High and low states of the system AM Herculis

Abstract

Context. We investigate the distribution of optically high and low states of the system AM Herculis (AM Her). Aims. We determine the state duty cycles, and their relationships with the mass transfer process and binary orbital evolution of the system. Methods. We make use of the photographic plate archive of the Harvard College Observatory between 1890 and 1953 and visual observations collected by the American Association of Variable Star Observers between 1978 and 2005. We determine the statistical probability of the two states, their distribution and recurrence behaviors. Results. We find that the fractional high state duty cycle of the system AM Her is 63%. The data show no preference of timescales on which high or low states occur. However, there appears to be a pattern of long and short duty cycle alternation, suggesting that the state transitions retain memories. We assess models for the high/low states for polars (AM Her type systems). We propose that the white-dwarf magnetic field plays a key role in regulating the mass transfer rate and hence the high/low brightness states, due to variations in the magnetic-field configuration in the system.

Activity of the Polar AM Her (RX J1816.2+4952): A Short Review

“…A typical strength of the active regions (large loops of magnetically confined gas near the donor due to its magnetic activity [9] and star spots [13,7]) during the high state is therefore needed (the times of the quite quiescent donor are thus very rare). In the scenario proposed by [21], this statistical distribution also suggests a typical configuration of the magnetic field of the whole system (donor+WD), around which the system balances during the high state of activity.…”

“…Its activity is dominated by the large-amplitude long-term variations: alternating high and low states of the optical brightness usually on the timescales of months and years (e.g. [8,21,10]). A small accreting region (about 0.1 of the radius of the white dwarf (WD)) near the magnetic pole of this object is the source of radiation and polarization of polar via several processes: cyclotron (accretion column -mainly optical and IR emission), thermal (soft X-rays -surface of the WD heated by the accreting region), bremsstrahlung (accretion column -hard X-rays) (e.g.…”

MAGNETIC CVs AS A BRIGHT REPRESENTATIVE OF CLOSE BINARIES

“…Monitoring of eclipsing polars provides a good chance to study the properties of mass transfer in close binary stars. However, no eclipsing polars have been monitored for a long time, and only a few non-eclipsing polars have been monitored photometrically, including the brightest polar AM Her (e.g., Hessman et al 2000;Kafka & Honeycutt 2003, 2005Wu & Kiss 2008). Three of the white-light eclipse profiles of HU Aqr obtained in 2011 using the 2.4-m telescope in Yunnan observatories are shown in Fig.…”

Long-Term Monitoring of Polars

“…AM Her 4.2-4.7 minutes (Bonnet-Bidaud et al 1991, Kalomeni et al 2005) and V1309 Ori 10 minutes (Katajainen et al 2003)). In the literature especially during the last decade long period observations of polars have been intensively studied (Hessman et al 2000, Wu & Kiss 2008, Kalomeni & Yakut 2008, Kafka & Hoard 2009, Sanad 2010. Studies of the long term variations are especially important to reveal any characteristics based on stellar activity and/or mass transfer.…”