CBA Center for Backyard Astrophysics



Orbital Light Curves of CVs

averaged from CBA observations

One of the great breakthroughs in our understanding of CVs occurred with the re-interpretation of the U Gem orbital light curve as dominated by an aspect-dependent bright spot at the edge of the disk (Smak 1971, Warner & Nather 1971). Thirty years later, our physical understanding of CVs has vastly grown; but observers still bumble along citing and re-citing U Gem as defining the "standard model". Can it really be that there is nothing else to learn after U Gem? By now we should know orbital light curves of ~100 stars, enabling us to understand how the various orbital effects (bright spot emission or absorption, reprocessing in the secondary, "ellipsoidal" waves, etc.) are distributed among the various classes of CVs. But no such study exists. Why not?

Mainly it's because the light curves are so erratic. Most photometric studies are based on just a few orbits; the light curves are of good quality but contaminated by flickering and other variations of non-orbital origin. These contaminations are frequently as large as, or larger than, the true orbital effect. The only way to overcome them is to average over many orbits. This is the spécialité du CBA. Thus, ironically, our network is much more sensitive to small-amplitude periodic signals than traditional studies based on large telescopes. The accompanying figures show samples of orbital light curves we have measured. Flickering disappears, because we average over hundreds of orbits. Roughly, our typical campaign of ~300 hr coverage gives a sensitivity to periodic signals ~0.02 mag (Porb/hr). This is about 4 times more sensitive than we could achieve with a week on a large professional telescope.

With data like this, we intend to carry out the study whose absence we deplore. We are preparing an atlas of orbital light curves, a much expanded version of these figures, including also nondetections (about half of all CVs have no discernible photometric signal at Porb). This should be excellent fodder for theoretical models. Please click on atlas to follow this growing collection.

Calibration

These magnitudes purport to be V magnitudes. However, there are some hefty caveats.

  1. The actual observations are most commonly differential photometry in white light. This calls for a correction, meaning a systematic uncertainty of up to ~0.3 mag. We will eventually reduce this to ~0.1 mag, when we have accurate measures of V and R for all the comparison stars.

  2. In a few cases we simply don't know the mean light level to a reasonable accuracy -- in which case we merely express the signal with a fixed mean of zero.

  3. The majority of the observations were obtained while the star was in its most common luminosity state -- "high state" for novalike variables, and "low state" for dwarf novae. In other luminosity states, the light curve differs drastically! (There's no confusion here, though, since the observations are sufficiently extensive to define the high and low states.)

  4. These are average light curves -- not merely folded light curves (which would have all the flickering still present, and hence hide the systematic features).