against the intense microwave
background substantially reduce the ages and numbers of sources that are
detected in flux-limited surveys. The realization that the galaxy forming
material in those epochs was concentrated in filaments occupying a small
fraction of the total volume then leads to the conclusion that radio lobes
permeated much of the volume occupied by the protogalactic material during that
era. The sustained overpressure in these extended lobes is likely to have
played an important role in triggering the high inferred rate of galaxy
formation at z > 1.5 and in the magnetization of the cosmic network of
filaments.Comment: 5 pages, 0 figures, submitted to ApJ Letters; uses emulateapj
Study of Four Young TeV Pulsar Wind Nebulae with a Spectral Evolution Model
We study four young Pulsar Wind Nebulae (PWNe) detected in TeV gamma-rays,
G21.5-0.9, G54.1+0.3, Kes 75, and G0.9+0.1, using the spectral evolution model
developed and applied to the Crab Nebula in our previous work. We model the
evolution of magnetic field and particle distribution function inside a
uniformly expanding PWN considering a time-dependent injection from the pulsar
and radiative and adiabatic losses. Considering uncertainties in the
interstellar radiation field (ISRF) and their distance, we study two cases for
each PWN. Because TeV PWNe have a large TeV gamma-rays to X-rays flux ratio,
the magnetic energy of the PWNe accounts for only a small fraction of the total
energy injected (typically a few x 10^{-3}). The gamma-ray emission is
dominated by inverse Compton scattering off the infrared photons of the ISRF. A
broken power-law distribution function for the injected particles reproduces
the observed spectrum well, except for G0.9+0.1. For G0.9+0.1, we do not need a
low energy counterpart because adiabatic losses alone are enough to reproduce
the radio observations. High energy power-law indices at injection are similar
(2.5 -- 2.6), while low energy power-law indices range from 1.0 to 1.6. The
lower limit of the particle injection rate indicates that the pair multiplicity
is larger than 10^4. The corresponding upper limit of the bulk Lorentz factor
of the pulsar winds is close to the break energy of the broken power-law
injection, except for Kes 75.Comment: Accepted for publication in ApJ; 56pages, 15figure
The Radio Properties and Magnetic Field Configuration in the Crab-like Pulsar Wind Nebula G54.1+0.3
We present a multifrequency radio investigation of the Crab-like pulsar wind
nebula (PWN) G54.1+0.3 using the Very Large Array. The high resolution of the
observations reveals that G54.1+0.3 has a complex radio structure which
includes filamentary and loop-like structures that are magnetized, a diffuse
extent similar to the associated diffuse X-ray emission. But the radio and
X-ray structures in the central region differ strikingly, indicating that they
trace very different forms of particle injection from the pulsar and/or
particle acceleration in the nebula. No spectral index gradient is detected in
the radio emission across the PWN, whereas the X-ray emission softens outward
in the nebula. The extensive radio polarization allows us to image in detail
the intrinsic magnetic field, which is well-ordered and reveals that a number
of loop-like filaments are strongly magnetized. In addition, we determine that
there are both radial and toroidal components to the magnetic field structure
of the pulsar wind nebula. Strong mid-IR emission detected in Spitzer Space
Telescope data is closely correlated with the radio emission arising from the
southern edge of G54.1+0.3. In particular, the distributions of radio and X-ray
emission compared with the mid-IR emission suggest that the PWN may be
interacting with this interstellar cloud. This may be the first PWN where we
are directly detecting its interplay with an interstellar cloud that has
survived the impact of the supernova explosion associated with the pulsar's
progenitor.Comment: 35 pages, 19 figures, accepted for publication in the Astrophysical
Journa
Three-dimensional magnetohydrodynamic simulations of the evolution of magnetic fields in Fanaroff-Riley class II radio sources
Radio observations of Fanaroff-Riley class II sources often show correlations
between the synchrotron emission and the linear-polarimetric distributions.
Magnetic position vectors seem to align with the projected emission of both the
radio jets and the sources' edges. Using statistics we study such relation as
well as its unknown time evolution via synthetic polarisation maps of model FR
II sources formed in 3D-MHD numerical simulations of bipolar, hypersonic and
weakly magnetised jets. The magnetic field is initially random with a
Kolmogorov power spectrum, everywhere. We investigate the structure and
evolution of magnetic fields in the sources as a function of the power of jets
and the observational viewing angle. Our synthetic polarisation maps agree with
observations, showing B-field vectors which are predominantly aligned with the
jet axis, and show that magnetic fields inside sources are shaped by the jets'
backflow. Polarimetry is found to correlate with time, the viewing angle and
the jet-to-ambient density contrast. The magnetic structure inside thin
elongated sources is more uniform than inside more spherical ones. We see jets
increase the magnetic energy in cocoons in proportion to the jet velocity and
the cocoon width. Filaments in the synthetic emission maps suggest turbulence
develops in evolved sources.Comment: Accepted for publication in the MNRAS. 21 pages, 11 figure
A super massive black hole binary in 3C66B: future observational perspectives
Supermassive black hole binaries (SMBHBs) may exist in the centers of
galaxies and active galactic nuclei (AGN) and are expected to be fairly common
in the Universe as a consequence of merging processes between galaxies. The
existence of SMBHBs can be probed by looking for double nuclei in galaxy
centers or, more easily, detecting periodic behavior in the observed radio
light curves. In a recent paper, Sudou et al. \cite{sudou2003} announced the
first direct observation of an SMBHB. Using VLBI observations they found that
the unresolved radio core of the radio galaxy 3C66B shows a well defined
elliptical motion with a period of yrs, implying the presence of
a couple of massive black holes in the center of the galaxy. In the present
paper we study the astrophysical implications of the existence of such an SMBHB
in 3C66B. In particular we focus on the information that can be obtained from
detecting a signal periodicity either in the -ray and/or -ray light
curves as a consequence of the motion of the black holes. These observations
could be used to extract further information on the physical parameters of the
SMBHB and partially solve the system parameter degeneracy. The detection of the
gravitational wave spectrum emitted by such system may be used to completely
determine the physical parameters of the binary system.Comment: 8 pages, 8 figures, in Press on Astronomy & Astrophysic
On the Physical Conditions in AGN Optical Jets
The energy budget of all known optical jets is discussed. It is found that to
power the extended radio lobes of radio galaxies, the jet bulk motion on kpc
scales must be relativistic, on average. Based on various constraints, a ``most
probable'' region centered around bulk Lorentz factor ~7.5 and jet inclination
\~20 degrees is found. Because of the consequent relativistic beaming, the rest
frame magnetic field is lower and electron lifetimes longer. Combining the
effects of time dilation and lower emission rate, the electron diffusion length
becomes fully consistent with the deprojected jet length, without the need for
reacceleration.Comment: To appear in the proceeding of "Lifecycle of Radio Galaxies"
symposium, New Astronomy Review, in press Typos corrected. In particular, in
the expression for dE/dt (section 2 line 6), B should be square
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