We report on the X-ray observations of the eclipsing polar HY Eri ( RX J0501–0359 ) , along with its photometric , spectrophotometric , and spectropolarimetric optical variations , collected over 30 years .
With an orbital period of 2.855 h , HY Eri falls near the upper edge of the 2 - 3 h period gap .
After 2011 , the system went into a prolonged low state , continuing to accrete at a low level .
We present an accurate alias-free long-term orbital ephemeris and report a highly significant period change by 10 ms that took place over the time interval from 2011 to 2018 .
We acquired a high-quality eclipse spectrum that shows the secondary star as a dM5–6 dwarf at a distance d = 1050 \pm 110 pc .
Based on phase-resolved cyclotron and Zeeman spectroscopy , we identify the white dwarf ( WD ) in HY Eri as a two-pole accretor with nearly opposite accretion spots of 28 and 30 MG .
The Zeeman analysis of the low state spectrum reveals a complex magnetic field structure , which we fit by a multipole model .
We detected narrow emission lines from the irradiated face of the secondary star , of which Mg I \lambda 5170 with a radial velocity amplitude of K _ { 2 } ^ { \prime } = 139 \pm 10 km s ^ { -1 } ( 90 % confidence ) tracks the secondary more reliably than the narrow H \alpha line .
Based on the combined dynamical analysis and spectroscopic measurement of the angular radius of the WD , we obtain a primary mass of M _ { 1 } = 0.42 \pm 0.05 M _ { \odot } ( 90 % confidence errors ) , identifying it as a probable He WD or hybrid HeCO WD .
The secondary is a main sequence star of M _ { 2 } = 0.24 \pm 0.04 M _ { \odot } that seems to be slightly inflated .
The large distance of HY Eri and the lack of similar systems suggest a very low space density of polars with low-mass primary .
According to current theory , these systems are destroyed by induced runaway mass transfer , suggesting that HY Eri may be doomed to destruction .
Over the last 30 years , HY Eri experienced high and low states with mass transfer rates that differed by three orders of magnitude , varying between \dot { M } \simeq 10 ^ { -9 } M _ { \odot } yr ^ { -1 } and 10 ^ { -12 } M _ { \odot } yr ^ { -1 } .
At a galactic latitude of -26.1° , it is located about 500 pc below the galactic plane .