Hα Time Series Photometry of Cataclysmic Variables

Cataclysmic variable stars (white dwarf binaries) have multiple light sources that significantly vary in intensity over time. The most significant are the secondary star, the white dwarf, the accretion disk around the white dwarf, the hot spot1) where the inflowing matter from the secondary impinges on the accretion disk, and the gas cloud surrounding the binary pair. Depending on the dynamics of the system, these sources can show significant Hα absorption or emission spectra. Plotting these changes in time can be helpful in studying the dynamics of the system.

To obtain a significantly short time series resolution (needed to evaluate short period CVs), exposures need to be comparably short. And for intensity measurement purposes, the Hα intensity of comparison stars has to be measured as well. This seems to preclude the use of spectrographs. But since the significant data is expressed in the relatively narrow band of the Hα spectral line, photometric evaluation of the system using a narrowband (≤ 7 nm) Hydrogen Alpha filter (656 nm) can provide useful information.

To evaluate the Hα photometry with respect to the orbital position of the WD, it's best to have simultaneous observations in the visible light spectrum and Hα spectra. If this isn't feasible, and when the period of the system is well established, then a series of observations in visible light can be used to establish a reference time for the current period of rotation, followed by Hα observations, preferably overlapping at least one whole cycle, to observe the relative changes of Hα intensity.

References


1) Warner, B. & Peters, W. L., The location and size of the hot spot in cataclysmic variable stars., http://articles.adsabs.harvard.edu//full/1972MNRAS.160...15W/0000015.000.html

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