74 research outputs found
The milliarcsecond-scale jet of PKS 0735+178 during quiescence
We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae
object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous
and later observations suggests that the overall kinematic and structural
properties of the jet are greatly influenced by its activity. Time intervals of
enhanced activity, as reported before 1993 and after 2000 by other studies, are
followed by highly superluminal motion along a rectilinear jet. In contrast the
less active state in which we performed our observations, shows subluminal or
slow superluminal jet features propagating through a twisted jet with two sharp
bends of about 90 deg. within the innermost three-milliarcsecond jet structure.
Proper motion estimates from the data presented here allow us to constrain the
jet viewing angle to values < 9 deg., and the bulk Lorentz factor to be between
2 and 4.Comment: 11 pages, 12 figures. Accepted for publication in A&
A possible jet precession in the periodic quasar B0605-085
The quasar B0605-085 (OH 010) shows a hint for probable periodical
variability in the radio total flux-density light curves. We study the possible
periodicity of B0605-085 in the total flux-density, spectra and opacity changes
in order to compare it with jet kinematics on parsec scales. We have analyzed
archival total flux-density variability at ten frequencies (408 MHz, 4.8 GHz,
6.7 GHz, 8 GHz, 10.7 GHz, 14.5 GHz, 22 GHz, 37 GHz, 90 GHz, and 230 GHz)
together with the archival high-resolution very long baseline interferometry
data at 15 GHz from the MOJAVE monitoring campaign. Using the Fourier transform
and discrete autocorrelation methods we have searched for periods in the total
flux-density light curves. In addition, spectral evolution and changes of the
opacity have been analyzed. We found a period in multi-frequency total
flux-density light curves of 7.9+-0.5 yrs. Moreover, a quasi-stationary jet
component C1 follows a prominent helical path on a similar time scale of 8
years. We have also found that the average instantaneous speeds of the jet
components show a clear helical pattern along the jet with a characteristic
scale of 3 mas. Taking into account average speeds of jet components, this
scale corresponds to a time scale of about 7.7 years. Jet precession can
explain the helical path of the quasi-stationary jet component C1 and the
periodical modulation of the total flux-density light curves. We have fitted a
precession model to the trajectory of the jet component C1, with a viewing
angle phi=2.6+-2.2 degrees, aperture angle of the precession cone
Omega=23.9+-1.9 degrees and fixed precession period (in the observers frame) P
= 7.9 yrs.Comment: 14 pages, 16 figures, 5 tables, accepted for publication in A&
Catching the radio flare in CTA 102 I. Light curve analysis
Context: The blazar CTA 102 (z=1.037) underwent a historical radio outburst
in April 2006. This event offered a unique chance to study the physical
properties of the jet. Aims: We used multifrequency radio and mm observations
to analyze the evolution of the spectral parameters during the flare as a test
of the shock-in-jet model under these extreme conditions. Methods: For the
analysis of the flare we took into account that the flaring spectrum is
superimposed on a quiescent spectrum. We reconstructed the latter from archival
data and fitted a synchrotron self-absorbed distribution of emission. The
uncertainties of the derived spectral parameters were calculated using Monte
Carlo simulations. The spectral evolution is modeled by the shock-in-jet model,
and the derived results are discussed in the context of a geometrical model
(varying viewing angle) and shock-shock interaction. Results: The evolution of
the flare in the turnover frequency-turnover flux density plane shows a double
peak structure. The nature of this evolution is dicussed in the frame of
shock-in-jet models. We discard the generation of the double peak structure in
the turnover frequency-turnover flux density plane purely based on geometrical
changes (variation of the Doppler factor). The detailed modeling of the
spectral evolution favors a shock-shock interaction as a possible physical
mechanism behind the deviations from the standard shock-in-jet model.Comment: 15 pages, 12 figure
The kinematics in the pc-scale jets of AGN The case of S5 1803+784
We present a kinematic analysis of jet component motion in the VLBI jet of
the BL Lac object S5 1803+784, which does not reveal long-term outward motion
for most of the components. Understanding the complex kinematic phenomena can
possibly provide insights into the differences between quasars and BL Lac
objects. The blazar S5 1803+784 has been studied with VLBI at =1.6, 2.3,
5, 8.4, and 15 GHz between 1993.88 and 2005.68 in 26 observing runs. We
(re)analyzed the data and present Gaussian model-fits. We collected the already
published kinematic information for this source from the literature and
re-identified the components according to the new scenario presented in this
paper. Altogether, 94 epochs of observations have been investigated. A careful
study of the long-term kinematics reveals a new picture for component motion in
S5 1803+784. In contrast to previously discussed motion scenarios, we find that
the jet structure within 12 mas of the core can most easily be described by the
coexistence of several bright jet features that remain on the long-term at
roughly constant core separations (in addition to the already known {\it
stationary} jet component 1.4 mas) and one faint component moving with
an apparent superluminal speed ( 19c, based on 3 epochs). While most of
the components maintain long-term roughly constant distances from the core, we
observe significant, smooth changes in their position angles. We report on an
evolution of the whole jet ridge line with time over the almost 12 years of
observations. The width of the jet changes periodically with a period of
8 to 9 years. We find a correlation between changes in the position angle and
maxima in the total flux-density. We present evidence for a geometric origin of
the phenomena and discuss possible models.Comment: The manuscript will be published by A&
Multiwavelength Observations of the Gamma-Ray Blazar PKS 0528+134 in Quiescence
We present multiwavelength observations of the ultraluminous blazar-type
radio loud quasar PKS 0528+134 in quiescence during the period July to December
2009. Significant flux variability on a time scale of several hours was found
in the optical regime, accompanied by a weak trend of spectral softening with
increasing flux. We suggest that this might be the signature of a contribution
from the accretion disk at the blue end of the optical spectrum. The optical
flux is weakly polarized with rapid variations of the degree and direction of
polarization, while the polarization of the 43 GHz radio core remains steady.
Optical spectropolarimetry suggests a trend of increasing degree of
polarization with increasing wavelength, providing additional evidence for an
accretion disc contribution towards the blue end of the optical spectrum. We
constructed four SEDs indicating that even in the quiescent state, the
bolometric luminosity of PKS 0528+134 is dominated by its gamma-ray emission. A
leptonic single-zone jet model produced acceptable fits to the SEDs with
contributions to the high-energy emission from synchrotron self-Compton
radiation and Comptonization of direct accretion disk emission. Fit parameters
close to equipartition were obtained. The moderate variability on long time
scales implies the existence of on-going particle acceleration, while the
observed optical polarization variability seems to point towards a turbulent
acceleration process. Turbulent particle acceleration at stationary features
along the jet therefore appears to be a viable possibility for the quiescent
state of PKS 0528+134.Comment: Accepted for Publication in The Astrophysical Journal. -
Acknowledgement adde
Structure and flux variability in the VLBI jet of BL Lacertae during the WEBT campaigns (1995--2004)
BL Lacertae has been the target of several observing campaigns by the Whole
Earth Blazar Telescope (WEBT) collaboration and is one of the best studied
blazars at all accessible wavelengths. A recent analysis of the optical and
radio variability indicates that part of the radio variability is correlated
with the optical light curve. Here we present an analysis of a huge VLBI data
set including 108 images at 15, 22, and 43 GHz obtained between 1995 and 2004.
The aim of this study is to identify the different components contributing to
the single-dish radio light curves. We obtain separate radio light curves for
the VLBI core and jet and show that the radio spectral index of single-dish
observations can be used to trace the core variability. Cross-correlation of
the radio spectral index with the optical light curve indicates that the
optical variations lead the radio by about 100 days at 15 GHz. By fitting the
radio time lags vs. frequency, we find that the power law is steeper than
expected for a freely expanding conical jet in equipartition with energy
density decreasing as the square of the distance down the jet as in the
K\"onigl model. There is a section of the compact radio jet where the emission
is weak such that flares propagating down the jet are bright first in the core
region with a secondary increase in flux about 1.0 mas from the core. This
illustrates the importance of direct imaging to the interpretation of
multi-wavelength light curves that can be affected by several distinct
components at any given time. We discuss how the complex behaviour of the light
curves and correlations can be understood within the framework of a precessing
helical jet model.Comment: 13(+5) pages, 12 figures, accepted for publication in A&A, replaced
because of layout problem
Another look at the BL Lacertae flux and spectral variability
The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope
(WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical
frequencies. During this period, high-energy observations were performed by
XMM-Newton, Swift, and Fermi. We analyse these data with particular attention
to the calibration of Swift UV data, and apply a helical jet model to interpret
the source broad-band variability. The GASP-WEBT observations show an optical
flare in 2008 February-March, and oscillations of several tenths of mag on a
few-day time scale afterwards. The radio flux is only mildly variable. The UV
data from both XMM-Newton and Swift seem to confirm a UV excess that is likely
caused by thermal emission from the accretion disc. The X-ray data from
XMM-Newton indicate a strongly concave spectrum, as well as moderate flux
variability on an hour time scale. The Swift X-ray data reveal fast (interday)
flux changes, not correlated with those observed at lower energies. We compare
the spectral energy distribution (SED) corresponding to the 2008 low-brightness
state, which was characterised by a synchrotron dominance, to the 1997 outburst
state, where the inverse-Compton emission was prevailing. A fit with an
inhomogeneous helical jet model suggests that two synchrotron components are at
work with their self inverse-Compton emission. Most likely, they represent the
radiation from two distinct emitting regions in the jet. We show that the
difference between the source SEDs in 2008 and 1997 can be explained in terms
of pure geometrical variations. The outburst state occurred when the
jet-emitting regions were better aligned with the line of sight, producing an
increase of the Doppler beaming factor. Our analysis demonstrates that the jet
geometry can play an extremely important role in the BL Lacertae flux and
spectral variability.Comment: 12 pages, 10 figures, accepted for publication in A&
The structure and emission model of the relativistic jet in the quasar 3C 279 inferred from radio to high-energy gamma-ray observations in 2008-2010
We present time-resolved broad-band observations of the quasar 3C 279
obtained from multi-wavelength campaigns conducted during the first two years
of the Fermi Gamma-ray Space Telescope mission. While investigating the
previously reported gamma-ray/optical flare accompanied by a change in optical
polarization, we found that the optical emission appears delayed with respect
to the gamma-ray emission by about 10 days. X-ray observations reveal a pair of
`isolated' flares separated by ~90 days, with only weak gamma-ray/optical
counterparts. The spectral structure measured by Spitzer reveals a synchrotron
component peaking in the mid-infrared band with a sharp break at the
far-infrared band during the gamma-ray flare, while the peak appears in the
mm/sub-mm band in the low state. Selected spectral energy distributions are
fitted with leptonic models including Comptonization of external radiation
produced in a dusty torus or the broad-line region. Adopting the interpretation
of the polarization swing involving propagation of the emitting region along a
curved trajectory, we can explain the evolution of the broad-band spectra
during the gamma-ray flaring event by a shift of its location from ~ 1 pc to ~
4 pc from the central black hole. On the other hand, if the gamma-ray flare is
generated instead at sub-pc distance from the central black hole, the
far-infrared break can be explained by synchrotron self-absorption. We also
model the low spectral state, dominated by the mm/sub-mm peaking synchrotron
component, and suggest that the corresponding inverse-Compton component
explains the steady X-ray emission.Comment: 23 pages, 18 figures 5 tables, Accepted for publication in The
Astrophysical Journa
WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007-2008. Unveiling different emission components
In 2007-2008 we carried out a new multiwavelength campaign of the Whole Earth
Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the
XMM-Newton satellite, to study its emission properties. The source was
monitored in the optical-to-radio bands by 37 telescopes. The brightness level
was relatively low. Some episodes of very fast variability were detected in the
optical bands. The X-ray spectra are well fitted by a power law with photon
index of about 2 and photoelectric absorption exceeding the Galactic value.
However, when taking into account the presence of a molecular cloud on the line
of sight, the data are best fitted by a double power law, implying a concave
X-ray spectrum. The spectral energy distributions (SEDs) built with
simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations
suggest that the peak of the synchrotron emission lies in the near-IR band, and
show a prominent UV excess, besides a slight soft-X-ray excess. A comparison
with the SEDs corresponding to previous observations with X-ray satellites
shows that the X-ray spectrum is extremely variable. We ascribe the UV excess
to thermal emission from the accretion disc, and the other broad-band spectral
features to the presence of two synchrotron components, with their related SSC
emission. We fit the thermal emission with a black body law and the non-thermal
components by means of a helical jet model. The fit indicates a disc
temperature greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s.Comment: 11 pages, 7 figures, accepted for publication in A&
The Spectral Energy Distribution of Fermi bright blazars
(Abridged) We have conducted a detailed investigation of the broad-band
spectral properties of the \gamma-ray selected blazars of the Fermi LAT Bright
AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray
spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray
data, collected within three months of the LBAS data taking period, we were
able to assemble high-quality and quasi-simultaneous Spectral Energy
Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is
similar to that of blazars discovered at other wavelengths, clearly showing, in
the usual Log - Log F representation, the typical broad-band
spectral signatures normally attributed to a combination of low-energy
synchrotron radiation followed by inverse Compton emission of one or more
components. We have used these SEDs to characterize the peak intensity of both
the low and the high-energy components. The results have been used to derive
empirical relationships that estimate the position of the two peaks from the
broad-band colors (i.e. the radio to optical and optical to X-ray spectral
slopes) and from the gamma-ray spectral index. Our data show that the
synchrotron peak frequency is positioned between 10 and
10 Hz in broad-lined FSRQs and between and Hz in
featureless BL Lacertae objects.We find that the gamma-ray spectral slope is
strongly correlated with the synchrotron peak energy and with the X-ray
spectral index, as expected at first order in synchrotron - inverse Compton
scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton
(SSC) models cannot explain most of our SEDs, especially in the case of FSRQs
and low energy peaked (LBL) BL Lacs. (...)Comment: 85 pages, 38 figures, submitted to Ap
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