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

Thursday, June 29, 2017

More on the AAVSO trends for Boyajian's Star

My earlier post on the dimming of KIC 8462852 that might be observable in the AAVSO photometry was looking for a single trend line, which seemed to be just observable above the noise. I hedge there, because there are always assumptions not far below the surface that might spoil the result. The human brain and randomness are old enemies, and often when we want to see a pattern, it's just nature playing tricks on us.

As more and more data came in, my visual impression of the data suggested to me that the light curve was fairly flat for the first few hundred days after AAVSO started taking data in 2015, and then sloped downward for about 100 or more days. In May 2017, we had a small dip, and this month (June of 2017) we've had another shallow but prolonged dip:
Plot by Tabetha Boyajian of the May and June 2017 dips
These dips are really below the AAVSO noise level and are just barely discernible in their data if we hold our breath and look cross-eyed at their light curves:
AAVSO data plotted at times of two dips
This plot of the "V" and "B" bands from the AAVSO data (averaged into 1 day bins for each of the 19 observers used), shows just what I mean. A 1% or 2% dip is just too subtle. As I write this (29 June 2017), it's not clear that the second dip is over yet. The black line drawn through the points is the R script's best effort at fitting a linear spline through the points while trying not to overfit - i.e. trying not to chase noise. The two red dotted lines represent the 18th of May and 11th of June 2017 - about when each dip started.

However, it is possible to see long term trends. Here's what happens when we ask the the linear spline algorithm (called earth()) to limit the wiggles in the fit and just look for the big trends with the same 19 AAVSO observers.
Plot over 638 days of AAVSO B and V data + pruned earth() spline fit
You can see a clear dimming trend. We have the most data in "V" band, and you can see there the curve is flat for about 286 days (early August 2016), when it turns downward at a rate of almost 3% per year (0.028 magnitudes/year). Even by eye, the trend appears to be unmistakable. In "B" the turning point seems to be coming a bit earlier, but the rate of decline is similar - about 2% per year. The trends in "R" data are similar. This is not that different from what Montet and Simon saw in the Kepler Full Frame images, just before the big series of dips in the stars light curve near the end of the Kepler primary mission.

So, the notion that the long term dimming and the dips are related may be true, but the long term dimming isn't a constant. there may be long periods when the lightcurve is flat. I have a sense we're about to find out.

All my data and scripts are on github. Feel free to have a look and reach your own conclusions.

Tuesday, April 4, 2017

A conclusive non-conclusion about dimming in the AAVSO data

I'm spending too much time on this, so will have to bring it to a close until the summer's observing is done.

I took one more look at the AAVSO data, this time doing something called binning, similar to what Brad Schaefer did with the DASCH data in his paper on dimming in the historic photographic plates. Binning takes several observations within a defined time period and averages them before attempting to fit a model to them. In this case, the model is a simple straight line. This has the effect of giving each time period an equal "vote" in the best fit to the model, even if there is much less data in one time period than another. In the case of the AAVSO data, some observers would report many observations over a short period of time, which tended to overweight their observations. Binning mitigates that.

Of course, you have to decide what period of time you will use for binning the roughly 500 day span we have so far. I arbitrarily picked 10 days, and averaged the observations for each observer over that time period. There were 47 AAVSO observers in all whose measurements survived the filtering process in the "V" passband.  There were 48 observers, but I identified one who temporarily had apparent problems with respect to the others, so was filtered out to make it simple.
The V Band Fit with 10 day binning

Friday, March 31, 2017

Brute Force and The AAVSO Data on Boyajian's Star

We have more than 500 days span of data from the AAVSO data on Boyajian's star now. I thought it might be worth a closer look to see if any of the secular dimming seen by either Schaefer in the archival plates or Montent and Simon in the Kepler full frame images might still be going on.

I am not a world class statistician, but sometimes a naive approach is interesting if we employ standard method knowing that our the systematics in the data are not well characterized yet.

A little background information


So, a brief explanation of what the AAVSO does. Many of their members have equipped their telescopes with special electrically cooled digital cameras and optical filters that together can measure the brightness of a star in a particular color, or band, of light with respect to standard comparison stars.  The colors we concern ourselves with right now are known as Blue, Visual, Red, and Infrared, or B,V, R and I for short.

Over many decades, the AAVSO has done a great deal of careful work finding and observing comparison stars, which are in turn compared to each other. Each observer procures his or her own equipment, pays for access to training materials, and is responsible for making sure their gear is in good working order. They are supplied with AAVSO software that turns the digital counts on the cameras into a brightness, or as it is known, a magnitude.

There are really only two things you need to know about magnitude to avoid being confused with what is to come. Some of it is historical accident, but it still makes sense in a way - unlike English spelling, which is all historical accident and little of it makes sense anymore:

  1. A higher magnitude means the source is dimmer.  The brightest things in the sky have a negative magnitude, and the dimmest thing you can see with your naked eye on a dark, moonless night is around magnitude 6. This is why the Y axis of the points you will see seems to be upside down, with the higher numbers lower on the Y axis.
  2. A small difference in magnitude is a big difference in brightness, because the scale is logarithmic. This actually makes sense, since the brightness of astronomical objects varies over a huge range. A decrease in brightness by a factor of 100 is 5 magnitudes.

The AAVSO Data so Far

I want to start with spoilers. No one should get too excited about this yet. We need more data taken over a longer time span to confirm that the Schaefer dimming is still going on. There are several possibilities left standing, including that there is no dimming going on, although my unconfirmed hunch is that there is some dimming taking place. Permit me to explain.

Friday, December 2, 2016

The Absolute, Definitive Truth About Alien Megastructures

The title of this post is a joke, or taken literally, an outright lie. The only definitive truth is that no one knows if anyone has ever built a megastructure, or even if they would if they could. I have persistent doubts if such things exist anywhere in the universe, but I can't yet tell you if such doubts are reasonable.

Update 8 December 2016: I left out one type of motivation for building a megastructure - planetary climate control. Although these "Dyson Dots" would be relatively small, they might be detectable for transiting planets. I need to run the numbers...


The Usual Disclaimer


So, we're going to be speculative here yet again, and very probably wrong. I won't be able to cite many facts, so if that is the sort of thing you like to read, perhaps now would be a good time to hit the back button.

But I'm Completely Serious

We are interested in the conjectured alien megastructures because we might have a chance to observe them with technology we have or could well have in the near future. These structures would be bigger than planets (my definition), and since we can observe planets about other stars, we might well be able to observe these things, and so looking for them is a kind of SETI. I've written before about why I think SETI is worthwhile.