Phase-resolved HST/STIS ultraviolet spectroscopy of the high-field polar AR UMa confirms that the white dwarf photospheric { Ly \alpha } Zeeman features are formed in a magnetic field of \sim 200 MG .
In addition to the { Ly \alpha } \pi and \sigma ^ { + } components , we detect the forbidden hydrogen 1 s _ { 0 } \rightarrow 2 s _ { 0 } transition , which becomes “ enabled ” in the presence of both strong magnetic and electric fields .
Overall , the combined ultraviolet and optical low state spectrum is similar to that of the single white dwarf PG 1031+234 , in that the optical continuum has a steeper slope than the ultraviolet continuum and that the depth of the { Ly \alpha } Zeeman lines reaches only 30 - 50 % of the continuum level .
Our attempt in fitting the low state data with single temperature magnetic white dwarf models remains rather unsatisfactory , indicating either a shortcoming in the present models or a new physical process acting in AR UMa .
As a result , our estimate of the white dwarf temperature remains somewhat uncertain , \mbox { $T _ { wd } $ } = 20 000 \pm 5000 K. We detect a broad emission bump centered at \sim 1445 Å and present throughout the entire binary orbit , and a second bump near \sim 1650 Å , which appears only near the inferior conjunction of the secondary star .
These are suggestive of low harmonic cyclotron emission produced by low-level ( \dot { M } \sim 10 ^ { -13 } \mbox { $M _ { \odot } $ } yr ^ { -1 } ) accretion onto both magnetic poles .
However , there is no evidence in the power spectrum of light variations for accretion in gas blobs .
The derived field strengths are B \sim 240 MG and B \gtrsim 160 MG for the northern and the southern pole , respectively , broadly consistent with the field derived from the Zeeman lines .
The observed { Ly \alpha } emission line shows a strong phase dependence with maximum flux and redshift near orbital phase \phi \sim 0.3 , strongly indicating an origin on the trailing hemisphere of the secondary star .
An additional { Ly \alpha } absorption feature with similar phasing as the { Ly \alpha } emission , but a \sim 700 km s ^ { -1 } blueshift could tentatively be ascribed to absorption of white dwarf emission in a moderately fast wind .
Finally , the high signal-to-noise STIS data provide important information on the intergalactic absorption toward AR UMa .
We derive a column density of neutral hydrogen of \mbox { $N _ { H } $ } = ( 1.1 \pm 1.0 ) \times 10 ^ { 18 } cm ^ { -2 } , the lowest of any known polar , making AR UMa an excellent candidate for further EUV observations .