R. Feiler

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 programme 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 data set 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 during non-rotating periods have significantly larger amplitude and faster variations of polarization angle than blazars without rotations. Our simulations show that the full set of observed rotations is not a likely outcome (probability ≤1.5 × 10^(−2)) of a random walk of the polarization vector simulated by a multicell model. Furthermore, it is highly unlikely (∼5 × 10^(−5)) that none of our rotations is physically connected with an increase in gamma-ray activity.

The RoboPol pipeline and control system

We describe the data reduction pipeline and control system for the RoboPol project. The RoboPol project is monitoring the optical R-band magnitude and linear polarization of a large sample of active galactic nuclei that is dominated by blazars. The pipeline calibrates and reduces each exposure frame, producing a measurement of the magnitude and linear polarization of every source in the 13 arcmin × 13 arcmin field of view. The control system combines a dynamic scheduler, real-time data reduction, and telescope automation to allow high-efficiency unassisted observations.

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.

30 GHz flux density measurements of the Caltech-Jodrell flat-spectrum sources with OCRA-p

To measure the 30-GHz flux densities of the 293 sources in the Caltech-Jodrell Bank flat-spectrum (CJF) sample. The measurements are part of an ongoing programme to measure the spectral energy distributions of flat spectrum radio sources and to correlate them with the milliarcsecond structures from VLBI and other measured astrophysical properties.Methods.The 30-GHz data were obtained with a twin-beam differencing radiometer system mounted on the Torun 32-m telescope. The system has an angular resolution of 1.2�.Results.Together with radio spectral data obtained from the literature, the 30-GHz data have enabled us to identify 42 of the CJF sources as Giga-hertz Peaked Spectrum (GPS) sources. Seventeen percent of the sources have rising spectra (

30 GHz observations of sources in the Very Small Array fields

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Optical polarization map of the Polaris Flare with RoboPol

> 0) between 5 and 30 GHz.

One Centimetre Receiver Array-prototype observations of the CRATES sources at 30 GHz

Small angular scale (high l) studies of cosmic microwave background (CMB) anisotropies require accurate knowledge of the statistical properties of extragalactic sources at cm-mm wavelengths. We have used a 30 GHz dual-beam receiver (One Centimetre Receiver Array prototype) on the Torun 32-m telescope to measure the flux densities of 121 sources in Very Small Array fields selected at 15 GHz with the Ryle Telescope. We have detected 57 sources above a limiting flux density of 5 mJy, of which 31 sources have a flux density greater than 10 mJy, which is our effective completeness limit. From these measurements we derive a surface density of sources above 10 mJy at 30 GHz of 2.2 ± 0.4 deg -2 . This is consistent with the surface density obtained by Mason et al. who observed a large sample of sources selected at a much lower frequency (1.4 GHz). We have also investigated the dependence of the spectral index distribution on flux density by comparing our results with those for sources above 1 Jy selected from the Wilkinson Microwave Anisotropy Probe 22 GHz catalogue. We conclude that the proportion of steep spectrum sources increases with decreasing flux density, qualitatively consistent with the predictions of de Zotti et al. We find no evidence for an unexpected population of sources above our completeness limit of 10 mJy whose spectra rise towards high frequencies, which would affect our ability to interpret current high-resolution CMB observations at 30 GHz and above.

30 GHz observations of sources in the Very Small Array fields: 30 GHz observations of sources in VSA fields

The stages before the formation of stars in molecular clouds are poorly understood. Insights can be gained by studying the properties of quiescent clouds, such as their magnetic field structure. The plane-of-the-sky orientation of the field can be traced by polarized starlight. We present the first extended, wide-field (∼10 deg^2) map of the Polaris Flare cloud in dust-absorption induced optical polarization of background stars, using the Robotic Polarimeter (RoboPol) polarimeter at the Skinakas Observatory. This is the first application of the wide-field imaging capabilities of RoboPol. The data were taken in the R band and analysed with the automated reduction pipeline of the instrument. We present in detail optimizations in the reduction pipeline specific to wide-field observations. Our analysis resulted in reliable measurements of 641 stars with median fractional linear polarization 1.3 per cent. The projected magnetic field shows a large-scale ordered pattern. At high longitudes it appears to align with faint striations seen in the Herschel-Spectral and Photometric Imaging Receiver (SPIRE) map of dust emission (250 μm), while in the central 4–5 deg^2 it shows an eddy-like feature. The overall polarization pattern we obtain is in good agreement with large-scale measurements by Planck of the dust emission polarization in the same area of the sky.

The realignment of a black hole misaligned with its accretion disc

Knowledge of the population of radio sources in the range �?2-200 GHz is important for understanding their effects on measurements of the cosmic microwave background power spectrum. We report measurements of the 30-GHz flux densities of 605 radio sources from the Combined Radio All-sky Targeted Eight-GHz Survey (CRATES), which have been made with the One Centimetre Receiver Array-prototype (OCRA-p) on the Toru�? 32-m telescope. The flux densities of sources that were also observed by Wilkinson Microwave Anisotropy Probe (WMAP) and previous OCRA surveys are in broad agreement with those reported here, however a number of sources display intrinsic variability. We find a good correlation between the 30 GHz and Fermi gamma-ray flux densities for common sources. We examine the radio spectra of all observed sources and report a number of gigahertz-peaked and inverted spectrum sources. These measurements will be useful for comparison to those from the Low Frequency Instrument of the Planck satellite, which will make some of its most sensitive observations in the region covered here.

Survey of planetary nebulae at 30 GHz with OCRA-p

Small angular scale (high l) studies of cosmic microwave background (CMB) anisotropies require accurate knowledge of the statistical properties of extragalactic sources at cm-mm wavelengths. We have used a 30 GHz dual-beam receiver (One Centimetre Receiver Array prototype) on the Torun 32-m telescope to measure the flux densities of 121 sources in Very Small Array fields selected at 15 GHz with the Ryle Telescope. We have detected 57 sources above a limiting flux density of 5 mJy, of which 31 sources have a flux density greater than 10 mJy, which is our effective completeness limit. From these measurements we derive a surface density of sources above 10 mJy at 30 GHz of 2.2 ± 0.4 deg -2 . This is consistent with the surface density obtained by Mason et al. who observed a large sample of sources selected at a much lower frequency (1.4 GHz). We have also investigated the dependence of the spectral index distribution on flux density by comparing our results with those for sources above 1 Jy selected from the Wilkinson Microwave Anisotropy Probe 22 GHz catalogue. We conclude that the proportion of steep spectrum sources increases with decreasing flux density, qualitatively consistent with the predictions of de Zotti et al. We find no evidence for an unexpected population of sources above our completeness limit of 10 mJy whose spectra rise towards high frequencies, which would affect our ability to interpret current high-resolution CMB observations at 30 GHz and above.