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.

Tuesday, September 6, 2016

Almost certainly wrong: an alien megastructure speculation about KIC 8462852

Update: 20 September 2016 - with the Gaia DR1, we didn't really know which way the 300 micro arcsecond systematics would push, us, but now there is some evidence that the parallax measurements are systematically underestimated. Another nail in the coffin.

Update: 14 September 2016 - it would seem that today's Gaia data release invalidates this, as the the star is no further from us than what Boyajian, et. al., estimated from its brightness, and possibly a fair bit closer.  So what we are seeing is a real dimming.

OK, what follows is highly speculative, but as far as I can tell, is at least internally consistent and doesn't require any exotic new physics. I've got some facts in here, but if all you care about is the facts, this isn't for you.

As I pointed out recently, to find ET technological civilizations, we're going to have to be wrong a lot - unless they are trying to make it easy for us, which they very well may not be. So, I am a long term optimist but short term pessimist. Unfortunately, being persistently wrong is very painful for some people, many of which might be the most qualified to try and set out the theoretical parameters for ET technology.

So, let me have a crack at it for the case of the star KIC 8462852, commonly referred to on this blog as "Tabby's Star," and I could well be proven wrong in a few days with the first Gaia data release. I will stick to known physics exploited with unknown technology, and perhaps it may take a bit longer to prove me wrong.

The conjectured megastructure is actually a swarm (conceivably millions) of light sails flying close to the star, using light pressure in clever ways to maintain their positions (I won't detail this yet, because my model of "near field" stellar sailing isn't very good). The megastructure is a shell of reflectors, perhaps within one or two stellar radii (a few million kilometers) of the star's atmosphere. These sails are steered in a coordinated way such that they concentrate the star's light in a particular direction by a high magnification, for the purpose of accelerating (or possibly deaccelerating) a very large light sail and its payload up to interstellar speeds - perhaps a few percent of the speed of light. It would concentrate the star's light by several orders of magnitude.

Thursday, September 1, 2016

Moving this blog

I've really had all I can take from Blogger, so will be moving this blog soon to a new platform. I will keep this site up, but it will lose its domain name, although there will be a link to it form the new site.

I don't yet know what will happen to RSS feeds - please stay tuned, I will let you know everything I know as soon as I know it.