The observational properties of the ten known AM Her systems are reviewed. A multitude of components of continuum and line radiation from these objects are outlined. We discuss the important physical processes involved in the accretion flow and funnel shock, with emphasis on the properties of the emission line regions. Other topics include the properties of the white dwarf primaries and M dwarf secondaries, the maintenance of synchronous rotation, and the implications of “dumpings” in the known orbital periods (80–115 minutes) and magnetic field strengths (20-35 megagauss).

The AM Herculis Magnetic Variables

A photometric monitoring campaign has been conducted in order to investigate the character and origin of flaring events occasionally seen in the long-term low-state light curve of the polar prototype AM Her. Four telescopes employed during 2004 May?July revealed that the events have typical duty cycles of 2%?35%, amplitudes of 0.2?0.6 mag, and typical durations of 15?90 minutes. A striking concentration of the 2004 events appears near inferior and superior conjunction of the secondary star. Interestingly, in the long-term RoboScope-monitored light curve (1990?2003), similar events are uniformly distributed in phase. AM Her's accretion geometry and the nature of its low states allow for two likely origins for the observed events, namely, residual accretion during low states and activity (flares) on the secondary star. Considering that AM Her is likely a one-pole accretor in the low states, the former requires irregular mass transfer from the secondary, resulting in random accretion bursts, with cyclotron beaming concentrating the flares into two phase intervals. On the other hand, considering the stability of the magnetic poles, this cannot address the random distribution of the events during the long-term light curve. Drifting active regions on the secondary star could explain the random distribution of the events; however, coincidence must be invoked to explain their occurrence at the observed phases of orbital conjunction. A combination of the two ideas is also discussed, in which stellar activity on the secondary star induces random accretion bursts, with cyclotron beaming then concentrating the flares into two phase intervals.

Low-State Flaring Events in AM Herculis

The binary system AM Herculis = 4U 1814 + 50 gives the first well ascertained example of an X-ray emitting magnetic white dwarf. The orbital period (3.1h) is apparent from X-ray to IR frequencies and in linear and circular polarization. Since the time of the identification of the X-ray source the system has been extensively studied. The observations (which range from 1 MeV to 20 μm) are reviewed and compared with the present theory of X-ray emitting white dwarfs.

The system AM Her = 4U 1814 + 50

Twenty-one new optical light curves, including five curves obtained in 2009 and sixteen curves detected from the AAVSO International Database spanning from 1977 to 2011, demonstrate 16 new primary minimum light times in the high state. Furthermore, seven newly found low-state transient events from 2006 to 2009 were discovered, consisting of five Gaussian-shaped events and two events with an exponential form with decay timescales of > tau((R) over dot2), which is required by numerical calculations of the secular evolution of cataclysmic variables. In order to explain the prominent periodic modulation, three plausible mechanisms-spot motion, the light travel-time effect, and magnetic active cycles-are discussed in detail.

http://iopscience.iop.org/article/10.1088/0004-637X/774/2/153/pdf

Updated photometry and orbital period analysis for the polar am herculis on the upper edge of the period gap

The mechanisms of “Noisar” phenomenon in AM Herculis-type stars are discussed. In an accretion column above the surface of the magnetized degenerate star the instability of some types may be excited, such as axi-symmetrical quasi-periodical penetration of the low-density “bulks” from the column axis to the outer parts; “boiling” with “bulks” moving inside or outside the column; “tornado” with low-density region rapidly rotating around the column axis; “switchings” of the accretion from one half of the “polar cap” to another and vice versa. The oscillations of different plasma clots (“spaghetti”) may interfer causing flux changes as well. Such “Noisar” oscillations appear in different regions of the accretion column (at the upper shock and near the column base, respectively), so hard and soft X-ray fluxes might not have correlation in their variability. The observations are in qualitative agreement with the models.



Die moglichen Ursachen des bei Sternen vom Typ AM Herculis auftretenden “Noisar”-Phanomens werden erortert. Wie gezeigt wird, kann es in der Akkretionssaule uber der Oberflache eines degenerierten Sterns mit Magnetfeld zur Anregung verschiedener Instabilitaten kommen. Dazu gehoren die axialsymmetrische quasiperiodische Verringerung der Dichte, ausgehend von der Saulenachse; ein “Kochen” mit Bewegung von Materie in die bzw. aus der Akkretionssaule; wirbelsturmartige Erscheinungen, bei denen ein Gebiet niedriger Dichte schnell um die Achse der Akkretionssaule rotiert; Umschalten der Akkretion von einer Halfte des polaren Gebiets auf die andere und umgekehrt. Die Interferenz der Oszillationen verschiedener Plasmaschlauche kann ebenfalls Anderungen des Rontgenflusses hervorrufen. Derartige “Noisar”-Oszillationen treten in unterschiedlichen Gebieten der Akkretionssaule auf (am oberen Schock und in der Nahe der Saulenbasis). Deshalb ist es moglich, das keine Korrelation zwischen der Variation von harter und weicher Rontgenstrahlung besteht. Die Beobachtungen bestatigen qualitativ die Richtigkeit des Modells.

On the mechanism of the “Noisar” phenomenon in magnetic close binary systems

Via simultaneous multicolor photometry and polarimetry of AM Herculis, we find correlations among polarization, flux, and color in the V and I bands in periodic and nonperiodic (i.e., flickering) activity. The primary minimum observed in both bands is accompanied by a blueward shift of the (B - R) color and a decrease in the absolute value of the percent circular polarization in the V band. Outside primary minimum, peaks of flickering activity tend to be associated with an increase in the absolute value of the circular polarization and the concurrent reddening of the (B - R) and (R - I) colors. The visual light curve of AM Her can be explained by a flickering, circularly polarized light source which is obscured at primary minimum, and is redder than the total system color in the URVRI bands.

W. Krzeminski

Papers

Correlated photometric and polarimetric phenomena in AM Herculis