CBA Center for Backyard Astrophysics

AM Canum Venaticorum: The 
1998 Campaign

AM CVn figure 1

O-C diagram of the principal superhump of AM CVn in 1998. Each point is a nightly pulse timing, with an average length of about 3.6 hr. The test period is 525.55 s, so the very slight downward trend indicates a mean period very slightly less. This is in sharp contrast to 1997, when the superhump occurred at the highest period (525.65 s) seen in any season of dense observation. The point-to-point scatter is ±26 s. Measurement error is estimated at ~15-20 s. So the extra error incurred by using nightly runs (rather than full 13.38 hr runs which fully resolve any sideband structure) is around 15 s -- essentially buried in the noise. This is why pulse period changes in this star are so easy to follow! We'll continue the campaign through early July. This is a great, great target for regular monitoring with small telescopes, because the shortness of the period makes it pretty easy to get a decent pulse timing with just a few hours per night.

AM CVn figure 2

O-C diagram of the 1011 s ("negative") superhump of AM CVn in 1998. The test period is 1011.40 s, so the probable decline signifies a slightly shorter period. The timing errors are about 3 times greater here, because: the signal is weaker; the period is longer (fewer cycles to average over); and there is a neighboring signal at 1028 s which is hard to separate. The general drift in the period (given by the amplitude and timescale of the O-C wiggles) is fairly similar to that of the 525 s superhump (which is much more richly documented since it does a basically similar dance every year). But we want to explore in detail how the two sets of wiggles correlate. To succeed in this we need to observe the star very frequently. As you can see a little wiggle occurred near E = 2000 which is not defined with certainty by our data -- there is some chance that the points belong exactly one cycle lower, or even half a cycle (with sudden transitions). We want to eliminate (or confirm!) such possibilities to get a really definitive test of the correlation between the two O-Cs. It looks to me like diligent observation for the rest of the season will preserve cycle count and hence define the wiggles. The perpetrators of this most promising work have been: Jonathan Kemp, Dave Skillman, Dave Harvey, Bob Fried, Tonny Vanmunster, and Gianluca Masi. We'd love to get more help!

AM CVn figure 3

Here are the three most interesting regions of the power spectrum during JD 945-952. These are essentially "cleaned" power spectra, i.e. with the aliases of each detected signal removed. The upper frame shows all three primary signals: the orbital signal at 1028.7 s, the positive superhump at 1051.2 s, and the negative superhump at 1011.4 s. These are the familiar basic signals, although we never previously saw them manifest at the same time. In our terminology, we call w the orbital frequency, W the frequency of apsidal advance, and N the frequency of nodal regression. The superhump frequencies then correspond to w-W and w+N. The middle frame shows the vicinity of the strongest signal at 525.6 s (the first harmonic of the positive superhump), which rises to a power of 255. There are signals at 520.0 and 515.4 s (166.17 and 167.63 c/d). The former is the 2w-W signal, occasionally seen in superhumping CVs. But the latter is certainly surprising. Probably it is 2w-W+N. The third frame shows the second harmonic of the positive superhump, at 350.4 s, and a weak extra signal at 345.3 s (250.19 c/d). The latter is 3w-W. The semi-amplitude of the 525.6 s signal is 0.012 mag. Since power scales with (amplitude)2, this means that the 1028 and 1051 s signals have a semi-amplitude of 0.0046 mag, and the weakest signal flagged has a semi-amplitude of 0.0018 mag. The phase of the orbital signal was consistent with the ephemeris of Harvey et al. (1998). The phase of the other signals wandered slowly throughout the season, as usual.