Polarization position angle standard stars: a reassessment of θ and its variability for seventeen stars based on a decade of observations

Observations of polarization position angle (θ) standards made from 2014 to 2023 with the High Precision Polarimetric Instrument (HIPPI) and other HIPPI-class polarimeters in both hemispheres are used to investigate their variability. Multiband data were first used to thoroughly recalibrate the instrument performance by bench-marking against carefully selected literature data. A novel co-ordinate difference matrix (CDM) approach – which combines pairs of points – was then used to amalgamate monochromatic (g band) observations from many observing runs and re-determine θ for 17 standard stars. The CDM algorithm was then integrated into a fitting routine and used to establish the impact of stellar variability on the measured position angle scatter. The approach yields variability detections for stars on long time-scales that appear stable over short runs. The best position angle standards are Car, o Sco, HD 154445, HD 161056, and ι¹ Sco, which are stable to ≤0.123^◦ . Position angle variability of 0.27–0.82^◦ , significant at the 3σ level, is found for 5 standards, including the Luminous Blue Variable HD 160529 and all but one of the other B/A-type supergiants (HD 80558, HD 111613, HD 183143, and 55 Cyg), most of which also appear likely to be variable in polarization magnitude (p) – there is no preferred orientation for the polarization in these objects, which are all classified as α Cygni variables. Despite this we make six key recommendations for observers – relating to data acquisition, processing and reporting – that will allow them to use these standards to achieve < 0.1^◦ precision in the telescope position angle with similar instrumentation, and allow data sets to be combined more accurately.