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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]

After three years of polarimetric monitoring of blazars, the RoboPol project has uncovered several key characteristics of polarimetric rotations in the optical for these most variable sources. The most important of these is that polarization properties of the synchrotron emission in the optical appear to be directly linked with gamma-ray activity. In this paper, we discuss the evidence for this connection, as well as the broader features of polarimetric behavior in blazars that are key in making progress with theoretical modeling of blazar emission. [ABSTRACT FROM AUTHOR]