103 research outputs found
The presence of interstellar scintillation in the 15 GHz interday variability of 1158 OVRO-monitored blazars
We have conducted the first systematic search for interday variability in a large sample of extragalactic radio sources at 15 GHz. From the sample of 1158 radio-selected blazars monitored over an ∼10 yr span by the Owens Valley Radio Observatory 40-m telescope, we identified 20 sources exhibiting significant flux density variations on 4-d time-scales. The sky distribution of the variable sources is strongly dependent on the line-of-sight Galactic H α intensities from the Wisconsin H α Mapper Survey, demonstrating the contribution of interstellar scintillation (ISS) to their interday variability. 21 per cent of sources observed through sightlines with H α intensities larger than 10  rayleighs exhibit significant ISS persistent over the ∼10 yr period. The fraction of scintillators is potentially larger when considering less significant variables missed by our selection criteria, due to ISS intermittency. This study demonstrates that ISS is still important at 15 GHz, particularly through strongly scattered sightlines of the Galaxy. Of the 20 most significant variables, 11 are observed through the Orion–Eridanus superbubble, photoionized by hot stars of the Orion OB1 association. The high-energy neutrino source TXS 0506+056 is observed through this region, so ISS must be considered in any interpretation of its short-term radio variability. J0616−1041 appears to exhibit large ∼20 per cent interday flux density variations, comparable in magnitude to that of the very rare class of extreme, intrahour scintillators that includes PKS0405−385, J1819+3845, and PKS1257−326; this needs to be confirmed by higher cadence follow-up observations
RoboPol: First season rotations of optical polarization plane in blazars
We present first results on polarization swings in optical emission of
blazars obtained by RoboPol, a monitoring program of an unbiased sample of
gamma-ray bright blazars specially designed for effective detection of such
events. A possible connection of polarization swing events with periods of high
activity in gamma rays is investigated using the dataset obtained during the
first season of operation. It was found that the brightest gamma-ray flares
tend to be located closer in time to rotation events, which may be an
indication of two separate mechanisms responsible for the rotations. Blazars
with detected rotations have significantly larger amplitude and faster
variations of polarization angle in optical than blazars without rotations. Our
simulations show that the full set of observed rotations is not a likely
outcome (probability ) of a random walk of the
polarization vector simulated by a multicell model. Furthermore, it is highly
unlikely () that none of our rotations is physically
connected with an increase in gamma-ray activity.Comment: 16 pages, 9 figure
Starlight-polarization-based tomography of the magnetized ISM: Pasiphae's line-of-sight inversion method
We present the first Bayesian method for tomographic decomposition of the
plane-of-sky orientation of the magnetic field with the use of stellar
polarimetry and distance. This standalone tomographic inversion method presents
an important step forward in reconstructing the magnetized interstellar medium
(ISM) in 3D within dusty regions. We develop a model in which the polarization
signal from the magnetized and dusty ISM is described by thin layers at various
distances. Our modeling makes it possible to infer the mean polarization
(amplitude and orientation) induced by individual dusty clouds and to account
for the turbulence-induced scatter in a generic way. We present a likelihood
function that explicitly accounts for uncertainties in polarization and
parallax. We develop a framework for reconstructing the magnetized ISM through
the maximization of the log-likelihood using a nested sampling method. We test
our Bayesian inversion method on mock data taking into account realistic
uncertainties from and as expected for the optical polarization survey
PASIPHAE according to the currently planned observing strategy. We demonstrate
that our method is effective in recovering the cloud properties as soon as the
polarization induced by a cloud to its background stars is higher than , for the adopted survey exposure time and level of systematic
uncertainty. Our method makes it possible to recover not only the mean
polarization properties but also to characterize the intrinsic scatter, thus
opening ways to characterize ISM turbulence and the magnetic field strength.
Finally, we apply our method to an existing dataset of starlight polarization
with known line-of-sight decomposition, demonstrating agreement with previous
results and an improved quantification of uncertainties in cloud properties.Comment: 28 pages, including 2 appendices, submitted to A&
Bright-Moon Sky as a Wide-Field Linear Polarimetric Flat Source for Calibration
Next-generation wide-field optical polarimeters like the Wide-Area Linear
Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes.
For efficient and accurate calibration of these instruments, wide-field
polarimetric flat sources will be essential. Currently, no established
wide-field polarimetric standard or flat sources exist. This paper tests the
feasibility of using the polarized sky patches of the size of around ten-by-ten
arcminutes, at a distance of up to 20 degrees from the Moon, on bright-Moon
nights as a wide-field linear polarimetric flat source. We observed 19 patches
of the sky adjacent to the bright-Moon with the RoboPol instrument in the
SDSS-r broadband filter. These were observed on five nights within two days of
the full-Moon across two RoboPol observing seasons. We find that for 18 of the
19 patches, the uniformity in the measured normalized Stokes parameters and
is within 0.2 %, with 12 patches exhibiting uniformity within 0.07 % or
better for both and simultaneously, making them reliable and stable
wide-field linear polarization flats. We demonstrate that the sky on
bright-Moon nights is an excellent wide-field linear polarization flat source.
Various combinations of the normalized Stokes parameters and can be
obtained by choosing suitable locations of the sky patch with respect to the
MoonComment: 8 pages including appendix, 6 figures and 3 tables. Submitted to
Astronomy and Astrophysics for review. Comments are welcom
Extreme photometric and polarimetric variability of blazar S4 0954+65 at its maximum optical and γ-ray brightness levels
In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the
optical and γ -ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope
(WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying
an upper limit to the size of the emitting region as low as 10−4 parsec. The WEBT data show rapid variability in both the degree
and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting
jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle.
All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with
the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic
reconnection. The WEBT optical data are compared to γ -ray data from the Fermi satellite. These are analysed with both fixed
and adaptive binning procedures. We show that the strong correlation between optical and γ -ray data without measurable delay
assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly
see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates
The Relativistic Jet Orientation and Host Galaxy of the Peculiar Blazar PKS 1413+135
PKS 1413+135 is one of the most peculiar blazars known. Its strange properties led to the hypothesis almost four decades ago that it is gravitationally lensed by a mass concentration associated with an intervening galaxy. It exhibits symmetric achromatic variability, a rare form of variability that has been attributed to gravitational milli-lensing. It has been classified as a BL Lac object, and is one of the rare objects in this class with a visible counterjet. BL Lac objects have jet axes aligned close to the line of sight. It has also been classified as a compact symmetric object, which have jet axes not aligned close to the line of sight. Intensive efforts to understand this blazar have hitherto failed to resolve even the questions of the orientation of the relativistic jet, and the host galaxy. Answering these two questions is important as they challenge our understanding of jets in active galactic nuclei and the classification schemes we use to describe them. We show that the jet axis is aligned close to the line of sight and PKS 1413+135 is almost certainly not located in the apparent host galaxy, but is a background object in the redshift range 0.247 < z < 0.5. The intervening spiral galaxy at z=0.247 provides a natural host for the putative lens responsible for symmetric achromatic variability and is shown to be a Seyfert 2 galaxy. We also show that, as for the radio emission, a "multizone" model is needed to account for the high-energy emission
Multiwavelength behaviour of the blazar 3C 279: decade-long study from γ-ray to radio
We report the results of decade-long (2008–2018) γ-ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, Fermi and Swift data, as well as polarimetric and spectroscopic data. The X-ray and γ-ray light curves correlate well, with no delay ≳3 h, implying general cospatiality of the emission regions. The γ-ray–optical flux–flux relation changes with activity state, ranging from a linear to a more complex dependence. The behaviour of the Stokes parameters at optical and radio wavelengths, including 43 GHz Very Long Baseline Array images, supports either a predominantly helical magnetic field or motion of the radiating plasma along a spiral path. Apparent speeds of emission knots range from 10 to 37c, with the highest values requiring bulk Lorentz factors close to those needed to explain γ-ray variability on very short time-scales. The Mg II emission line flux in the ‘blue’ and ‘red’ wings correlates with the optical synchrotron continuum flux density, possibly providing a variable source of seed photons for inverse Compton scattering. In the radio bands, we find progressive delays of the most prominent light-curve maxima with decreasing frequency, as expected from the frequency dependence of the τ = 1 surface of synchrotron self-absorption. The global maximum in the 86 GHz light curve becomes less prominent at lower frequencies, while a local maximum, appearing in 2014, strengthens toward decreasing frequencies, becoming pronounced at ∼5 GHz. These tendencies suggest different Doppler boosting of stratified radio-emitting zones in the jet
Searches for neutrinos in the direction of radio-bright blazars with the ANTARES telescope
Active galaxies, especially blazars, are among the most promising neutrino
source candidates. To date, ANTARES searches for these objects considered
GeV-TeV -ray bright blazars. Here, a statistically complete
radio-bright blazar sample is used as the target for searches of origins of
neutrinos collected by the ANTARES neutrino telescope over 13 years of
operation. The hypothesis of a neutrino-blazar directional correlation is
tested by pair counting and by a complementary likelihood-based approach. The
resulting post-trial -value is ( in the two-sided
convention), possibly indicating a correlation. Additionally, a time-dependent
analysis is performed to search for temporal clustering of neutrino candidates
as a mean of detecting neutrino flares in blazars. None of the investigated
sources alone reaches a significant flare detection level. However, the
presence of 18 sources with a pre-trial significance above indicates
a ( in the two-sided convention) detection of a
time-variable neutrino flux. An \textit{a posteriori} investigation reveals an
intriguing temporal coincidence of neutrino, radio, and -ray flares of
the J0242+1101 blazar at a ( in the two-sided convention)
level. Altogether, the results presented here suggest a possible connection of
neutrino candidates detected by the ANTARES telescope with radio-bright
blazars
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