Friday, January 5, 2018

The Elsie Paper

Noteafter this was nearly done,  couple of people pointed out to me the simultaneous release of a preprint by Deeg+ that reaches essentially the same conclusion as the Boyajian+ paper, but uses a different method, and covers all 4 2017 dips.

This post is a slightly updated text version of Wow! Signal Burst 25, which was was being released on the 3rd of January 2018, almost simultaneously with a press release announcing a new paper on Tabby’s Star by Boyajian, et. al., titled The First Post-Kepler Brightness Dips of KIC8462852 

In a previous post, I went over the events of last summer and into the Fall of 2017.  I recommend that one first if you are unfamiliar with those events, and also to Tabby's Star for the Perplexed. We also had a conversation with astrophysicist Eva Bodman on the Unseen Podcast in October 2017 in which we discussed recent developments.

To provide a very quick summary there were 4 brightness dips observed from Boyajian’s Star over the summer, with increasing depth and duration. These were named, in order, Elsie, Celeste, Skara Brae, and Angkor. Following the Angkor dip, there was a prolonged period during which the star’s brightness increased, informally dubbed “the blip” - and now the blip may be repeating.  In Burst 24 we also discussed the published evidence we had to date that whatever is causing both the dips and the long term dimming must have as a major component, very fine dust.

The new paper is about observations taken over that period of time in 2017, but is primarily focused on Elsie. Since no one knew at the time that there would be more dips after Elsie, a lot of resources were martialed to monitor that one dip.

The paper is in 5 Sections and an appendix. It discusses a large number of observations taken over the summer, including SETI observations, and discusses their implications for models explaining the star’s behavior.  What I intend to do in this burst is briefly visit each section and explain what’s going on in more depth than you’ll get in a press release, but probably less than an undergraduate astrophysics class. I’ll try to explain a few concepts that are important to the paper, but I won’t cover every detail.  Any mistakes or missions are my fault alone.

Also, note that at several points in this paper, papers currently in progress are mentioned. More information is coming. In particular, the infrared observations are still being written up. I hope there will be some interesting findings there.


Section 1

Let’s start with the Introduction. This goes over pretty much the same ground as covered in our previous posts addressing Tabby’s Star. It begins with the Planethunters discovery of the oddities of this star based on the Kepler space telescope data, and takes us up to the start of Elsie, although some of the literature reviewed in this section was first released post-Elsie.


Section 2

Section 2 is the big section, covering photometry, infrared photometry, spectroscopy, polarimetry, and radio SETI observations. I’ll explain what each of those is when we get to it.

Let’s start with section 2.1, which is photometry.

Photometry is a measurement of how the brightness of a star varies, and how this variation depends on which band of light wavelengths you use. The brightness in turn depends on a number of things, including how much interstellar dust and gas the light has to travel through to get to Earth. A graph of the measured brightness vs. time is called a light curve. The first part discusses the monitoring that was going on since 2016 using the crowdfunded Las Cumbres telescope network. When photometry using the Las Cumbres network first noticed Elsie beginning on the 19th of May, a number of other telescopes joined in on the observations. Figure 1 in the paper shows Elsie light curves from 12 different observatories. These are all detailed in the Appendix. They all show the same basic shape for Elsie, but vary in depth depending on the color band of light observed.

Section 2.1.1 was one of the most interesting to me, because it describes the dips in more detail than we’ve seen before. For example, the paper notes the very shallow decline in brightness between between Celeste and Skara Brae - which I called DWAIN at the time (Dip Without an Interesting Name), although I have also thought could be interpreted as the long term dimming bottoming out. The paper leaves open the interpretation of DWAIN.

I am particularly intrigued by the third dip known as Skara Brae, and details on the “twinkle” event at the center of Skara Brae, which they describe as a very short duration dip (see Figure 3) that bounces to a higher brightness. They compare the twinkle to the Kepler Day 1540 dip, which does look similar, but they caution against reading too much into that. It’s still not clear to me how any model out there now is going to explain Skara Brae, but most likely I’m not applying enough imagination.

There is also some interesting discussion of the steepest dip, Angkor, and how brightness hovered just below normal before it fully recovered.

Section 2.2 - Spectroscopy  - is what we’ve only had limited information about so far.

In photometry, we look at broad bands of color (or equivalently, wavelength) and measure the brightness of the star in that band. In spectroscopy, we are interested in a very detailed account of how the brightness varies with wavelength -its spectrum - since different substances either emit or absorb light at very specific wavelengths. Astronomers have been doing spectroscopy for generations, and can learn a great deal about a star from studying its spectrum, including how fast it is moving toward or away from Earth - also known as the radial velocity.

You might guess that because we are dividing the light up into many small pieces, we need a more powerful telescope to get good data, and you are correct. The big telescopes used for spectroscopy are far more expensive and harder to schedule, so we can’t just monitor the star all the time with spectroscopes. The smallest telescope detailed here was the 3 meter Shane telescope, and the other was the 10 meter Keck telescopes in Hawaii.

Although more details are forthcoming about the spectroscopy, one of the things discussed in the paper was an effort to measure any radial velocity difference between the dips and the times when there were no dips. Long story short, no statistically significant difference was found. We already knew from earlier observations that Boyajian’s Star did not have a close companion, and thi confirms it, although a big planet could be still be fairly close to the star.

Also, the team notes that the usual absorption lines you would expect to see from the interstellar medium are there, and they don’t detectably vary during the dips. It is suggested that we need to see a deeper dip in order to really measure this well.

The bottom line for spectroscopy - so far - is that nothing obviously strange happened to the star’s spectrum during the dips.

Section 2.3 is a short section about the photometry in infrared light taken by the NEOWISE space telescope. All we know is that no change in brightness during the dips was observed at all. Once again, we are told this is more information coming in two new papers.

Section 2.4 talks about polarimetry, or measurements of the polarization of light coming from the star. Essentially, the polarization observed is very probably due to interstellar dust and has nothing to do with the star or anything happening close to the star.

Section 2.5 describes radio SETI efforts during the dips. Here we are looking for very powerful, artificial radio signals coming from the star using the SETI Institute’s Allen Telescope Array in California. In the radio band that they searched, they found no such signals.We have discussed on this podcast before why this is not surprising, even if you think an ET civilization is operating near Tabby’s Star.

Section 3

Section 3 is analysis, and Section 3.1 is very important. The results are summarized in Figure 7 of the paper, which shows after some analysis of the photometry data, that there are distinct differences in the depths of the light curves at different color bands.  Between the shortest and longest wavelength bands, the ratio of the dip depths is almost a factor of two - 1.94. The word astronomers will use for this color variation as that the dips are chromatic. That is very hard to explain with a solid object of any sort.

Section 3.2 takes the color ratios observed and uses this to perform a detailed analysis of the particle sizes that could be scattering the star’s light differently at different wavelengths. This gets into some detailed mineralogy that I won’t detail here.

A number of different minerals are considered, but all require that the particle sizes are much less than 1 micron, and optically thin. Optically thin, simply put, means that the light is shining through the material, and is not being so much blocked by it, but scattered. This sort of small dust particle can not orbit the star, because the star’s radiation pressure will blow it out of the system, so whatever is producing the dust is producing it more or less continually. This is one problem I have with the symmetry of the dips - I would expect the cloud of dust to look more like a comet’s tail. I suspect that what we are seeing isn’t just a transit signature, but a burst of dust production that corresponds with a transit.

Section 4

Section 4 is the Discussion Section, in which the team compares the new data to several models. They show, for example, that it’s hard if not impossible for a giant artificial megastructure to produce the kind of color variations seen (or just any very large opaque object).  Just because we think that there is dust blocking the star doesn’t make it easy to figure out where that dust came from, though.

They lend some encouragement to a model that involves a lot of exocomets or dust-shrouded planetesimals - something has to be producing the dust after all, and they strongly hint that we would need target-of-opportunity time on the James Webb Space Telescope to really narrow down the parameters of such a model. This telescope, if we are lucky, will launch some time next year, although Boyajian’s star is not one of its early science objectives.

Also discussed is a simple model to determine whether stellar cool spots might be responsible for the Elsie family of dips. The result is a firm “maybe,” and they don’t rule out more complex models of how the star might be cooling or changing radius.What it will take to rule those models out isn’t clear to me.

They also caution against any assumption that the events seen to date are periodic with the information we have to date. It certainly seems to me that the system we are observing is evolving a great deal, and the recent “blips” throw a new wrinkle into that. Perhaps, they note, observations in June of 2019 might help us to see a repeat of the first big dip in the Kepler light curve.

Section 5

Section 5 is the conclusion section, and takes a look at the bigger picture. The use of crowdfunding is one element they highlight, and they summarize the key results, especially the fact that the 2017 dips were chromatic.  Finally, they make the case for further monitoring over the long term. You can do something about that by going over to wherestheflux.com and making a donation to pay for telescope time.

I have had some e-mails and other communications that worry that Alien Megastructures are now out the door because of the fine dust. As I’ve discussed before, there is a major problem with the classic sort of Dyson Swarm megastructure hypothesis for Boyajian’s Star that have nothing to do with the chromatic dimming - the constraints on excess infrared brightness that Dyson predicted in his 1960 Science paper "Search for Artificial Stellar Sources of Infra-Red Radiation". These constraints get just a bit tighter now with the NEOWISE observations, although the details are in work. However, if ET is operating around this star, what are the chances they would be kicking up a considerable amount of fresh dust? What other observables might their activities have? I don’t have an answer to those questions, but I think they are intriguing questions, and I plan to look further into them


Tuesday, November 28, 2017

I'm still perplexed - Tabby's Star Update for November 2017

Update: 29 November 2017

I've been meaning to put out an update for the last several months, and just when I am poised to do so, something else happens. So, here it is is, and I may need another update soon. It' s been an eventful few months, and if you haven't been following closely, you may want to read this.

The tl;dr


Kickstarter-funded observations of the star by the Las Cumbres telescope network began in 2016. There was a Winter interruption when the star was too close to the sun, but observations resumed in the Spring. From about mid-2016 there was a prolonged dimming episode which I am tempted to assume was related to what followed. In May, we saw our first of four dips, during which the overall slow dimming stopped and turned into a slow brightening. After the last dip in mid September, the star brightened for about one month, levelled off in brightness, and lately has been slowly dimming again. There are some new preprints out that contain some interesting tidbits.

The Quiet, Slowly Dimming Period

Before the first dip in May of 2017, we already had strong reason to believe that the star had been slowly dimming for the better part of a year from the AAVSO data. I posted this tentative conclusion of 1-2% per year dimming in April of 2017. I compared this to the dimming Montet and Simon has mined out of the Kepler Full Field Image data (see Wow! Signal Episode 33), which was a precursor to a series of dips, and wondered if a dip might be coming soon.  

The Sequence of Dips 

Things started to get really interesting in Mid May of 2017, just as the Las Cumbres Observatory was getting into full swing monitoring the star.


Elsie

The first dip of 2017 started around the 18th of May 2017, and lasted for about 5 or 6  days, during which the star’s brightness declined more than 1%, and even closer to 2% at peak. 1% may not seem like much, but that is a measurable decline in flux and not something that is normally seen with main sequence stars like this, as we discussed in Tabby's Star for the Perplexed. Also, 1% is way too big to be a transiting planet for a star this size.  This dip was later named “Elsie” after a vote by the Kickstarter supporters.

Since no one knew whether there would be more dips or not, an Astronomer’s Telegram went out, and a number of astronomical instruments pointed at the star in those few days. We don’t have all the data made public yet, but we do know that this dip resulted in a slight reddening of the star - that is, the dip was definitely deeper in blue light than in red. This is strong evidence that whatever was blocking the star was not a single solid object, but had very fine dust as a major component. These dust particles would have to be well under 1 micron in size, which is typical of dust seen in the interstellar medium or comet dust, but such small particles in orbit around the star would not last long before the pressure of starlight (which exceeds the force of gravity) drove them away.

Celeste

  A second dip started on the 11th of June, and was eventually named Celeste. Celeste lasted about 2 weeks, and was also a bit more than a  1% drop in brightness. We don’t yet have a full report of the observations taken during Celeste, but so far, I don’t believe there was much difference from Elsie. Following Celeste, there was as period during which the brightness of the star seemed unsettled. I called this DWAIN, but it wasn’t a dip. What we can see now in retrospect was the brightness of the star - neglecting the dips - was bottoming out.

Skara Brae

The third dip was, to my mind, the strangest. And began about the 2nd of August. This dip was named Skara Brae, and lasted about 16 days until August 18th. In addition to its duration (a typical planetary transit is well under 1 day), what makes Skara Brae stand out is its symmetry, the linear slopes of its sides,  and a high degree of photometric activity in the exact center of the dip, when the brightness was briefly down 3%. We’ll have links in the show notes where you can see the light curve plots of all the dips.

Angkor

The fourth and largest dip - Angkor - started around the 28th of August, dipped to about 2%, and lasted until the 14th of September - the exact times when a dip starts and ends are a little fuzzy - so it lasted about as long as Skara Brae, and was deeper on average, although it seems to have been a bit more ragged and not quite as cleanly symmetrical - it seemed to bottom out (sharply, like Skara Brae) on around September 10th. One notable thing about Angkor is that it was the first dip that the AAVSO data clearly caught. This is probably because of its depth.
A recent LCO light curve from http://www.wherestheflux.com/



The Post-Dip Brightening and Dimming Again


After Angkor, it became apparent that the star was brightening slowly in the shorter wavelength. In B band, this brightening may have been as much as 2% from the minimum between Celeste and Skara Brae. We don’t have as much data in the longer wavelength I band data from AAVSO, but the I band brightness appears to be roughly flat after Angkor. I say “appears” because the scatter in the I data is around 2%, so it will take a while to see a trend emerge.

I would have enjoyed it if the star had just kept brightening for a long time, but in mid November, Bruce Gary noticed a rapid decrease in brightness over 1 day, and a slow decline thereafter, and for now we have seen the star give up about half of its brightness gain since Angkor.


Some New Papers in Process Made Public

Scientific papers come out fast in preprint these days, as the recent flurry of papers addressing the hyperbolic asteroid Oumuamua made clear. This is partly because astronomers can reduce their data very rapidly, and also because they can collaborate electronically. This has also been happening with recent developments on Boyajian's Star.

In August, a couple of new preprints came out, and both dealt with the long term dimming of the star. As always, there will be links in the show notes. They generally agreed that there had been a slow dimming trend since 2016, but the paper by Simon et. al. dug up a 4000 day span of data from the All Sky Automated Survey, and found that there had also been two periods of brightening. This isn’t surprising that the star doesn’t just dim all the time, but no one had found an example until now.  The second preprint, by Meng, et. al., also looked closely at the recent slow dimming using data from the Swift space telescope as well as ground based observations. They found a reddening in the dimming which was different than reddening we see from interstellar medium, suggesting that whatever is causing the dimming is orbiting around the star.

In September, there was an interesting preprint by Steele, et. al., that looks at the polarization of light during the period of the dips. I'm not yet sure if these measurements constrain any hypotheses that much, but I'm glad someone took a look at it.

Another preprint, by Wyatt, et. al. published in October, looks at the Exocomet interpretation in terms of the Elsie dip. This paper revealed that there is infrared space telescope data from NEOWISE (not yet made public) during Elsie, and that no increase in emission was detected during Elsie. They give the integrated depth of Elsie as 6.5 %-days. Angkor  and  Skara Brae had much greater integrated depths, and so there would be more hope of detecting an infrared excess. If there are such data, they haven’t been released yet.

What We're Looking Forward to Now

As the star comes together with our sun, photometric observations from the ground will get harder to come by until early Spring of 2018. Only observers in the more northern latitudes will have a good look at it when the sun is down. We hope that some space-based observing time will be made available, as it was last Winter, so that we can keep monitoring for more dips.
A lot of people think they know something about the periodicity of the dips, but this remains speculative until someone can clearly demonstrate at least three closely similar events with an even spacing between them. To me, the system is clearly evolving, and fine dust is blocking at least some of the flux, and this dust will not be in a periodic orbit - it must be produced afresh by something else that may or may not be doing so periodically.
In the near term, we await a paper by Boyajian, et. al. that brings together all the observational data from the dips, or at least from Elsie. We have reason to believe that is in work,  but it's likely be time consuming to get it right, and will be worth waiting for.

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