415 research outputs found
The connection between the 15 GHz radio and gamma-ray emission in blazars
Since mid-2007 we have carried out a dedicated long-term monitoring programme
at 15 GHz using the Owens Valley Radio Observatory 40 meter telescope. One of
the main goals of this programme is to study the relation between the radio and
gamma-ray emission in blazars and to use it as a tool to locate the site of
high energy emission. Using this large sample of objects we are able to
characterize the radio variability, and study the significance of correlations
between the radio and gamma-ray bands. We find that the radio variability of
many sources can be described using a simple power law power spectral density,
and that when taking into account the red-noise characteristics of the light
curves, cases with significant correlation are rare. We note that while
significant correlations are found in few individual objects, radio variations
are most often delayed with respect to the gamma-ray variations. This suggests
that the gamma-ray emission originates upstream of the radio emission. Because
strong flares in most known gamma-ray-loud blazars are infrequent, longer light
curves are required to settle the issue of the strength of radio-gamma
cross-correlations and establish confidently possible delays between the two.
For this reason continuous multiwavelength monitoring over a longer time period
is essential for statistical tests of jet emission models.Comment: To appear in the Proceedings of the IAU Symposium No. 313:
"Extragalactic jets from every angle," Galapagos, Ecuador, 15-19 September
2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska (Eds.),
Cambridge University Pres
The RoboPol optical polarization survey of gamma-ray-loud blazars
We present first results from RoboPol, a novel-design optical polarimeter operating at the Skinakas Observatory in Crete. The data, taken during the 2013 May–June commissioning of the instrument, constitute a single-epoch linear polarization survey of a sample of gamma-ray-loud blazars, defined according to unbiased and objective selection criteria, easily reproducible in simulations, as well as a comparison sample of, otherwise similar, gamma-ray-quiet blazars. As such, the results of this survey are appropriate for both phenomenological population studies and for tests of theoretical population models. We have measured polarization fractions as low as 0.015 down to R-mag of 17 and as low as 0.035 down to 18 mag. The hypothesis that the polarization fractions of gamma-ray-loud and gamma-ray-quiet blazars are drawn from the same distribution is rejected at the 3σ level. We therefore conclude that gamma-ray-loud and gamma-ray-quiet sources have different optical polarization properties. This is the first time this statistical difference is demonstrated in optical wavelengths. The polarization fraction distributions of both samples are well described by exponential distributions with averages of ⟨p⟩=6.4^(+0.9)_(−0.8)×10^(−2) for gamma-ray-loud blazars, and ⟨p⟩=3.2^(+2.0)_(−1.1)×10^(−2) for gamma-ray-quiet blazars. The most probable value for the difference of the means is 3.4^(+1.5)_(−2.0)×10^(−2). The distribution of polarization angles is statistically consistent with being uniform
The F-GAMMA program: Multi-frequency study of Active Galactic Nuclei in the Fermi era. Program description and the first 2.5 years of monitoring
To fully exploit the scientific potential of the Fermi mission, we initiated
the F-GAMMA program. Between 2007 and 2015 it was the prime provider of
complementary multi-frequency monitoring in the radio regime. We quantify the
radio variability of gamma-ray blazars. We investigate its dependence on source
class and examine whether the radio variability is related to the gamma-ray
loudness. Finally, we assess the validity of a putative correlation between the
two bands. The F-GAMMA monitored monthly a sample of about 60 sources at up to
twelve radio frequencies between 2.64 and 228.39 GHz. We perform a time series
analysis on the first 2.5-year dataset to obtain variability parameters. A
maximum likelihood analysis is used to assess the significance of a correlation
between radio and gamma-ray fluxes. We present light curves and spectra
(coherent within ten days) obtained with the Effelsberg 100-m and IRAM 30-m
telescopes. All sources are variable across all frequency bands with amplitudes
increasing with frequency up to rest frame frequencies of around 60 - 80 GHz as
expected by shock-in-jet models. Compared to FSRQs, BL Lacs show systematically
lower variability amplitudes, brightness temperatures and Doppler factors at
lower frequencies, while the difference vanishes towards higher ones. The time
scales appear similar for the two classes. The distribution of spectral indices
appears flatter or more inverted at higher frequencies for BL Lacs. Evolving
synchrotron self-absorbed components can naturally account for the observed
spectral variability. We find that the Fermi-detected sources show larger
variability amplitudes as well as brightness temperatures and Doppler factors,
than non-detected ones. Flux densities at 86.2 and 142.3 GHz correlate with 1
GeV fluxes at a significance level better than 3sigma, implying that gamma rays
are produced very close to the mm-band emission region.Comment: Accepted for publication in section 4. Extragalactic astronomy of
Astronomy and Astrophysics (18 pages, 9 figures
Far infrared properties of PbTe doped with Hg
Single crystal samples of PbTe doped with Hg were grown using the Bridgman method. Far infrared reflectivity spectra were measured at room temperature for samples with 0.5 at. % Hg; 0.9 at. % Hg and 1.4 at. % Hg. The plasma frequency decreased when PbTe was doped with Hg and it was lowest for the PbTe sample doped with 0.5 at. % Hg. The values of the determined optical free carrier mobility increased and was the highest for PbTe doped with 0.5 at. % Hg
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&
A luminosity constraint on the origin of unidentified high energy sources
The identification of point sources poses a great challenge for the high
energy community. We present a new approach to evaluate the likelihood of a set
of sources being a Galactic population based on the simple assumption that
galaxies similar to the Milky Way host comparable populations of gamma-ray
emitters. We propose a luminosity constraint on Galactic source populations
which complements existing approaches by constraining the abundance and spatial
distribution of any objects of Galactic origin, rather than focusing on the
properties of a specific candidate emitter. We use M31 as a proxy for the Milky
Way, and demonstrate this technique by applying it to the unidentified EGRET
sources. We find that it is highly improbable that the majority of the
unidentified EGRET sources are members of a Galactic halo population (e.g.,
dark matter subhalos), but that current observations do not provide any
constraints on all of these sources being Galactic objects if they reside
entirely in the disk and bulge. Applying this method to upcoming observations
by the Fermi Gamma-ray Space Telescope has the potential to exclude association
of an even larger number of unidentified sources with any Galactic source
class.Comment: 18 pages, 4 figures, to appear in JPhys
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&
15 GHz Monitoring of Gamma-ray Blazars with the OVRO 40 Meter Telescope in Support of Fermi
We present results from the first two years of our fast-cadence 15 GHz
gamma-ray blazar monitoring program, part of the F-GAMMA radio monitoring
project. Our sample includes the 1158 blazars north of -20 degrees declination
from the Candidate Gamma-Ray Blazar Survey (CGRaBS), which encompasses a
significant fraction of the extragalactic sources detected by the Fermi
Gamma-ray Space Telescope. We introduce a novel likelihood analysis for
computing a time series variability amplitude statistic that separates
intrinsic variability from measurement noise and produces a quantitative error
estimate. We use this method to characterize our radio light curves. We also
present results indicating a statistically significant correlation between
simultaneous average 15 GHz radio flux density and gamma-ray photon flux.Comment: 5 pages, 7 figures; 2009 Fermi Symposium; eConf Proceedings C09112
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
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