136 research outputs found
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
The Relation between Radio Polarization and Gamma-ray Emission in AGN Jets
We have compared the parsec-scale jet linear polarization properties of the
Fermi LAT-detected and non-detected sources in the complete
flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE
sources, 30 were detected by the LAT during its first three months of
operation. We find that during the era since the launch of Fermi, the
unresolved core components of the LAT-detected jets have significantly higher
median fractional polarization at 15 GHz. This complements our previous
findings that these LAT sources have higher apparent jet speeds, brightness
temperatures and Doppler factors, and are preferentially found in higher
activity states.Comment: 6 pages, 3 figures, to appear in the proceedings of "High Energy
Phenomena In Relativistic Outflows II" (Buenos Aires, Argentina, October
26-30, 2009) International Journal of Modern Physics
Locating the γ-ray emission site in Fermi/LAT blazars – II. Multifrequency correlations
In an attempt to constrain and understand the emission mechanism of γ-rays, we perform a cross-correlation analysis of 15 blazars using light curves in millimetre, optical and γ-rays. We use discrete correlation function and consider only correlations significant at the 99 per cent level. A strong correlation was found between 37 and 95 GHz with a near-zero time delay in most of the sources, and ∼1 month or longer in the rest. A similar result was obtained between the optical and γ-ray bands. Of the 15 sources, less than 50 per cent showed a strong correlation between the millimetre and γ-ray or millimetre and optical bands. The primary reason for the lack of statistically significant correlation is the absence of a major outburst in the millimetre bands of most of the sources during the 2.5 yr time period investigated in our study. This may indicate that only the long-term variations or large flares are correlated between these bands. The variability of the sources at every waveband was also inspected using fractional rms variability (F_(var)). The F_(var) displays an increase with frequency reaching its maximum in the γ-rays
Multiband optical polarimetry of the BL Lac object PKS 2155--304. Intranight and long term variability
The polarized and total flux of the BL Lac object PKS 2155-304 were monitored
intensively and simultaneously in the optical UBVRI bands with the Turin
photopolarimeter at the CASLEO 2.15 m telescope during 4 campaigns in June,
August, November 1998 and August 1999. The effective observation time amounted
to ~47 hours. PKS 2155-304 showed a linear polarization percentage (P) usually
ranging between 3% and 7% and a polarization position angle (PA) mainly between
70 and 120 deg. The highest temporal resolution of our observations, 15
minutes, is unprecedented for polarimetric monitoring of this source, and has
allowed us to detect amplitude variations of the linear polarization percentage
from 6 to 7.5% in time scales of hours. In some nights the polarization
percentage seems to increase toward shorter wavelengths, however the polarized
spectrum does not vary significantly with time. The most remarkable variability
event occurred on 1998 June 18, when the degree of linear polarization
decreased by more than a factor 2 in one day in all bands, while the PA rotated
by 90 deg. This is consistent with the presence of two emission components, of
different polarization degree and position angle. Intranight variability of P
and PA can be interpreted with small amplitude physical or geometrical changes
within the jet. Measurements of the circular polarization over time intervals
of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin
Photopolarimeter and with a CCD camera at Cordoba Astronomical Observatory did
not show light variations correlated with those of the linearly polarized flux.Comment: 21 pages, 6 eps figures. Accepted for The Astrophysical Journal
Supplement Serie
Multiband optical polarimetry of BL Lac objects with the Nordic Optical Telescope
Optical polarization of seven selected BL Lac objects in UBVRI bands was
studied with the Nordic Optical Telescope from December 10-14, 1999. Two of
them, 3C 66A and PKS 0735+178, were monitored for 4 nights for a total
integration time of 4.75 and 5.5 hours, respectively. Other objects (1Jy
0138-097, H 0414+009, PKS 0823-223, OJ287 and BL Lac) were observed sparsely
during the run. Apart from PKS 0823-223 (more polarized than observed in the
past), the sources show levels of flux and polarization consistent with results
at previous epochs. 3C 66A and PKS 0735+178 were intensively observed during
December 11 and 12 and exhibited variability of polarization, both on
internight and intranight time scales. Wavelength dependence of polarization
has been investigated, as well as circular polarization. The results are
discussed within the standard model for BL Lacs.Comment: 11 pages, 7 figures, accepted to Astronomy & Astrophysic
Multi-frequency VLBA study of the blazar S5 0716+714 during the active state in 2004: I. Inner jet kinematics
We observed the blazar \object{0716+714} with the
VLBA during its active state in 2003-2004. In this paper we discuss
multi-frequency analysis of the inner jet (first 1 mas) kinematics. The
unprecedentedly dense time sampling allows us to trace jet components without
misidentification and to calculate the component speeds with good accuracy. In
the smooth superluminal jet we were able to identify and track three components
over time moving outwards with relatively high apparent superluminal speeds
(8.5-19.4 ), which contradicts the hypothesis of a stationary oscillating
jet in this source. Component ejections occur at a relatively high rate (once
in two months), and they are accompanied by mm-continuum outbursts.
Superluminal jet components move along wiggling trajectories, which is an
indication of actual helical motion. Fast proper motion and rapid decay of the
components suggest that this source should be observed with the VLBI at a rate
of at least once in one or two months in order to trace superluminal jet
components without confusion.Comment: 4 pages, 3 figures, Astronomy & Astrophysics Letter, language
corrections adde
Relation between millimeter wavelengths emission and high-energy emission for active galactic nuclei
After comparing the flux densities of a sample of active galactic nuclei
detected by energetic gamma-ray experiment telescope at 90 and 230 GHz with the
-ray emissions detected by Compton Gamma Ray Observatory and x-ray
emission, a strong correlation between the emissions at the millimeter
wavelength and the -ray emission is found. The average flux density of
x-ray is almost proportional to the average flux density at the millimeter
wavelength for quasars detected by energetic gamma-ray experiment telescope,
which strongly supports the previous idea that the x-ray emissions of this kind
sources are mainly produced by Synchrotron Self-Compton process.Comment: 6 pages, Chinese Physics Letters in pres
Estimating black hole masses of blazars
Estimating black hole masses of blazars is still a big challenge. Because of
the contamination of jets, using the previously suggested size -- continuum
luminosity relation can overestimate the broad line region (BLR) size and black
hole mass for radio-loud AGNs, including blazars. We propose a new relation
between the BLR size and emission line luminosity and present
evidences for using it to get more accurate black hole masses of radio-loud
AGNs. For extremely radio-loud AGNs such as blazars with weak/absent emission
lines, we suggest to use the fundamental plane relation of their elliptical
host galaxies to estimate the central velocity dispersions and black hole
masses, if their velocity dispersions are not known but the host galaxies can
be mapped. The black hole masses of some well-known blazars, such as OJ 287, AO
0235+164 and 3C 66B, are obtained using these two methods and the M -
relation. The implications of their black hole masses on other related studies
are also discussed.Comment: 7 pages, invited talk presented in the workshop on Multiwavelength
Variability of Blazars (Guangzhou, China, Sept. 22-24, 2010). To be published
in the Journal of Astrophysics and Astronom
Multifrequency Observations of the Gamma-Ray Blazar 3C 279 in Low-State during Integral AO-1
We report first results of a multifrequency campaign from radio to hard X-ray
energies of the prominent gamma-ray blazar 3C 279 during the first year of the
INTEGRAL mission. The variable blazar was found at a low activity level, but
was detected by all participating instruments. Subsequently a multifrequency
spectrum could be compiled. The individual measurements as well as the compiled
multifrequency spectrum are presented. In addition, this 2003 broadband
spectrum is compared to one measured in 1999 during a high activity period of
3C 279.Comment: 4 pages including 6 figures, to appear in: 'Proc. of the 5th INTEGRAL
Workshop', ESA SP-552, in pres
Cm-Wavelength Total Flux and Linear Polarization Properties of Radio-Loud BL Lacertae Objects
Results from a long-term program to quantify the range of behavior of the
cm-wavelength total flux and linear polarization variability properties of a
sample of 41 radio-loud BL Lac objects using weekly to tri-monthly observations
with the University of Michigan 26-m telescope operating at 14.5, 8.0, and 4.8
GHz are presented; these observations are used to identify class-dependent
differences between these BL Lacs and QSOs in the Pearson-Readhead sample. The
BL Lacs are found to be more highly variable in total flux density than the
QSOs, exhibiting changes that are often nearly-simultaneous and of comparable
amplitude at 14.5 and 4.8 GHz in contrast to the behavior in the QSOs and
supporting the existence of class-dependent differences in opacity within the
parsec-scale jet flows. Structure function analyses of the flux observations
quantify that a characteristic timescale is identifiable in only 1/3 of the BL
Lacs. The time-averaged fractional linear polarizations are only on the order
of a few percent and are consistent with the presence of tangled magnetic
fields within the emitting regions. In many sources a preferred long-term
orientation of the EVPA is present; when compared with the VLBI structural
axis, no preferred position angle difference is identified. The polarized flux
typically exhibits variability with timescales of months to a few years and
shows the signature of a propagating shock during several resolved outbursts.
The observations indicate that the source emission is predominately due to
evolving source components and support the occurrence of more frequent shock
formation in BL Lac parsec-scale flows than in QSO jets. The differences in
variability behavior and polarization between BL Lacs and QSOs can be explained
by differences in jet stability.Comment: 1 LaTex (aastex) file, 21 postscript figure files, 2 external LaTex
table files. To appear in the Astrophysical Journa
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