Analysis of the light curves - 1
Photometric observer Richard Miles reported a V band magnitude of 3.02 for epsilon Aurigae this past week, which essentially matches the pre-eclipse average and signals the end of optical eclipse (4th contact). The long march through eclipse is over. However, we still need your observations for at least the balance of this year in order to more precisely define 4th contact after-the-fact, and to characterize the magnitude of out of eclipse variations (~0.1 mag in V). Information on the latter will be helpful in comparison with planned post-eclipse observations with both the CHARA Array MIRC imager (with exciting new 6 telescope mode) and the IRTF SpeX infrared instrument.
What do we learn from the fine light curves now collected?
Step one: timing of events in the light curves.
Step two: analysis of "fine structure" present in the light curves.
(a) Timings: More careful analysis is in progress, but the working times in reference to available visual and V band light curves are:
Start of eclipse, JD 2,455,050 (give or take a week)
Start of total phase, JD 2,455,250
End of total phase, JD 2,455,620
End of eclipse, JD 2,455,750
(b) Shapes: classical eclipsing binary star light curve analysis involves the assumption of spherical stars and deals with "times of contact" - defined as First (I) when the stars are tangentially 'touching' [ oO ]. Second (II) has one star completely hidden [ O ]. For unequal size stars, a Third Contact (III) happens when the smaller star is ready to reappear from behind the larger. Fourth contact is the final tangential contact between stars [ Oo ] and marks the end of eclipse.
Now that we've seen the companion in epsilon Aurigae and it is not a simple spherical star, but rather a long, thin dark "brick-like" disk that, at most, only eclipses the southern hemisphere of the F star, 2nd and 3rd contacts aren't quite the same as in the case of simple spheres eclipsing each other. Hence, for the "brick" - second contact probably refers to the moment when coverage of the southern hemisphere reaches a maximum, even though the leading edge crossed the F star west limb, moving east to west. Similarly, third contact reflects the end of maximal coverage of the F star southern hemisphere as the disk trailing edge approaches the east limb of the F star. Note in the light curve how much more steep egress appears than ingress - suggesting an opacity change in the disk. More about this later. Also notice that relative to generally agreed time of mid-eclipse, JD 2,455,400, egress started only 120 days after mid-eclipse, while ingress ended nearly 150 days prior to mid-eclipse. the eclipse is asymmetric. By that measure, with ingress starting 350 days prior to mid-eclipse, one could expect by symmetry that egress should end circa JD 2,455,750 - which is essentially what we are seeing lately.
(c) More about shapes: totality is not flat. the simple spherical stars model produces a flat-bottomed total eclipse for unequal size stars. The bumps and wiggles during totality in epsilon Aurigae are very real and very significant! More about that later too.
(d) What about other wavelengths? We are forutnate to have not just V band, but UBVRIJH light curves. Next time, we'll compare and contrast these, and dig deeper into the significance of shapes at these differing wavelengths.
Meanwhile, keep observing!