Neglected X-ray discovered polars

Abstract

We report results on the ROSAT-discovered noneclipsing short-period polars RX J0154.0−5947, RX J0600.5−2709, RX J0859.1+0537, RX J0953.1+1458, and RX J1002.2−1925 collected over 30 years. We present accurate linear orbital ephemerides that allow a correct phasing of data taken decades apart. Three of the systems show cyclotron and Zeeman lines that yield magnetic field strengths of 36 MG, 19 MG, and 33 MG for the last three targets, respectively. RX J0154.0−5947, RX J0859.1+0537, and RX J1002.2−1925 show evidence for part-time accretion at both magnetic poles, while RX J0953.1+1458 is a polar with a stable one-pole geometry. RX J1002.2−1925 shows large variations in the shapes of its light curves that we associate with an unstable accretion geometry. Nevertheless, it appears to be synchronized. We determined the bolometric soft and hard X-ray fluxes and the luminosities at the Gaia distances of the five stars. Combined with estimates of the cyclotron luminosities, we derived high-state accretion rates that range from Ṁ = 2.9 × 10−11 M⊙ yr−1 to 9.7 × 10−11 M⊙ yr−1 for white dwarf masses between 0.61 and 0.82 M⊙, in agreement with predictions based on the observed effective temperatures of white dwarfs in polars and the theory of compressional heating. Our analysis lends support to the hypothesis that different mean accretion rates appply for the subgroups of short-period polars and nonmagnetic cataclysmic variables.