Discovery of an Unusual High State in the Long-Term Light Curve of AM Herculis

• Published in: Publications of the Astronomical Society of the Pacific 2009-12, Vol.121 (886), p.1352-1358
• Main Authors: Kafka, S., Hoard, D. W.
• Format: Article
• Language: English
• Subjects: Accretion; Astronomical magnitude; Companion stars; Cyclotrons; Hemic system; Histograms; Light curves; Mass transfer; Starspots; Variable stars
• ISSN: 0004-6280; 1538-3873
• DOI: 10.1086/648579

The magnetic cataclysmic variable archetype AM Herculis is known to display high and low photometric states, in which it switches from Roche lobe-overflow accretion (high) to wind accretion (low). The origin of this behavior is still unknown (although it is believed to be linked to the presence of starspots located near the L1 point on the secondary star), as are the specifics of duration and frequency of the two states. During the recent (2007) transition from an exceptionally long period of persistent low state behavior back to the high state, AM Her entered a short (∼2 ∼ 2 month) intermediate state, in which its brightness remained at∼0.5 mag ∼ 0.5     mag fainter than the normal high-state level. Within the recorded history of AM Her, this was the first time the system displayed such behavior. Using data from the American Association of Variable Star Observers, we compare the characteristics of this faint high state to the subsequent normal high state, and a prior “failed high state,” in which the system briefly reached a brightness level near that of the normal high state before rapidly returning to the low state. The distribution of magnitude values attained during each state, along with the shapes of the orbital light curve in each state, support a scenario in which the specific configuration of the accretion flow between the secondary star and the white dwarf must be rapidly established at the end of a low state, even before the mass transfer rate has reached the normal high-state level.