Polarimetric observations of white dwarfs (arXiv, tables)

Based on our research it turns out that most of white dwarfs from our sample show very small or even no polarisation at all. Thus, we were not able to contuct a statistical study of their polarisation degree. However, all white dwarfs will be described in upcoming publication on dimm polarisation standard stars.

We present polarimetric observations of selected white dwarfs. Our sample consists of white dwarfs (WD) of DA and DC spectral types in the R magnitude range from 13 to 17. We performed polarimetric observations with the RoboPol polarimeter attached to the 1.3-m telescope at the Skinakas Observatory. Together we have 74 WDs in our sample, of which almost all are low polarized WDs with polarization degree (PD) smaller than 1%, while only 2 have PD higher than 1%. There is a strong evidence that on average the isolated WDs of DC type have higher PD (with meadian PD of 0.78%) than the isolated DA type WDs (with median PD of 0.36%). On the other hand the median of the PD of isolated DA type WDs is almost the same, i.e. 0.36% as the median PD of DA type white dwarfs in binary systems, i.e. 0.33%. This shows, as expected, that there is no contribution to the PD from the companion that in most cases of WDs binary systems is a red dwarf.
Because 97% of WDs in our sample have PD lower than 1%, they can be used as faint zero–polarized standard star candidates in the R magnitude range from 13 up to 17. They cover the Northern sky between 13 hour to 24 hour in right ascension and from

Figure 1: Brightness R distribution of our white dwarfs sample. That consists of 74 WDs in total. There are 53 isolated WSs and 21 WDs in binary sysems. Isolated WDs group consists of 49 DA type WDs and 4 DC type WDs. Red vertical lines denote isolated DC type WDs.

Figure 2: Distribution of WDs in the right ascension and the declination, as well as in the Galactic longitude and the Galactic latitude in the left and right columns, respectively. There is a clear lack of WDs in our sample located in the Galactic latitude range between -20 and +20 degrees, i.e. in the location of Galactic disk. It is caused by the sky surveys that very often avoid this sky region. Red vertical lines denote isolated DC type WDs.

Figure 3: Calculated PD, PD error as well as the PD / PD error as a function of brightness in R band in the top, middle, and bottom panel, respectively. Black points denote measurements with PD / PD error > 1.41, while red circles indicate the DC type WDs. The horizontal lines show 1.41 and 1.7 levels.

Figure 4: Comparison of polarisation degree between DA and DC type white dwarfs. These two groups are clearly different. The DC type WDs show higher polarisation degree. One should be aware that there are only 4 white dwarfs of DC spectral type in the sample.

Polarimetric observations of high mass X-ray binaries

For reasons beyond our control (calibration of RINGO3) the data collected during the observation campaign in 2013 and 2014 could not be used to analyze the variability of polarization as a function of orbital phase. Only based on data collected from June 2015 to December 2015 we managed to get orbital parameters for the SAX J2103.5+4545 binary. The publication describing the results is in preparation. Apart SAX J2103.5+4545 we observed also other BeXBs: 4U 0115+634, RX J0240.4+6112, V 0332+53 and KS 1947+300.

Figure 5: Polarimetric measurements of SAX J2103.5+4545 as a function of orbital phase in blue and red RINGO3 filters along with the fitted model. Resulting orbital parameters are shown above each plot.