Tuesday, July 18, 2017

July 2017 Update on Tabby's Star

If you want to know what is going on day to day with Tabby's Star, then the site Where's The Flux is an excellent resource. If you want to catch up on the basic info with sourced facts, you might want to check out the Wiki on /r/kic8462852. It includes a timeline of what has happened so far and a list of information sources - both the professional literature and more accessible materials as well.

In this post I'll try to create a bit more context without going overboard on the speculation. People love speculating on this star (as do I), but really very little of it is justified at this point. The hard work of observing and phenomenology has to take precedence. My main focus has been on figuring out the broad strokes of what it is we've been seeing since October of 2015 when this ordinary star suddenly became the focus of intense study.

The Elsie Complex

As we documented earlier, in May of 2017 the star had its first real dip in brightness, since dubbed "Elsie." In June, there was another dip, "Celeste," and that was shortly followed by another shallow dip of long duration, which does not yet have a name (let's call it "DWAIN," or Dip Without an Interesting Name).
Elsie, Celeste, and DWAIN, which continues. Based on LCO photometry data

DWAIN was only about 0.5% below normal, but went on for a long time - at least two weeks - and we're still not sure it's over. The WTF paper reported two 0.5% dips (1 and 2) as observed by Kepler, and another one of 0.4% (dip 6). However, none of them lasted as long as DWAIN. Here's what Dip 1 looked like - about 2-3 days in duration. Note the abrupt entry, the peaked minimum and smooth, gradual exit. This sure doesn't look like a planet, where we'd expect a fair bit of symmetry and a flatter bottom.
Dip 1 from the Kepler Space Telescope data

Dip 4 (D425) looks more like what we've been seeing lately. Apparently chaotic fluctuation with small dips. It was about a year after dip 4 that we see anything else in the Kepler data. Now, the current complex started about 2640 days after Dip 4, so if it's a repeat, the represents a period of about 7.2 years (or possibly half that, since we would have missed out on the last repeat in that case).

Dip 4 (0.2%) from the Kepler Space Telescope Data

DWAIN doesn't look like a planet. In order to be a planet, to last this long it would have to be a planet far from the star with a really long period, so we would have to hit both the geometric and temporal lotteries to see it.

So, What is the best Analog?

Maybe there isn't a good analog in the Kepler light curve, but we should keep the long term dimming in mind, especially what Montet and Simon derived from the Full Frame Images. I have plotted their data below marking a couple of key dip events, using my own spline fit showing a steeper decline just after Day 1000:
FFI Data showing dimming during Kepler Mission
Note that after about 200 days of decline there is a small dip and a period of chaotic, shallow fluctuations:
D1200 Dip from Kepler PDCSAP data with some quality flagged points removed
The actual dip was at day 1206. So, if this is an analog, the dimming should continue for a while longer after being masked by the fluctuations, then level off before the next big event, about 300 days away from May 2017, or some time in March 2018.

The Long Term Dimming, Further Updated

I had a look at the latest ASASSN sky patrol data. Ohio State's ASASSN is a supernova hunting project using small telescopes covering as much of the sky as possible. They don't specialize in precision photometry (you don't need to to tell a supernova has popped off), but they have taken some photometric data on KIC 8462852. This is on github if you want to have a look at it yourself, or you can go straight to the source.

Although ASASSN seems to be biased a bit dim with respect to AAVSO, their data do show a similar pattern, although because of their southern locations their winter gap is fairly long. Their data was fairly flat until late in 2016, then dimmed a bit until recently. 

The spline fit shown here an contains a recent upward trend, but I'm not sure this isn't just chasing a couple of bright points.

Ohio State ASASSN data on KIC 8462852

Compare this to the latest AAVSO data (with better coverage across the winter gap): 

AAVSO V band data fit with same spline algorithm

So, although it's just above the noise, there does seem to be some corroboration that there was about 200-300 days of dimming at a level of about 2% per year before the current complex of small dips.

It's hard to tell for sure, but it appears that the dimming we saw in the winter and spring of 2017 has stopped.  It may also be continuing, but there is too much activity to clearly discern it over the last two months. Various dimming models are in play, and telling them apart is tricky, but the null hypothesis seems to be hanging by a thread.

What to Expect Soon 

I understand that Tabby and her team are working on a paper documenting the Elsie observations. We know there were IR and spectral observations at the minimum. I'm looking forward to the preprint, but it will take a while longer. Whether I undertake the exercise of unpacking that paper here will depend in how much content is there.

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