141 research outputs found
Blazar sequence - an artefact of Doppler boosting
The blazar sequence is a scenario in which the bolometric luminosity of the
blazar governs the appearance of its spectral energy distribution. The most
prominent result is the significant negative correlation between the
synchrotron peak frequencies and the synchrotron peak luminosities of the
blazar population. Observational studies of the blazar sequence have, in
general, neglected the effect of Doppler boosting. We study the dependence of
both the synchrotron peak frequency and luminosity with Doppler-corrected
quantities. We determine the spectral energy distributions of 135 radio-bright
AGN and find the best-fit parabolic function for the distribution to quantify
their synchrotron emission. The corresponding measurements of synchrotron peak
luminosities and frequencies are Doppler-corrected with a new set of Doppler
factors calculated from variability data. The relevant correlations for the
blazar sequence are determined for these intrinsic quantities. The Doppler
factor depends strongly on the synchrotron peak frequency, the lower energy
sources being more boosted. Applying the Doppler correction to the peak
frequencies and luminosities annuls the negative correlation between the two
quantities, which becomes positive. For BL Lacertae objects, the positive
correlation is particularly strong. The blazar sequence, when defined as the
anticorrelation between the peak frequency and luminosity of the synchrotron
component of the spectral energy distribution, disappears when the intrinsic,
Doppler-corrected values are used. It is an observational phenomenon created by
variable Doppler boosting across the synchrotron peak frequency range.Comment: 9 pages, 5 figures + 2 tables. The published version with minor
changes, the main conclusions are unchange
Statistical analyses of long-term variability of AGN at high radio frequencies
We present a study of variability time scales in a large sample of Active
Galactic Nuclei at several frequencies between 4.8 and 230 GHz. We investigate
the differences of various AGN types and frequencies and correlate the measured
time scales with physical parameters such as the luminosity and the Lorentz
factor. Our sample consists of both high and low polarization quasars, BL
Lacertae objects and radio galaxies. The basis of this work is the 22 GHz, 37
GHz and 87 GHz monitoring data from the Metsahovi Radio Observatory spanning
over 25 years. In addition,we used higher 90 GHz and 230 GHz frequency data
obtained with the SEST-telescope between 1987 and 2003. Further lower frequency
data at 4.8 GHz, 8 GHz and 14.5 GHz from the University of Michigan monitoring
programme have been used. We have applied three different statistical methods
to study the time scales: The structure function, the discrete correlation
function and the Lomb-Scargle periodogram. We discuss also the differences and
relative merits of these three methods. Our study reveals that smaller flux
density variations occur in these sources on short time scales of 1-2 years,
but larger outbursts happen quite rarely, on the average only once in every 6
years. We do not find any significant differences in the time scales between
the source classes. The time scales are also only weakly related to the
luminosity suggesting that the shock formation is caused by jet instabilities
rather than the central black hole.Comment: 19 pages, 12 figures, Accepted for publication in A&
Variability of the Spectral Energy Distribution of the Blazar S5 0716+714
The emission from blazars is known to be variable at all wavelengths. The
flux variability is often accompanied by spectral changes. Spectral energy
distribution (SED) changes must be associated with changes in the spectra of
emitting electrons and/or the physical parameters of the jet. Meaningful
modeling of blazar broadband spectra is required to understand the extreme
conditions within the emission region. Not only is the broadband SED crucial,
but also information about its variability is needed to understand how the
highest states of emission occur and how they differ from the low states. This
may help in discriminating between models. Here we present the results of our
SED modeling of the blazar S5 0716+714 during various phases of its activity.
The SEDs are classified into different bins depending on the optical brightness
state of the source.Comment: 4 pages, 3 figures, contributed talk presented at the conference
Multifrequency Variability of Blazars, Guangzhou, China, September 22-24,
2010. To appear in Journal of Astrophysics and Astronomy (JAA
Multi-epoch VLBA observations of 3C 66A
We present the results of six-epoch Very Long Baseline Array (VLBA)
observations of 3C~66A. The high-resolution Very Long Baseline Interferometer
(VLBI) maps obtained at multi-frequency (2.3, 8.4, and 22.2 GHz) simultaneously
enabled us to identify the brightest compact component with the core. We find
that the spectrum of the core can be reasonably fitted by the synchrotron
self-absorption model. Our VLBA maps show that the jet of 3C~66A has two
bendings at about 1.2 and 4 mas from the core. We also give possible
identifications of our jet components with the components in previous VLBA
observations by analysing their proper motions. We find consistent differences
of the position from the core in one component between different frequencies at
six epochs.Comment: 10 pages, 5 figures, received 30 January 2007, accepted 22 March 200
Multifrequency VLBA Monitoring of 3C 273 during the INTEGRAL Campaign in 2003 - I. Kinematics of the Parsec Scale Jet from 43 GHz Data
In this first of a series of papers describing polarimetric multifrequency
Very Long Baseline Array (VLBA) monitoring of 3C 273 during a simultaneous
campaign with the INTEGRAL gamma-ray satellite in 2003, we present 5 Stokes I
images and source models at 7 mm. We show that a part of the inner jet (1-2
milliarcseconds from the core) is resolved in a direction transverse to the
flow, and we analyse the kinematics of the jet within the first 10 mas. Based
on the VLBA data and simultaneous single-dish flux density monitoring, we
determine an accurate value for the Doppler factor of the parsec scale jet, and
using this value with observed proper motions, we calculate the Lorentz factors
and the viewing angles for the emission components in the jet. Our data
indicates a significant velocity gradient across the jet with the components
travelling near the southern edge being faster than the components with more
northern path. We discuss our observations in the light of jet precession model
and growing plasma instabilities.Comment: Accepted for publication in Astronomy & Astrophysics, 16 pages, 15
figure
Long-term radio variability of AGN: flare characteristics
We have studied the flare characteristics of 55 AGN at 8 different frequency
bands between 4.8 and 230 GHz. Our extensive database enables us to study the
various observational properties of flares in these sources and compare our
results with theoretical models. We visually extracted 159 individual flares
from the flux density curves and calculated different parameters, such as the
peak flux density and duration, in all the frequency bands. The selection of
flares is based on the 22 and 37 GHz data from Mets\"ahovi Radio Observatory
and 90 and 230 GHz data from the SEST telescope. Additional lower frequency
4.8, 8, and 14.5 GHz data are from the University of Michigan Radio
Observatory. We also calculated variability indices and compared them with
earlier studies. The observations seem to adhere well to the shock model, but
there is still large scatter in the data. Especially the time delays between
different frequency bands are difficult to study due to the incomplete sampling
of the higher frequencies. The average duration of the flares is 2.5 years at
22 and 37 GHz, which shows that long-term monitoring is essential for
understanding the typical behaviour in these sources. It also seems that the
energy release in a flare is independent of the duration of the flare.Comment: 11 pages, 9 figures, 2 tables, accepted for publication in A&
A multifrequency analysis of radio variability of blazars
We have carried out a multifrequency analysis of the radio variability of
blazars, exploiting the data obtained during the extensive monitoring programs
carried out at the University of Michigan Radio Astronomy Observatory (UMRAO,
at 4.8, 8, and 14.5 GHz) and at the Metsahovi Radio Observatory (22 and 37
GHz). Two different techniques detect, in the Metsahovi light curves, evidences
of periodicity at both frequencies for 5 sources (0224+671, 0945+408, 1226+023,
2200+420, and 2251+158). For the last three sources consistent periods are
found also at the three UMRAO frequencies and the Scargle (1982) method yields
an extremely low false-alarm probability. On the other hand, the 22 and 37 GHz
periodicities of 0224+671 and 0945+408 (which were less extensively monitored
at Metsahovi and for which we get a significant false-alarm probability) are
not confirmed by the UMRAO database, where some indications of ill-defined
periods about a factor of two longer are retrieved. We have also investigated
the variability index, the structure function, and the distribution of
intensity variations of the most extensively monitored sources. We find a
statistically significant difference in the distribution of the variability
index for BL Lac objects compared to flat-spectrum radio quasars (FSRQs), in
the sense that the former objects are more variable. For both populations the
variability index steadily increases with increasing frequency. The
distribution of intensity variations also broadens with increasing frequency,
and approaches a log-normal shape at the highest frequencies. We find that
variability enhances by 20-30% the high frequency counts of extragalactic
radio-sources at bright flux densities, such as those of the WMAP and Planck
surveys.Comment: A&A accepted. 12 pages, 16 figure
Wavelet analysis of a large sample of AGN at high radio frequencies
We have studied the characteristic timescales of 80 AGNs at 22, 37 and 90 GHz
examining the properties of the wavelet method and comparing them to
traditional Fourier-based methods commonly used in astronomy. We used the
continuous wavelet transform with the Morlet wavelet to study the
characteristic timescales. We also gain information when the timescale is
present in the flux curve and if it is persistent or not. Our results show that
the sources are not periodic and changes in the timescales over a long time are
common. The property of wavelets to be able to distinguish when the timescale
has been present is superior to the Fourier-based methods. Therefore we
consider it appropriate to use wavelets when the quasi-periodicities in AGNs
are studied.Comment: 14 pages, 10 figures, A&A in pres
Relative Timing of Variability of Blazars at XâRay and Lower Frequencies
The rich Xâray light curves of blazars obtained with RXTE allow meaningful correlation analyses with longer wavelengths. This reveals strong connections of variations across the electromagnetic spectrum. In 3C 279, PKS 1510â089, and BL Lac, the characteristics of the Xâray variability change along with the projected direction of the compact jet. Outbursts in the radio, IR, or optical often precede flares at high energies. A period of pronounced variability in BL Lac in late 2000 occurs at both optical and Xâray frequencies, with the Xâray spectral index steepening. A superluminal radio knot is ejected during this event. The implication of our monitoring results is that the IR to Xâray (as well as Îłâray) emission is cospatial with the compact radio jet, most likely occurring in the superluminal knots. In the radio galaxy 3C 120, in which the Xârays probably come mainly from a hot accretionâdisk corona, the appearance of superluminal radio knots follows (by 4 weeks) dips in the Xâray emission, as in microquasars but on longer timescales. The delay implies that the core of the radio jet, as seen in mmâwave VLBA images, lies at least 0.4 pc from the central engine, consistent with models in which the jet flow accelerates far from the black hole. The quasar 3C 273 may be an interesting hybrid case in which contributions to the Xâray emission may come from both the jet and corona. The power spectral density has a lowâfrequency break that, in analogy with blackâhole binary systems, implies a mass of the central black hole of 3 â 6 Ă 108 Mâ, similar to that obtained by reverberation mapping of emissionâline variability. © 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87866/2/167_1.pd
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&
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