Browsing by Subject "superhumps"

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Long-Term Optical Observations Of Two Lmxbs: Uw Crb (=Ms 1603+260) And V1408 Aql (=4U 1957+115)
(2012-10) Mason, Paul A.; Robinson, Edward L.; Bayless, Amanda J.; Hakala, Pasi J.; Robinson, Edward L.; Bayless, Amanda J.
We present new optical photometry of two low-mass X-ray binary stars, UW CrB (MS 1603+260) and V1408 Aql (4U 1957+115). UW CrB is an eclipsing binary and we refine its eclipse ephemeris and measure an upper limit to the rate of change of its orbital period, vertical bar P vertical bar < 4.2 x 10(-11) (unitless). The light curve of UW CrB shows optical counterparts of type I X-ray bursts. We tabulate the times, orbital phases, and fluences of 33 bursts and show that the optical flux in the bursts comes primarily from the accretion disk, not from the secondary star. The new observations are consistent with a model in which the accretion disk in UW CrB is asymmetric and precesses in the prograde direction with a period of similar to 5.5 days. The light curve of V1408 Aql has a low-amplitude modulation at its 9.33 hr orbital period. The modulation remained a nearly pure sine curve in the new data as it was in 1984 and 2008, but its mean amplitude was lower, 18% against 23% in the earlier data. A model in which the orbital modulation is caused by the varying aspect of the heated face of the secondary star continues to give an excellent fit to the light curve. We derive a much improved orbital ephemeris for the system.
The Orbital Period And Time-Variable Asymmetric Accretion Disk In The X-Ray Binary MSs 1603.6+2600 (=UW Coronae Borealis)
(2008-09) Mason, Paul A.; Robinson, Edward L.; Gray, Candace L.; Hynes, Robert I.; Robinson, Edward L.; Gray, Candace L.
We present CCD photometry of the low-mass X-ray binary UW Coronae Borealis (UW CrB). Its light curve shows eclipses at a period near 111 minutes, but the eclipses vary in depth and shape and often disappear. Restricting our analysis to the deeper eclipses, we find the orbital period to be 110: 976722 +/- 0: 000012 minutes, but the times of mideclipse can deviate by more than 0.025 in phase from the best-fit ephemeris. There is an additional large-amplitude variation with a period of 112: 58 +/- 0: 03 minutes reminiscent of the superhumps seen in the light curves of some cataclysmic variables. The variations of the eclipse morphology are not random, repeating at a period near 5.5 days, and the shape of the superhump-like modulation also varies at this period. We interpret the light curve as the eclipse of the accretion disk around the neutron star by the secondary star. The surface brightness of the accretion disk is strongly asymmetric and highly variable, producing the variations of the eclipse morphology and times of mideclipse. A model in which the distribution of surface brightness is elliptical and precesses at the 5.5 day period reproduces the eclipse depths and the times of mideclipse reasonably well. As 112.6 minutes is the beat period between 110.97672 minutes and 5.5 days, the superhump-like variability is closely related to the precessing elliptical disk, but the causal relationship is unclear.