I. Papamastorakis

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.

RoboPol: optical polarization-plane rotations and flaring activity in blazars

We present measurements of rotations of the optical polarization of blazars during the second year of operation of RoboPol, a monitoring programme of an unbiased sample of gamma-ray bright blazars specially designed for effective detection of such events, and we analyse the large set of rotation events discovered in two years of observation. We investigate patterns of variability in the polarization parameters and total flux density during the rotation events and compare them to the behaviour in a non-rotating state. We have searched for possible correlations between average parameters of the polarization-plane rotations and average parameters of polarization, with the following results: (1) there is no statistical association of the rotations with contemporaneous optical flares; (2) the average fractional polarization during the rotations tends to be lower than that in a non-rotating state; (3) the average fractional polarization during rotations is correlated with the rotation rate of the polarization plane in the jet rest frame; (4) it is likely that distributions of amplitudes and durations of the rotations have physical upper bounds, so arbitrarily long rotations are not realized in nature.

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.

RoboPol: the optical polarization of gamma-ray-loud and gamma-ray-quiet blazars

We present average R-band optopolarimetric data, as well as variability parameters, from the first and second RoboPol observing season. We investigate whether gamma- ray--loud and gamma-ray--quiet blazars exhibit systematic differences in their optical polarization properties. We find that gamma-ray--loud blazars have a systematically higher polarization fraction (0.092) than gamma-ray--quiet blazars (0.031), with the hypothesis of the two samples being drawn from the same distribution of polarization fractions being rejected at the 3{\sigma} level. We have not found any evidence that this discrepancy is related to differences in the redshift distribution, rest-frame R-band lu- minosity density, or the source classification. The median polarization fraction versus synchrotron-peak-frequency plot shows an envelope implying that high synchrotron- peaked sources have a smaller range of median polarization fractions concentrated around lower values. Our gamma-ray--quiet sources show similar median polarization fractions although they are all low synchrotron-peaked. We also find that the random- ness of the polarization angle depends on the synchrotron peak frequency. For high synchrotron-peaked sources it tends to concentrate around preferred directions while for low synchrotron-peaked sources it is more variable and less likely to have a pre- ferred direction. We propose a scenario which mediates efficient particle acceleration in shocks and increases the helical B-field component immediately downstream of the shock.

RoboPol: do optical polarization rotations occur in all blazars?

We present a new set of optical polarization plane rotations in blazars, observed during the third year of operation of RoboPol. The entire set of rotation events discovered during three years of observations is analysed with the aim of determining whether these events are inherent in all blazars. It is found that the frequency of the polarization plane rotations varies widely among blazars. This variation cannot be explained either by a difference in the relativistic boosting or by selection effects caused by a difference in the average fractional polarization. We conclude that the rotations are characteristic of a subset of blazars and that they occur as a consequence of their intrinsic properties.

The NGC 6426 RR Lyrae Variables and Horizontal-Branch Morphology

We present BVRI CCD photometry for 12 RR Lyrae variables, including three newly discovered ones in the Oosterhoff type II globular cluster NGC 6426. New light curves and ephemerides are presented. The mean periods of the RRab and RRc variables whose light curves are analyzed in this work are Pab = 0.70 ± 0.02 days and Pc = 0.34 ± 0.03 days, respectively. The number ratio of the RRc type variables to the total number of RR Lyrae type variables is n(c)/n(ab + c) = 0.36. The period-amplitude relation for fundamental-mode RR Lyrae variables (RRab) in NGC 6426 supports the recent conclusion of Clement & Shelton that this relation is not a function of metal abundance. Fourier decomposition of the light curves has been used to determine the mass, luminosity, and temperature for the RRc stars. Application of the formula of Jurcsik & Kovacs, which relates Fourier parameters of RRab stars to [Fe/H], yielded the value [Fe/H]= -2.16 ± 0.13 dex, which is compatible (given the errors) with the value -2.33 ± 0.15 dex, derived from the red giant branch color index developed by Hatzidimitriou et al. From star counts along the horizontal branch (HB), we obtain the Lee et al. HB color distribution index, (B-R)/(B+V+R) = 0.58 ± 0.18. The HB of NGC 6426 resembles that of NGC 5053 and M68. We find almost no faint blue stars analogous to the ones constituting the extended HB in M15.

The Age of NGC 6426, a Metal-poor Globular Cluster in the Galactic Halo

We present B, V, R, and I photometry of the metal-poor globular cluster in the galactic halo, NGC 6426. The observations were performed using the 1.3 m Telescope at Skinakas Observatory in Crete. We derived the reddening of the cluster to be E(B-V) = 0.39 ± 0.02 [E(V-I) = 0.53 ± 0.03]. The metal abundance of the cluster was estimated from the shape of the red giant branch following the techniques by Sarajedini and by Da Costa & Armandroff. It was found to be [Fe/H] = -2.33 ± 0.15 dex. The mean V magnitude of the RR Lyrae variables found in the cluster is 18.14 ± 0.02 (based on a paper in preparation). The resulting distance modulus of the cluster is 16.41 ± 0.07. Finally, we derived the relative age of NGC 6426. Using the methodology of Harris et al., we found that NGC 6426 is marginally older than M92 (by 0.7 Gyr). Following the Chaboyer, Demarque, & Sarajedini method we found NGC 6426 to have the same age as the mean metal-poor globular cluster of the galactic halo.

Optical polarization map of the Polaris Flare with RoboPol

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.

Early-time polarized optical light curve of GRB 131030A

We report the polarized optical light curve of a gamma-ray burst afterglow obtained using the RoboPol instrument. Observations began 655 s after the initial burst of gamma-rays from GRB 131030A, and continued uninterrupted for 2 h. The afterglow displayed a low, constant fractional linear polarization of p = (2.1 ± 1.6) per cent throughout, which is similar to the interstellar polarization measured on nearby stars. The optical brightness decay is consistent with a forward-shock propagating in a medium of constant density, and the low polarization fraction indicates a disordered magnetic field in the shock front. This supports the idea that the magnetic field is amplified by plasma instabilities on the shock front. These plasma instabilities produce strong magnetic fields with random directions on scales much smaller than the total observable region of the shock, and the resulting randomly-oriented polarization vectors sum to produce a low net polarization over the total observable region of the shock.