339 research outputs found
Optical polarisation variability of radio loud narrow line Seyfert 1 galaxies. Search for long rotations of the polarisation plane
Narrow line Seyfert 1 galaxies (NLSy1s) constitute the AGN subclass
associated with systematically smaller black hole masses. A few radio loud ones
have been detected in MeV -- GeV energy bands by Fermi and evidence for the
presence of blazar-like jets has been accumulated. In this study we wish to
quantify the temporal behaviour of the optical polarisation, fraction and
angle, for a selected sample of radio loud NLSy1s. We also search for rotations
of the polarisation plane similar to those commonly observed in blazars. We
have conducted R-band optical polarisation monitoring of a sample of 10 RL
NLSy1s 5 of which have been previously detected by Fermi. The dataset includes
observations with the RoboPol, KANATA, Perkins and Steward polarimeters. In the
cases where evidences for long rotations of the polarisation plane are found,
we carry out numerical simulations to assess the probability that they are
caused by intrinsically evolving EVPAs instead of observational noise. Even our
moderately sampled sources show indications of variability, both in
polarisation fraction and angle. For the four best sampled objects in our
sample we find multiple periods of significant polarisation angle variability.
In the two best sampled cases, namely J1505+0326 and J0324+3410, we find
indications for three long rotations. We show that although noise can induce
the observed behaviour, it is much more likely that the apparent rotation is
caused by intrinsic evolution of the EVPA. To our knowledge this is the very
first detection of such events in this class of sources. In the case of the
largest dataset (J0324+3410) we find that the EVPA concentrates around a
direction which is at 49.3\degr to the 15-GHz radio jet implying a projected
magnetic field at an angle of 40.7\degr to that axis.Comment: Accepted for publication in section 2. Astrophysical processes of
Astronomy and Astrophysic
F-GAMMA: On the phenomenological classification of continuum radio spectra variability patterns of Fermi blazars
The F-GAMMA program is a coordinated effort to investigate the physics of
Active Galactic Nuclei (AGNs) via multi-frequency monitoring of Fermi blazars.
In the current study we show and discuss the evolution of broad-band radio
spectra, which are measured at ten frequencies between 2.64 and 142 GHz using
the Effelsberg 100-m and the IRAM 30-m telescopes. It is shown that any of the
78 sources studied can be classified in terms of their variability
characteristics in merely 5 types of variability. It is argued that these can
be attributed to only two classes of variability mechanisms. The first four
types are dominated by spectral evolution and can be described by a simple
two-component system composed of: (a) a steep quiescent spectral component from
a large scale jet and (b) a time evolving flare component following the
"Shock-in-Jet" evolutionary path. The fifth type is characterised by an
achromatic change of the broad band spectrum, which could be attributed to a
different mechanism, likely involving differential Doppler boosting caused by
geometrical effects. Here we present the classification, the assumed physical
scenario and the results of calculations that have been performed for the
spectral evolution of flares.Comment: Proceedings of the conference: "The Central Kiloparsec in Galactic
Nucleic: Astronomy at High Angular Resolution 2011", August 29 - September 2,
2011, Bad Honnef, German
Full-Stokes polarimetry with circularly polarized feeds - Sources with stable linear and circular polarization in the GHz regime
We present a pipeline that allows recovering reliable information for all
four Stokes parameters with high accuracy. Its novelty relies on the treatment
of the instrumental effects already prior to the computation of the Stokes
parameters contrary to conventional methods, such as the M\"uller matrix one.
The instrumental linear polarization is corrected across the whole telescope
beam and significant Stokes and can be recovered even when the recorded
signals are severely corrupted. The accuracy we reach in terms of polarization
degree is of the order of 0.1-0.2 %. The polarization angles are determined
with an accuracy of almost 1. The presented methodology was applied
to recover the linear and circular polarization of around 150 Active Galactic
Nuclei. The sources were monitored from July 2010 to April 2016 with the
Effelsberg 100-m telescope at 4.85 GHz and 8.35 GHz with a cadence of around
1.2 months. The polarized emission of the Moon was used to calibrate the
polarization angle. Our analysis showed a small system-induced rotation of
about 1 at both observing frequencies. Finally, we identify five
sources with significant and stable linear polarization; three sources remain
constantly linearly unpolarized over the period we examined; a total of 11
sources have stable circular polarization degree and four of
them with non-zero . We also identify eight sources that maintain
a stable polarization angle over the examined period. All this is provided to
the community for polarization observations reference. We finally show that our
analysis method is conceptually different from the traditionally used ones and
performs better than the M\"uller matrix method. Although it was developed for
a system equipped with circularly polarized feeds it can easily be modified for
systems with linearly polarized feeds as well.Comment: 19 pages, 17 figures, accepted for publication in Astronomy &
Astrophysics on May 30, 201
F-GAMMA: Multi-frequency radio monitoring of Fermi blazars. The 2.64 to 43 GHz Effelsberg light curves from 2007-2015
The advent of the Fermi-GST with its unprecedented capability to monitor the
entire 4 pi sky within less than 2-3 hours, introduced new standard in time
domain gamma-ray astronomy. To explore this new avenue of extragalactic physics
the F-GAMMA programme undertook the task of conducting nearly monthly,
broadband radio monitoring of selected blazars from January 2007 to January
2015. In this work we release all the light curves at 2.64, 4.85, 8.35, 10.45,
14.6, 23.05, 32, and 43 GHz and present first order derivative data products
after all necessary post-measurement corrections and quality checks; that is
flux density moments and spectral indices. The release includes 155 sources.
The effective cadence after the quality flagging is around one radio SED every
1.3 months. The coherence of each radio SED is around 40 minutes. The released
dataset includes more than measurements. The median fractional
error at the lowest frequencies (2.64-10.45 GHz) is below 2%. At the highest
frequencies (14.6-43 GHz) with limiting factor of the atmospheric conditions,
the errors range from 3% to 9%, respectively.Comment: Accepted for publication in Section: Catalogs and data of Astronomy &
Astrophysic
Radio jet emission from GeV-emitting narrow-line Seyfert 1 galaxies
We studied the radio emission from four radio-loud and gamma-ray-loud
narrow-line Seyfert 1 galaxies. The goal was to investigate whether a
relativistic jet is operating at the source, and quantify its characteristics.
We relied on the most systematic monitoring of such system in the cm and mm
radio bands which is conducted with the Effelsberg 100 m and IRAM 30 m
telescopes and covers the longest time-baselines and the most radio frequencies
to date. We extract variability parameters and compute variability brightness
temperatures and Doppler factors. The jet powers were computed from the light
curves to estimate the energy output. The dynamics of radio spectral energy
distributions were examined to understand the mechanism causing the
variability. All the sources display intensive variability that occurs at a
pace faster than what is commonly seen in blazars. The flaring events show
intensive spectral evolution indicative of shock evolution. The brightness
temperatures and Doppler factors are moderate, implying a mildly relativistic
jet. The computed jet powers show very energetic flows. The radio polarisation
in one case clearly implies a quiescent jet underlying the recursive flaring
activity. Despite the generally lower flux densities, the sources appear to
show all typical characteristics seen in blazars that are powered by
relativistic jets.Comment: Accepted for publication in 4 - Extragalactic astronomy of Astronomy
and Astrophysic
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&
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
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