432 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
Demonstration of magnetic field tomography with starlight polarization towards a diffuse sightline of the ISM
The availability of large datasets with stellar distance and polarization
information will enable a tomographic reconstruction of the
(plane-of-the-sky-projected) interstellar magnetic field in the near future. We
demonstrate the feasibility of such a decomposition within a small region of
the diffuse ISM. We combine measurements of starlight (R-band) linear
polarization obtained using the RoboPol polarimeter with stellar distances from
the second Gaia data release. The stellar sample is brighter than 17 mag in the
R band and reaches out to several kpc from the Sun. HI emission spectra reveal
the existence of two distinct clouds along the line of sight. We decompose the
line-of-sight-integrated stellar polarizations to obtain the mean polarization
properties of the two clouds. The two clouds exhibit significant differences in
terms of column density and polarization properties. Their mean
plane-of-the-sky magnetic field orientation differs by 60 degrees. We show how
our tomographic decomposition can be used to constrain our estimates of the
polarizing efficiency of the clouds as well as the frequency dependence of the
polarization angle of polarized dust emission. We also demonstrate a new method
to constrain cloud distances based on this decomposition. Our results represent
a preview of the wealth of information that can be obtained from a tomographic
map of the ISM magnetic field.Comment: 25 pages, 14 figures, published in ApJ, data appear in journa
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
37th International Cosmic Ray Conference (ICRC2021)
The detection of the flaring gamma-ray blazar TXS 0506+056 in spatial and temporal coincidence with the high-energy neutrino IC-170922A represents a milestone for multi-messenger astronomy. The prompt multi-wavelength coverage from several ground- and space-based facilities of this special event was enabled thanks to the key role of the Fermi-Large Area Telescope (LAT), continuously monitoring the gamma-ray sky. Exceptional variable and transient events, such as bright gamma-ray flares of blazars, are regularly reported to the whole astronomical community to enable prompt multi-wavelength observations of the astrophysical sources. As soon as realtime IceCube high-energy neutrino event alerts are received, the relevant positions are searched, at multiple timescales, for gamma-ray activity from known sources and newly detected emitters positionally consistent with the neutrino localization.In this contribution, we present an overview of follow-up activities and strategies for the realtime neutrino alerts with the Fermi-LAT, focusing on some interesting coincidences observed with gamma-ray sources. We will also discuss future plans and improvements in the strategies for the identification of gamma-ray counterparts of single high-energy neutrinos.</p
The hunt for extraterrestrial high-energy neutrino counterparts
The origin of Petaelectronvolt (PeV) astrophysical neutrinos is fundamental
to our understanding of the high-energy Universe. Apart from the technical
challenges of operating detectors deep below ice, oceans, and lakes, the
phenomenological challenges are even greater than those of gravitational waves;
the sources are unknown, hard to predict, and we lack clear signatures.
Neutrino astronomy therefore represents the greatest challenge faced by the
astronomy and physics communities thus far. The possible neutrino sources range
from accretion disks and tidal disruption events, to relativistic jets and
galaxy clusters with blazar TXS~0506+056 the most compelling association thus
far. Since that association, immense effort has been put into proving or
disproving that jets are indeed neutrino emitters, but to no avail. By
generating simulated neutrino counterpart samples, we explore the potential of
detecting a significant correlation of neutrinos with jets from active galactic
nuclei. We find that, given the existing challenges, even our best experiments
could not have produced a result. Larger programs over the next few
years will be able to detect a significant correlation only if the brightest
radio sources, rather than all jetted active galactic nuclei, are neutrino
emitters. We discuss the necessary strategies required to steer future efforts
into successful experiments.Comment: 8 pages, 1 figure, 1 table, accepted for publication in A&
Compact Symmetric Objects -- I Towards a Comprehensive Bona Fide Catalog
Compact Symmetric Objects (CSOs) are jetted Active Galactic Nuclei (AGN) with
overall projected size <1 kpc. The classification was introduced to distinguish
these objects from the majority of compact jetted-AGN in centimeter wavelength
very long baseline interferometry observations, where the observed emission is
relativistically boosted towards the observer. The original classification
criteria for CSOs were: (i) evidence of emission on both sides of the center of
activity, and (ii) overall size <1 kpc. However some relativistically boosted
objects with jet axes close to the line of sight appear symmetric and have been
mis-classified as CSOs, thereby undermining the CSO classification. This is
because two essential CSO properties, pointed out in the original papers, have
been neglected: (iii) low variability, and (iv) low apparent speeds along the
jets. As a first step towards creating a comprehensive catalog of ``bona fide''
CSOs, we identify 79 bona fide CSOs, including 15 objects claimed as confirmed
CSOs here for the first time, that match the CSO selection criteria. This
sample of bona fide CSOs can be used for astrophysical studies of CSOs without
contamination by mis-classified CSOs. We show that the fraction of CSOs in
complete flux density limited AGN samples with S >700 mJy is
between % and %.Comment: 28 pages, 9 figures, 3 tables, accepted for publicatio
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
Compact Symmetric Objects -- II Confirmation of a Distinct Population of High-Luminosity Jetted Active Galaxies
Compact Symmetric Objects (CSOs) are compact (<1 kpc), jetted Active Galactic
Nuclei (AGN), whose jet axes are not aligned close to the line of sight, and
whose observed emission is not predominantly relativistically boosted towards
us. Two classes of CSOs have previously been identified: approximately one
fifth are edge-dimmed and designated as CSO 1s, while the rest are edge
brightened and designated as CSO 2s. This paper focuses almost exclusively on
CSO 2s. Using complete samples of CSO 2s we present three independent lines of
evidence, based on their relative numbers, redshift distributions, and size
distributions, which show conclusively that the vast majority (> 99%) of CSO 2s
do not evolve into larger-scale radio sources. These CSO 2s belong to a
distinct population of jetted-AGN, which should be characterized as
``short-lived'' compared to the classes of larger jetted-AGN, as opposed to
``young''. We show that there is a sharp upper cutoff in the CSO 2 size
distribution at pc. The distinct differences between most CSO 2s
and other jetted-AGN provides a crucial new time domain window on the formation
and evolution of relativistic jets in AGN and the supermassive black holes that
drive them.Comment: 29 pages, 10 figures, 7 tables, accepted for publicatio
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