168 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: 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
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
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
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&
High cadence, linear and circular polarization monitoring of OJ 287 - Helical magnetic field in a bent jet
We present a multi-frequency, dense radio monitoring program of the blazar
OJ287 using the 100m Effelsberg radio telescope. We analyze the evolution in
total flux density, linear and circular polarization to study the jet structure
and its magnetic field geometry. The total flux density is measured at nine
bands from 2.64 GHz to 43 GHz, the linear polarization parameters between 2.64
GHz and 10.45 GHz, and the circular polarization at 4.85 GHz and 8.35 GHz. The
mean cadence is 10 days. Between MJD 57370 and 57785, OJ287 showed flaring
activity and complex linear and circular polarization behavior. The radio EVPA
showed a large clockwise (CW) rotation by ~340 with a mean rate of
-1.04 /day. Based on concurrent VLBI data, the rotation seems to
originate within the jet core at 43 GHz (projected size 0.15 mas or 0.67
pc). Moreover, optical data show a similar monotonic CW EVPA rotation with a
rate of about -1.1 /day which is superposed with shorter and faster
rotations of about 7.8 /day. The observed variability is consistent
with a polarized emission component propagating on a helical trajectory within
a bent jet. We constrained the helix arc length to 0.26 pc and radius to
0.04 pc as well as the jet bending arc length projected on the plane of the sky
to 1.9-7.6 pc. A similar bending is observed in high angular resolution
VLBI images at the innermost jet regions. Our results indicate also the
presence of a stable polarized emission component with EVPA (-10)
perpendicular to the large scale jet, suggesting dominance of the poloidal
magnetic field component. Finally, the EVPA rotation begins simultaneously with
an optical flare and hence the two might be physically connected. That optical
flare has been linked to the interaction of a secondary SMBH with the inner
accretion disk or originating in the jet of the primary.Comment: 11 pages, 9 figures, 1 table, accepted for publication in section 4.
Extragalactic astronomy of Astronomy and Astrophysics on August 21, 201
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
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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