The Accretion Modes of IPs
The simplest picture of IPs is that they have a partial (and fully Keplerian) accretion disk, truncated by the magnetic field of the white dwarf. There is strong evidence of accretion disks (or at least disk-like structures) in many IPs (Hellier 1991). The X-ray orbital modulations seen in many IPs are also often (if not always) caused by azimuthal structure of the disk (Hellier et al. 1993; Parker, Norton & Mukai 2005).
The well known exception is V2400 Oph, which is a diskless accretor. That is, the primary's magnetic field is too strong to permit the formation of an accretion disk. X-ray modulations at one of two possible sidebands (\omega-\Omega or 2\omega-\Omega) are expected in diskless IPs (Wynn & King 1992), but not seen in most IPs (Hellier 1992).
There are three important numbers: the minimum distance between the ballistic trajectory from the L1 point and the white dwarf, rmin; the magnetospheric radius, rmag; and the circularization radius, rcir, where the specific angular momentum of the material in a Keplerian disk would equal that of the co-rotating material at the L1 point. A disk will definitely form if rmin > rmag, while a disk will definitely disappear if rmag > rcir. The case of rmin < rmag < rcir is tricky, since a disk should not be able to form but once formed, it can remain undisrupted. Since rmag is determined in part by the accretion rate, an episode of high accretion rate can establish a disk in this regime. Depending on the mass ratio of the binary, Pspin/Porb~0.1 corresponds to this regime.
This means that systems such as EX Hya are hard to understand in the standard, Keplerian, accretion disk picture. A series of papers based on the diamagnetic blob picture, however, explain such systems (Wynn & King 1995; Norton, Wynn & Somerscales 2004; Norton et al. 2008). So far, there are 2 other confirmed IPs in this regime; other candidates have been proposed, but not been confirmed (yet).
Then there is disk overflow accretion (Hellier 1993). For the overflow stream to directly accrete, something like this condition: rmin < rmag < rcir must be met.
In addition, a possibility that the partial disk may be misaligned with the orbital plane in IPs has been suggested.
Since this makes it a key parameter, we provide a table of confirmed IPs sorted by Pspin/Porb. We also show this graphically on the IP Home page.
Please send your comments, suggestions etc. to Koji.Mukai@nasa.gov and/or Koji.Mukai@umbc.edu