314 research outputs found
Doppler factors, Lorentz factors and viewing angles for quasars, BL Lacertae objects and radio galaxies
We have calculated variability Doppler boosting factors, Lorentz factors, and
viewing angles for a large sample of sources by using total flux density
observations at 22 and 37 GHz and VLBI data. We decomposed the flux curves into
exponential flares and determined the variability brightness temperatures of
the fastest flares. By assuming the same intrinsic brightness temperature for
each source, we calculated the Doppler boosting factors for 87 sources. In
addition we used new apparent jet speed data to calculate the Lorentz factors
and viewing angles for 67 sources. We find that all quasars in our sample are
Doppler-boosted and that the Doppler boosting factors of BL Lacertae objects
are lower than of quasars. The new Lorentz factors are about twice as high as
in earlier studies, which is mainly due to higher apparent speeds in our
analyses. The jets of BL Lacertae objects are slower than of quasars. There are
some extreme sources with very high derived Lorentz factors of the order of a
hundred. These high Lorentz factors could be real. It is also possible that the
sources exhibit such rapid flares that the fast variations have remained
undetected in monitoring programmes, or else the sources have a complicated jet
structure that is not amenable to our simple analysis. Almost all the sources
are seen in a small viewing angle of less than 20 degrees. Our results follow
the predictions of basic unification schemes for AGN.Comment: 12 pages, 14 figures, Accepted for publication in A&
Oblique Shocks As The Origin Of Radio To Gamma-ray Variability In AGN
The `shock in jet' model for cm-waveband blazar variability is revisited,
allowing for arbitrary shock orientation with respect to the jet flow
direction, and both random and ordered magnetic field. It is shown that oblique
shocks can explain events with swings in polarization position angle much less
than the 90 deg. associated with transverse structures, while retaining the
general characteristics of outbursts, including spectral behavior and level of
peak percentage polarization. Models dominated by a force-free, minimum energy
magnetic field configuration (essentially helical) display a shallow rise in
percentage polarization and frequency dependent swing in polarization position
angle not in agreement with the results of single-dish monitoring observations,
implying that the field is predominantly random in the quiescent state.
Outbursts well-explained by the `shock in jet' model are present during
gamma-ray flaring in several sources, supporting the idea that shock events are
responsible for activity from the radio to gamma-ray bands.Comment: 19 pages, 8 figures, accepted for publication in Ap
Locating the gamma-ray emission site in Fermi/LAT blazars from correlation analysis between 37 GHz radio and gamma-ray light curves
We address the highly debated issue of constraining the gamma-ray emission
region in blazars from cross-correlation analysis using discrete correlation
function between radio and gamma-ray light curves. The significance of the
correlations is evaluated using two different approaches: simulating light
curves and mixed source correlations. The cross-correlation analysis yielded 26
sources with significant correlations. In most of the sources, the gamma-ray
peaks lead the radio with time lags in the range +20 and +690 days, whereas in
sources 1633+382 and 3C 345 we find the radio emission to lead the gamma rays
by -15 and -40 days, respectively. Apart from the individual source study, we
stacked the correlations of all sources and also those based on sub-samples.
The time lag from the stacked correlation is +80 days for the whole sample and
the distance travelled by the emission region corresponds to 7 pc. We also
compared the start times of activity in radio and gamma rays of the correlated
flares using Bayesian block representation. This shows that most of the flares
at both wavebands start at almost the same time, implying a co-spatial origin
of the activity. The correlated sources show more flares and are brighter in
both bands than the uncorrelated ones.Comment: 15 pages, 8 figures and 4 tables. Published in MNRAS. Online-only
Figure 6 is available as ancillary file with this submissio
Relativistic Beaming and Flux Variability in Active Galactic Nuclei
We discuss the impact of special relativistic effects on the observed light
curves and variability duty cycles of AGNs. We model the properties of AGN
light curves at radio wavelengths using a simulated shot noise process in which
the occurrence of major flaring events in a relativistic jet is governed by
Poisson statistics. We show that flaring sources whose radiation is highly
beamed toward us are able to reach very high flux levels, but will in fact
spend most of their time in relatively low flaring states due to relativistic
contraction of flare time scales in the observer frame. The fact that highly
beamed AGNs do not return to a steady-state quiescent level between flares
implies that their weakly beamed counterparts should have highly stable flux
densities that result from a superposition of many long-term, low-amplitude
flares. The ``apparent'' quiescent flux levels of these weakly beamed AGNs
(identified in many unified models as radio galaxies) will be significantly
higher than their ''true'' quiescent (i.e., non-flaring) levels. We use Monte
Carlo simulations to investigate flux variability bias in the selection
statistics of flat-spectrum AGN samples. In the case of the Caltech-Jodrell
Flat-spectrum survey, the predicted orientation bias towards jets seen end-on
is weakened if the parent population is variable, since the highly beamed
sources have a stronger tendency to be found in low flaring states. This effect
is small, however, since highly beamed sources are relatively rare, and their
fluxes tend to be boosted sufficiently above the survey limit such that they
are selected regardless of their flaring level. We find that for larger
flat-spectrum AGN surveys with fainter flux cutoffs, variability should not be
an appreciable source of selection bias.Comment: Accepted for publication in the Astrophysical Journa
Long-term variability of radio-bright BL Lacertae objects
Radio-bright BL Lacertae objects (BLOs) are typically variable and exhibit
prominent flaring. We use a sample of 24 BLOs to get a clear idea of their
flaring behavior and to find possible commonalities in their variability
patterns. Our goal was to compare the results given by computational time
scales and the observed variability parameters determined directly from the
flux curves. Also, we wanted to find out if the BLO flares adhere to the
generalized shock model. We use long-term monitoring data from 4.8, 8, 14.5,
22, 37, 90 and 230 GHz. The structure function, discrete correlation function
and Lomb-Scargle periodogram time scales, calculated in a previous study, are
analyzed in more detail. We determine flare durations, rise and decay times,
absolute and relative peak fluxes from the monitoring data. We find that BLOs
demonstrate a wide range of variability behavior. BLOs include sources with
fast and strong variability, such as OJ 287, PKS 1749+096 and BL Lac, but also
sources with more rolling fluctuations like PKS 0735+178. The most extreme
flares can last for up to 13 years or have peak fluxes of approximately 12 Jy
in the observer's frame. When the Doppler boosting effect is taken into
account, the peak flux of a flare does not depend on the duration of the flare.
A rough analysis of the time lags and peak flux evolution indicates that BLO
flares in the mm - cm wavelengths are high-peaking, i.e., are in the adiabatic
stage. Thus, the results concur with the generalized shock model.Comment: Published in the Astronomical Journa
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&
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