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Fermi -Large Area Telescope (LAT) observations provide continuous and regularly sampled measurements of gamma-ray photon flux for hundreds of blazars. Many of these light curves, spanning almost 15 yr, have been thoroughly examined for periodicity in multiple studies. However, the possibility that blazars may exhibit irregularly repeating flaring patterns in their gamma-ray light curves has not been systematically explored. In this study, we aim to find repeating episodes of flaring activity in the 100 brightest blazars using Fermi-LAT light curves with various integration times. We use a Bayesian Blocks representation to convert the time series into strings of symbols and search for repeating sub-strings using a fuzzy search algorithm. As a result, we identify 27 repeated episodes in the gamma-ray light curves of 10 blazars. We find that the patterns are most likely produced in structured jets composed of a fast spine and a slower sheath. When individual emission features propagate in the spine, they scatter seed photons produced in the non-uniform sheath through the inverse Compton mechanism, resulting in a set of gamma-ray flares with a similar profile every such passage. Additionally, we explore the theoretically predicted possibility that the spine-sheath structure facilitates the production of high-energy neutrinos in blazar jets. Using the catalogue of track-like events detected by the IceCube neutrino telescope, we find evidence supporting this hypothesis at a 2.8σ significance level. [ABSTRACT FROM AUTHOR]

We performed observations of 28 white dwarfs (WDs) as the second part of our WD optical linear polarimetric survey. Our study shows that highly linear polarized WDs are very rare. However, three systems, WD1336+123 (DB+dM4), WD1415+234 (DBA) and WD1619+123 (DA+dG0), show linear polarization degree higher than 1 per cent. In particular, WD1415+234 with polarization degree ∼2.6 per cent is the strongest polarized WD in our total sample of 101 objects. We found that the median polarization degree of isolated DA WDs and DB WDs is similar but lower than the median polarization of isolated DC WDs. The DB WDs in binaries seem to be more polarized than binaries containing DA+dM or double-degenerated systems (DDSs) with DA component. Additionally, we present for the first time, the measurements of optical linear polarization of both components in two common proper motion binary systems (CPMB) systems: WD1336+123 and the Sirius-like system WD2129+000. Because the vast majority of WDs have very low polarization degree (<0.5 per cent), they can be used as faint linear polarization standards. Our sample covers SDSS r magnitude range from 13 to 17 and right ascension range from 13h to 24h, with the extension from 1h to 3h. [ABSTRACT FROM AUTHOR]

We present the first linear polarimetric survey of white dwarfs (WDs). Our sample consists of WDs of DA and DC spectral types in the SDSS r magnitude range from 13 to 17. We performed polarimetric observations using the RoboPol polarimeter attached to the 1.3-m telescope at Skinakas Observatory. We have 74 WDs in our sample, of which almost all are low-polarized WDs with a polarization degree (PD) lower than 1 per cent; only two have a PD higher than 1 per cent. There is evidence that on average isolated DC-type WDs have a higher PD (with a median PD of 0.78 per cent) than isolated DA-type WDs (with a median PD of 0.36 per cent). On the other hand, the median PD of isolated DA-type WDs is almost the same (i.e. 0.36 per cent) as the median PD of DA-type WDs in binary systems with red dwarfs (dM type; i.e. 0.33 per cent). This shows, as expected, that there is no contribution to the PD from the companion if the WD companion is a red dwarf, which is the most common situation for WDbinary systems. We do not find differences in the PD between magnetic and non-magnetic WDs. Because 97 per cent of WDs in our sample have a PD lower than 1 per cent, they can be used as faint zero-polarized standard stars in the magnitude range from 13 to 17 of the SDSS r filter. They cover the Northern sky between 13h and 23h in right ascension and between -11° and 78° in declination. In addition, we found that for low extinction values (<0.04), the best model that describes the dependence of the PD on E(B - V) is given by the equation PDmax,ISM[per cent] = 0.65 E(B - V)0.12. [ABSTRACT FROM AUTHOR]

The blazar 1156+295 was active at γ-ray energies, exhibiting three prominent flares during the year 2010. Here, we present results using the combination of broad-band (X-ray through mm single-dish) monitoring data and radio-band imaging data at 43 GHz on the connection of γ-ray events to the ejections of superluminal components and other changes in the jet of 1156+295. The kinematics of the jet over the interval 2007.0-2012.5 using 43 GHz Very Long Baseline Array observations reveal the presence of four moving and one stationary component in the inner region of the blazar jet. The propagation of the third and fourth components in the jet corresponds closely in time to the active phase of the source in γ-rays. We briefly discuss the implications of the structural changes in the jet for the mechanism of γ-ray production during bright flares. To localize the γ-ray emission site in the blazar, we performed the correlation analysis between the 43 GHz radio core and the γ-ray light curve. The time lag obtained from the correlation constrains the γ-ray emitting region in the parsec-scale jet. [ABSTRACT FROM AUTHOR]