The Remarkable Eclipsing Asynchronous AM Herculis Binary RX J19402-1025

Abstract

We report on two years of photometric and spectroscopic observation of the recently discovered AM Herculis star RX J19402-1025. A sharp eclipse feature is present in the optical and X-ray light curves, repeating with a period of 12116.290±0.003 s. The out-of-eclipse optical waveform contains approximately equal contributions from a signal at the same period and another signal at 12150 s. As these signals drift in and out of phase, the wave form of the light curve changes in a complex but predictable manner. After one entire "supercycle" of 50 days (the beat period between the shorter periods), the light curve returns to its initial shape. We present long-term ephemerides for each of these periods. It is highly probable that the eclipse period is the underlying orbital period, while the magnetic white dwarf rotates with P = 12150 s. The eclipses appear to be eclipses of the white dwarf by the secondary star. But there is probably also a small obscuring effect from cold gas surrounding the secondary, especially on the orbit-leading side where the stream begins to fall towards the white dwarf. The latter hypothesis can account for several puzzling effects in this star, as well as the tendency among most AM Her stars for the sharp emission-line components to slightly precede the actual motion of the secondary. The presence of eclipses in an asynchronous AM Her star provides a marvelous opportunity to study how changes in the orientation of magnetic field lines affect the accretion flows. Repeated polarimetric light curves and high-resolution studies of the emission lines are now critical to exploit this potential. Leighly et al. 1993). Among active galactic nuclei, this was the only strictly periodic signal reported at any wavelength that passes tests of persistence and phase stability. It has been difficult to understand this behavior. Madejski et al. (1993) have ended the mystery by showing that, although the period is indeed real, it belongs not to NGC 6814 but to another X-ray source 37 arcmin away in the sky. Follow-up spectroscopy and photometry of several stars in the smaller X-ray error circle revealed the nature of the interloper: a cataclysmic variable, almost certainly of the AM Herculis variety, and provisionally named RX J19402 -1025 (Madejski et al. 1993;Staubert et al. 1994; «2000= 19 h 40 m ll?6, <%ooo= -10°25'25'. '7).Although NGC 6814 is now relegated to obscurity, the new X-ray source will surely become one of the most-307

The unique magnetic cataclysmic variable V1432 Aql

“…Contrary to the other 3 systems (BY Cam, V1500 Cyg and CD Ind), the rotation period of the white dwarf exceeds the orbital period. Moreover, this object is the only one among the four objects for which eclipses and self-eclipses of the accretion column are observed (Patterson et al 1995). …”

“…For improvement of accuracy, we used the method of "artificial" (or "mean weighted") star (Andronov & Baklanov 2004;Kim et al 2004). The main comparison star is "C" of Patterson et al (1995), for which we determined V = 11. m 95 ± 0. m 03 and R = 11. m 02 ± 0. m 02 by linking to the standard near WZ Sge (Henden 2001) and applying a small color correction to the instrumental V system (Andronov et al 2006). The magnitudes are expressed as the instrumental differences between the variable star and the comparison star.…”

“…Rosen et al (1993) identified the variable ROSAT X-ray source RX J1940.1-1025 with an optical counterpart V1432 Aql. Strong He II emission lines at λ4686 and λ5411 in combination with X-ray radiation allowed investigators to classify the object as a magnetic cataclysmic variable (Patterson et al 1995). Light variations in the optical range show both the period of 12 150 s and cycle-to-cycle variability.…”

“…An extensive photometric campaign by Patterson et al (1995) allowed to determine a beat period between P spin and P orb , equal to ∼50 d . Mukai et al (2003) studied the X-ray variability as the function of phase of this beat period.…”

Time-Resolved Photometry of the Optical Counterpart of Swift J2319.4+2619

“…In such systems, of which there are currently only four known (BY Cam, V1500 Cyg, V1432 Aql, CD Ind), the white dwarf spin period and the orbital period differ by ∼1%-2% (Patterson et al 1995;Campbell & Schwope 1999). The cause of the slight asynchronism in these systems is unknown, but may be caused by recent (recorded or unrecorded) nova eruptions (V1500 Cyg is a known nova, Nova Cyg 1975), or, as suggested by Patterson et al (1995), by the difficulty in synchronizing white dwarfs at longer orbital periods (among the four known asynchronous polars, all but CD Ind have orbital periods in excess of 3.3 hr). The latter explanation would seem to be relevant to BY Cam and V1432 Aql, where the orbital periods (∼3:36 hr) and estimated white dwarf masses (∼1 M ⊙ ; Ramsay 2000) suggest via equation (2) that B ≳ 60 MG would be required to synchronize the white dwarfs in these systems.…”