468 research outputs found
Constraining the limiting brightness temperature and Doppler factors for the largest sample of radio bright blazars
Relativistic effects dominate the emission of blazar jets complicating our
understanding of their intrinsic properties. Although many methods have been
proposed to account for them, the variability Doppler factor method has been
shown to describe the blazar populations best. We use a Bayesian hierarchical
code called {\it Magnetron} to model the light curves of 1029 sources observed
by the Owens Valley Radio Observatory's 40-m telescope as a series of flares
with an exponential rise and decay, and estimate their variability brightness
temperature. Our analysis allows us to place the most stringent constraints on
the equipartition brightness temperature i.e., the maximum achieved intrinsic
brightness temperature in beamed sources which we found to be . Using our findings we estimated the
variability Doppler factor for the largest sample of blazars increasing the
number of available estimates in the literature by almost an order of
magnitude. Our results clearly show that -ray loud sources have faster
and higher amplitude flares than -ray quiet sources. As a consequence
they show higher variability brightness temperatures and thus are more
relativistically beamed, with all of the above suggesting a strong connection
between the radio flaring properties of the jet and -ray emission.Comment: 14 pages, 8 figures, accepted for publication in AP
F-GAMMA: Variability Doppler factors of blazars from multiwavelength monitoring
Recent population studies have shown that the variability Doppler factors can
adequately describe blazars as a population. We use the flux density variations
found within the extensive radio multi-wavelength datasets of the F-GAMMA
program, a total of 10 frequencies from 2.64 up to 142.33 GHz, in order to
estimate the variability Doppler factors for 58 -ray bright sources,
for 20 of which no variability Doppler factor has been estimated before. We
employ specifically designed algorithms in order to obtain a model for each
flare at each frequency. We then identify each event and track its evolution
through all the available frequencies for each source. This approach allows us
to distinguish significant events producing flares from stochastic variability
in blazar jets. It also allows us to effectively constrain the variability
brightness temperature and hence the variability Doppler factor as well as
provide error estimates. Our method can produce the most accurate (16\% error
on average) estimates in the literature to date.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Scale invariant jets: from blazars to microquasars
Black holes, anywhere in the stellar-mass to supermassive range, are often
associated with relativistic jets. Models suggest that jet production may be a
universal process common in all black hole systems regardless of their mass.
Although in many cases observations support such hypotheses for microquasars
and Seyfert galaxies, little is known on whether boosted blazar jets also
comply with such universal scaling laws. We use uniquely rich multiwavelength
radio light curves from the F-GAMMA program and the most accurate Doppler
factors available to date to probe blazar jets in their emission rest frame
with unprecedented accuracy. We identify for the first time a strong
correlation between the blazar intrinsic broad-band radio luminosity and black
hole mass, which extends over 9 orders of magnitude down to microquasars
scales. Our results reveal the presence of a universal scaling law that bridges
the observing and emission rest frames in beamed sources and allows us to
effectively constrain jet models. They consequently provide an independent
method for estimating the Doppler factor, and for predicting expected radio
luminosities of boosted jets operating in systems of intermediate or
tens-of-solar mass black holes, immediately applicable to cases as those
recently observed by LIGO.Comment: 13 pages, 4 figures, accepted for publication in AP
Search for AGN counterparts of unidentified Fermi-LAT sources with optical polarimetry: Demonstration of the technique
The third Fermi-LAT catalog (3FGL) presented the data of the first four years
of observations from the Fermi Gamma-ray Space Telescope mission. There are
3034 sources, 1010 of which still remain unidentified. Identifying and
classifying gamma-ray emitters is of high significance with regard to studying
high-energy astrophysics. We demonstrate that optical polarimetry can be an
advantageous and practical tool in the hunt for counterparts of the
unidentified gamma-ray sources (UGSs). Using data from the RoboPol project, we
validated that a significant fraction of active galactic nuclei (AGN)
associated with 3FGL sources can be identified due to their high optical
polarization exceeding that of the field stars. We performed an optical
polarimetric survey within uncertainties of four unidentified 3FGL
sources. We discovered a previously unknown extragalactic object within the
positional uncertainty of 3FGL J0221.2+2518. We obtained its spectrum and
measured a redshift of . Using these measurements and
archival data we demonstrate that this source is a candidate counterpart for
3FGL J0221.2+2518 and most probably is a composite object: a star-forming
galaxy accompanied by AGN. We conclude that polarimetry can be a powerful asset
in the search for AGN candidate counterparts for unidentified Fermi sources.
Future extensive polarimetric surveys at high galactic latitudes (e.g.,
PASIPHAE) will allow the association of a significant fraction of currently
unidentified gamma-ray sources.Comment: accepted to A&
RoboPol: Connection between optical polarization plane rotations and gamma-ray flares in blazars
We use results of our 3 year polarimetric monitoring program to investigate
the previously suggested connection between rotations of the polarization plane
in the optical emission of blazars and their gamma-ray flares in the GeV band.
The homogeneous set of 40 rotation events in 24 sources detected by {\em
RoboPol} is analysed together with the gamma-ray data provided by {\em
Fermi}-LAT. We confirm that polarization plane rotations are indeed related to
the closest gamma-ray flares in blazars and the time lags between these events
are consistent with zero. Amplitudes of the rotations are anticorrelated with
amplitudes of the gamma-ray flares. This is presumably caused by higher
relativistic boosting (higher Doppler factors) in blazars that exhibit smaller
amplitude polarization plane rotations. Moreover, the time scales of rotations
and flares are marginally correlated.Comment: 12 pages, 16 figures, accepted to MNRA
Using variability and VLBI to measure cosmological distances
In this paper, we propose a new approach to determining cosmological distances to active Galactic nuclei (AGNs) via light travel-time arguments, which can be extended from nearby sources to very high redshift sources. The key assumption is that the variability seen in AGNs is constrained by the speed of light and therefore provides an estimate of the linear size of an emitting region. This can then be compared with the angular size measured with very long baseline interferometryer to derive a distance. We demonstrate this approach on a specific well-studied low-redshift (z = 0.0178) source 3C 84 (NGC 1275), which is the bright radio core of the Perseus Cluster. We derive an angular diameter distance including statistical errors of D-A = 72(-6)(+5) Mpc for this source, which is consistent with other distance measurements at this redshift. Possible sources of systematic errors and ways to correct for them are discussed
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