AR Ursae Majoris: The First High‐Field Magnetic Cataclysmic Variable

Abstract

We identify the luminous soft X-ray source AR UMa as a magnetic cataclysmic variable containing a white dwarf with the highest Ðeld yet detected in an accreting binary. IUE and optical spectroscopy, optical photometry, and circular polarimetry and spectropolarimetry deÐne remarkably distinct accretion states of this binary. Circular polarization is nearly absent in the high state, but the low state exhibits values which vary between 2% and 5% on the orbital period of 1.932 hr. The UV continuum contains a broad absorption feature near 1300 while optical spectropolarimetry during the low state reveals a A , number of strongly polarized dips. These are interpreted as Zeeman components of hydrogen Lya and another atmospheric species, possibly He I, in a photospheric magnetic Ðeld of D230 MG.The radial velocity curve of the low-state optical emission lines shares the period of the optical photometry and polarimetry and is phased appropriately for an origin on the irradiated secondary star. While the high state exhibits prominent UV line emission typical of the magnetic variables, the strength of the UV continuum does not vary appreciably with a change in accretion state. This, combined with the high soft X-ray luminosity and lack of circular polarization, indicates that accretion occurs largely in the form of dense Ðlaments which avoid a stando † shock and thermalize their kinetic energy below the white dwarf photosphere. We suggest that these phenomena may play a role in the apparent lack of high-Ðeld systems with easily detectable circular polarization during high-accretion states.

Phase‐resolvedHubble Space Telescope/STIS Spectroscopy of the Exposed White Dwarf in the High‐Field Polar AR Ursae Majoris

“…1) are the broad absorption troughs of the Lya n and p`Zeeman components. This conÐrms the identiÐcation of the absorption line observed at D1300 in A the IUE low-state spectrum as Lya p`split in a Ðeld of D230 MG (Schmidt et al 1996). At such Ðeld strengths, the p~component falls shortward of the wavelength range covered by STIS, and the strong decrease in Ñux observed below 1145 is due to the rapid drop of the STIS sensitivity A toward shortest wavelengths.…”

“…Moderate circular polarization is, however, observed during the low state, and, if interpreted as dichroism in the photosphere of the white dwarf, indicates a magnetic Ðeld MG. B Z 200 ConÐrmation of an extremely high Ðeld (B D 230 MG) was provided by the detection of the Lya p`Zeeman component in the IUE low-state spectrum of AR UMa (Schmidt et al 1996). Additional optical ) and EUV/X-ray observations ) constrained the binary parameters and highlighted the central role of the strong magnetic Ðeld in the accretion process.…”

“…An analysis of the Harvard and Sonneberg plate material showed that AR UMa spends most of the time in a state of low accretion activity, V D 16.5, with sporadic high states reaching up to V D 13.5 (Remillard et al 1994 ;Wenzel 1993). Schmidt et al (1996) conÐrmed AR UMa as a strongly magnetic CV (polar) with unprecedented properties : in contrast to all 1 Based on observations made with the NASA/ESA Hubble Space T elescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy Inc., under NASA contract NAS 5-26555.…”

Circular polarization survey of intermediate polars I. Northern targets in the range 17 h < RA < 23 h

“…the orbital period decreases until ∼3 h when the accretion effectively "turns off" then it resumes at ∼2 h, allows a regime where an IP may have its "true" magnetic field resurface when the accretion turns off, allow synchronization, then reappear as a polar when the accretion resumes at a lower rate. This theory also ties in with studies of isolated WDs, where peak magnetic fields of ∼10 9 G are seen, whereas in binaries the field strength is typically an order of magnitude smaller (the maximum seen is 230 MG in AR UMa Schmidt et al 1996) (Wickramasinghe & Ferrario 2000).…”

An Unusual Low State of the Polar AR UMa

“…Due to the observed high X-ray luminosity, moderate (2-5%) circular polarization during the low state (Shakhowskoj & Havelin 2000), and IUE spectral features, AR UMa was classified as a polar but the confirmation of its extremely strong magnetic field of around 230 MGs was provided by the detection of Zeeman components of Lα and presence of forbidden lines (Schmidt et al 1996, Gansicke et al 2001.…”