Astronomers discover two new polars

Analyzing data from the Spektr-RG (SRG) space observatory and from the Zwicky Transient Facility (ZTF), astronomers at the California Institute of Technology (Caltech) and elsewhere have discovered two new polar poles. The discovery is reported in a paper published June 9 in arXiv’s pre-print repository.

Cataclysmic variables (CVs) are binary star systems made up of a white dwarf and a normal companion. They increase in brightness irregularly by a large factor and then fall back to a resting state. Polars are a subclass of catastrophic variables that are distinguished from other CVs by the presence of a very strong magnetic field in their white dwarfs†

Now, a team of astronomers led by Antonio C. Rodriguez of Caltech has found two new polars, designated ZTFJ0850+0443 and ZTFJ0926+0105. The detection is the result of comparing the eROSITA Final Equatorial Depth Survey (eFEDS) catalog with forced photometry from ZTF Data Release 5 (DR5).

“We discovered two polars: ZTFJ0850+0443 and ZTFJ0926+0105, through a crossmatch of the eFEDS dataset and ZTF archival photometry,” the researchers wrote in the paper.

According to the study, ZTFJ0850+0443 is an obscuring pole with a turnaround time of 1.72 hours, at a distance of about 3,260 light-years from Earth. Its white dwarf has a mass of about 0.81 solar masses, while the mass of its companion star has been estimated at about 0.12 solar masses. The results suggest that ZTFJ0850+0443 is probably a polar low field with a magnetic field strength of less than 10 MG.

At a distance of about 1,200 light years, ZTFJ0926+0105 is a non-eclipsing pole with an orbital period of approximately 1.48 hours. It has a more typical magnetic field strength of polars – at least 26 MG. Since ZTFJ0926+0105 is not eclipsing, the team was unable to measure the mass of its white dwarf.

The astronomers concluded that their discovery demonstrates the importance of eFEDS research as a complement to ZTF for the detection of new catastrophic variables. In addition, they added that by using ESA’s Gaia satellite, it will be possible to obtain precise brightness of the newly discovered polars. The recent Gaia Data Release 3 (DR3), published on June 13, can be very helpful in this context.

Schwope et al (2021) identified an eclipsing pool by an eROSITA/SRG crossmatch with Gaia using a proprietary eRASS dataset, the scientists noted.

Rodriguez’s team’s research is part of a larger follow-up analysis of the eFEDS/ZTF footprint. Such studies could be helpful in overcoming observational biases in previous optical-only searches for catastrophic variables, and would lead directly to accurate volume-limited studies of CVs.

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