Looking at the Light Curve II: Hints of an eclipse?
This is a follow up to my last post about looking at the light curve to see if the eclipse has started. If you haven't read it yet, please check it out first.
At right is the latest (as of this posting) data from our Quick Look file. Remember the most recent data is at the top. Again, if you look at it quickly it does seem like reports of the star are getting fainter. So let's check out a recent light curve:
Now the average line is starting to look like something real. The error bars of the 2-day bins are no longer always overlapping. Specifically, compare the last 2-day bin (at roughly 3.2 magnitudes) with the third to last 2-day bin (at roughly 3rd magnitude). So the raw data is starting to show a dip that is approaching statistical significance. Having photometric data here would be nice to confirm the visual data. But we don't have any yet. This illustrated a nice feature of visual observations - since we have so many observers and it is easier to report, we get data sooner and don't have to wait for photometric data reduction. If we didn't have any visual data, right now the light curve would look like this:
If we were going by the light curves alone we could not say the eclipse has started. That is, if we submitted this light curve to a journal by itself it would not pass the reviews of the referees. But we do have the added consideration that models based on past activity of this star predict the beginning of an eclipse right now. So when you add that to the equation then chances that this is the beginning of the eclipse make more sense.
This is an important lesson about data analysis. The human eye and common sense can be your best tool. Remember to always think back to your research question and not get blinded by numbers alone. Our research question is: "has the eclipse started?" So we want to consider more than just the numbers, but also see how they fit into the big picture to make a decision.
So what do you think? Is this evidence of the start of the eclipse?
*Note: Notice the blue crosshairs on the first light curve at the top of the post. Those indicate observations made by me. I appear to be consistently on the bright end of the submitted observations. This may be a bias in my eyes, my observing strategy, my local observing conditions, etc. I am tempted to consciously correct for it in future observations but that would be bad! One should never try to correct for bias in their personal visual observations unless you are absolutely sure what the cause is and how to fix it. In other words, if I knew that I was mixing up my comparison stars that would be one thing. But I don't know what is causing this bias, so I'm going to continue to report what I see. As long as I'm consistent and honest, that will be okay. As discussed before, many statistical routines count on spreads in the data.