G. Roberts

BK Lyncis: the oldest old nova?... and a Bellwether for cataclysmic variable evolution

We summarize the results of a 20-year campaign to study the light curves of BK Lyn, a nova-like star strangely located below the 2-3 hour orbital-period gap in the family of cataclysmic variables (CVs). Two apparent superhumps dominate the nightly light curves with periods 4.6% longer, and 3.0% shorter, than the orbital period. The first appears to be associated with the star’s brighter states (V � 14), while the second appears to be present throughout and becomes very dominant in the low state (V � 15.7). It is plausible that these arise, respectively, from a prograde apsidal precession and a retrograde nodal precession of the star’s accretion disk.

Time Series Spectroscopy of Pulsating Subdwarf B Stars: PG 1605+072

We report the detection of velocity variations in the pulsating subdwarf star PG 1605+072. Oscillations are detected at the same frequencies found from photometry and have amplitudes of up to 14 km s-1 for Hβ. The strongest oscillation found in previous photometric observations is not evident in our spectroscopy or photometry, and it may be absent because of the beating of closely spaced modes. Phase differences between spectroscopy and B-magnitude photometry imply that maximum brightness occurs not long after maximum radius. We have also found evidence of variation in the observed amplitudes of five Balmer lines, with a decrease in the amplitude of the strongest mode blueward from Hβ. This effect is not expected, and a longer time series will be needed to clarify it.

A Thousand Hours of GW Librae: The Eruption and Aftermath

We report the results of a worldwide observing campaign in 2007 April, which covered the second known outburst of the dwarf nova GW Librae, as well as the aftermath in 2008 and 2010. The data consist of time-series photometry obtained from several locations around the Earth. The main eruption lasted 26 days. It began with a sharp 1 day rise to maximum light at V = 8.3, after which the star dimmed slowly for a while, then dropped off suddenly, landing at V = 15. The main eruption was followed by a slow, shallow drop toward quiescence. As expected, GW Lib showed powerful and long-lasting superhumps in its light curve at a period slightly longer than Porb. These superhumps took a long time (10 days) to appear and continued for at least 90 days after the initial eruption. This time frame agrees with the general idea that cataclysmic variables (CVs) of very short Porb are actually quite old, with small secondaries that have been exhausted by mass transfer over eons of evolution. A superhump period excess of 1.3% suggests a secondary star mass near 0.06 M⊙. A bolometric accounting of accretion light during the outburst yields a fairly good estimate of the time-averaged accretion rate; for the measured distance of 100 pc, we estimate Lbol = 8( ± 2)1031 ergs s-1, which suggests dM/dt = 1.3( ± 0.3)10-11 M⊙ yr-1. We obtained an additional 102 nights of photometry during 2008 and 2010. During both years of posteruption observation, the star was near quiescence at V ~ 16.7. Its white dwarf pulsations, famous before outburst, were missing in both years. However, in 2008 a new pulsation appeared, with a period of ~20 minutes (~73 cycles day-1) and properties not previously seen in any other stars of this type. The signal wandered slightly in frequency and amplitude, suggesting an underlying poor coherence and/or unresolved multiplet structure. This might signal a new type of pulsation in the heated white dwarf, or it could arise from the accretion disk itself. The star also commonly showed the famous but mysterious 2.1 hr signal in posteruption data. It appears, however, that the dominant (fundamental) signal is actually at 4.2 hr (~5.5 cycles day-1). The origin of this signal remains as obscure as ever. This clock seems to lose phase in a few days or less; such a low coherence suggests an origin in the accretion disk.

Rapid Oscillations in Cataclysmic Variables. XVII. 1RXS J070407 + 262501

We present a study of the recently discovered intermediate polar 1RXS J070407 + 262501, distinctive for its large-amplitude pulsed signal at P = 480 s. Radial velocities indicate an orbital period of 0.1821(2) days, and the light curves suggest 0.18208(6) days. Time-series photometry shows a precise spin period of 480.6700(4) s, decreasing at a rate of 0.096(9) ms yr-1: i.e., on a time scale =2.5×106 yr. The light curves also appear to show a mysterious signal at P = 0.263 days, which could possibly signify the presence of a superhump in this magnetic cataclysmic variable.

Orbital, Superhump, and Superorbital Periods in the Cataclysmic Variables AQ Mensae and IM Eridani

We report photometric detections of orbital and superorbital signals, and negative orbital sidebands, in the light curves of the nova-like cataclysmic variables AQ Mensae and IM Eridani. The frequencies of the orbital, superorbital, and sideband signals are 7.0686 (3), 0.263 (3), and 7.332 (3) cycles per day (c d -1 ) in AQ Mensae, and 6.870 (1), 0.354 (7), and 7.226 (1) c d -1 in IM Eridani. We also find a spectroscopic orbital frequency in IM Eridani of 6.86649 (2) c d -1 . These observations can be reproduced by invoking an accretion disc that is tilted with respect to the orbital plane. This model works well for X-ray binaries, in which irradiation by a primary neutron star can account for the disc’s tilt. A likely tilt mechanism has yet to be identified in CVs, yet the growing collection of observational evidence indicates that the phenomenon of tilt is indeed at work in this class of object. The results presented in this paper bring the number of CVs known to display signals associated with retrograde disc precession to twelve. We also find AQ Men to be an eclipsing system. The eclipse depths are highly variable, which suggests that the eclipses are grazing. This finding raises the possibility of probing variations in disc tilt by studying systematic variations in the eclipse profile.

Accretion-disc precession in UX Ursae Majoris

We thank the National Science Foundation for support of this re search (AST12-11129), and also the Mount Cuba Astronomical Foundation. Finally, we thank the American Association of Variable Star Obs ervers (AAVSO) for providing the infrast ructure and continued inspirat ion which makes programs like this possible.

Superhumps and spin-period variations in the intermediate polar RX J2133.7+5107

We thank the National Science Foundation for support of this research (AST-1211129 and AST-1615456), and NASA through HST-G0-13630. EdM acknowledges financial support from the Ministerio de Educacion, Cultura y Deporte (Spain) under the Mobility Program Salvador de Madariaga (PRX15/00521).

A 16-yr photometric campaign on the eclipsing novalike variable DW Ursae Majoris

We present an analysis of photometric observations of the eclipsing novalike variable DW UMa made by the CBA consortium between 1999 and 2015. Analysis of 372 new and 260 previously published eclipse timings reveals a 13.6 year period or quasi-period in the times of minimum light. The seasonal light curves show a complex spectrum of periodic signals: both positive and negative superhumps, likely arising from a prograde apsidal precession and a retrograde nodal precession of the accretion disc. These signals appear most prominently and famously as sidebands of the orbital frequency but the precession frequencies themselves, at 0.40 and 0.22 cycles per day, are also seen directly in the power spectrum. The superhumps are sometimes seen together and sometimes separately. The depth, width and skew of eclipses are all modulated in phase with both nodal and apsidal precession of the tilted and eccentric accretion disc. The superhumps, or more correctly the precessional motions which produce them, may be essential to understanding the mysterious SW Sextantis syndrome. Disc wobble and eccentricity can both produce Doppler signatures inconsistent with the true dynamical motions in the binary, and disc wobble might boost the mass-transfer rate by enabling the hot white dwarf to directly irradiate the secondary star.

Imaging the Photosphere and Chromosphere of the RS CVn Star, II Peg

We present simultaneous starspot photometry and high resolution Hα spectroscopy of II Peg which gives evidence of differential motions in the chromosphere.