Friday, September 20, 2013

Going Where No Probe Has Gone Before

The headlines about Voyager's achievement are confusing: "In a Breathtaking First, NASA’s Voyager 1 Exits the Solar System" says the New York Times; "NASA confirm Voyager 1 has left the solar system" says the LA Times; "Voyager 1 Reaches Interstellar Space. But Has It Left the Solar System? Wellllll…" says Slate; "Where does the solar system end? Voyager isn't officially there yet" from NBC News; and "Stop Saying Voyager 1 Has Left the Solar System" says Motherboard. As Heidi Klum says on Project Runway, you're either in or you're out. Which is it?

An artist's impression of Voyager 1 passing into interstellar space, represented by the brown haze. Credit: NASA/JPL-Caltech

According to a new paper in Science and experts at NASA, Voyager did reach an important milestone in 2012, passing through the bubble of the solar wind - fast-moving particles that blow away from the sun - into a region that scientists are calling interstellar space. A graphic from the New York Times explains this well.

Is this the same as leaving the solar system? No, because the Oort Cloud - containing comets - is part of the outer solar system and extends out to around 100,000 astronomical units (AU), where one AU is the distance from the sun to the earth. By contrast, when Voyager 1 passed into interstellar space last year it was located at a distance of only about 122 AU. So, it has a long way to go before leaving the solar system, as shown in this figure from NASA. (Note that the distance scale is logarithmic, so the Oort cloud is much larger than it appears from glancing at this figure.)

A figure showing the heliosphere (containing the solar wind), and the region outside it containing interstellar space and the Oort Cloud. The x-axis is in astronomical units (AU) when 1 AU is equal to the distance from the Sun to the Earth. Credit: NASA/JPL-Caltech

As pointed out by Matthew Francis in a blog post, the outer boundary of the solar system is much more poorly defined than the interstellar boundary discussed above, and Voyager is not going to reach it any time soon.

I respect the effort NASA & some writers took to describe Voyager's milestone accurately. With tricky concepts like this it's tough to communicate a consistent story. For example, Alan Boyle carefully explained that Voyager has not left the solar system, but the video at the top of the web-page shows Brian Williams on the National News confidently declaring that Voyager left the solar system. Similarly Joel Achenbach gets the story right but a video's caption says "Voyager 1 has crossed a new frontier, becoming the first spacecraft ever to leave the solar system, NASA said Thursday". Also, at the bottom there's a link to a story in a different part of the Washington Post that's titled: "Voyager 1 just left the solar system using less computing power than your iPhone".

How did NASA handle this communication challenge? Alan Boyle's article says
""It's a very fine point, and many people don't realize the Oort Cloud is in interstellar space and it's considered part of the solar system," Veronica McGregor, JPL's news chief, wrote during a Reddit AMA chat session. "We knew many media would make the error, and we tried to make it clear in interviews. None of our materials say we've exited the solar system."" 
This is true, although it might have helped to make an explicit comment that Voyager 1 has not left the solar system in their press release. The nearest they get is this vague comment: "Scientists do not know when Voyager 1 will reach the undisturbed part of interstellar space where there is no influence from our sun." They do make a clear statement in this article, but it’s harder to find.

An artist's impression of the Oort Cloud, drawn to scale. Credit: NASA/JPL

The idea that Voyager 1 has passed into interstellar space and is still in the solar system is definitely confusing, as it seems like these two possibilities should be mutually exclusive. The definition of interstellar space used here refers to the region outside the influence of the solar wind, where the density of plasma is much higher. The Science paper doesn't even use the term "interstellar space" but "interstellar medium". In an informal sense we are living in interstellar space, because we're in a galaxy between the Sun and other stars.

There were other chances for the Voyager team members to explain the milestone. Mike Wall did an interview at space.com with Ed Stone, the project scientist for Voyager. Wall's first question was "So how do you feel now that Voyager 1 has finally left the solar system?" and Stone answered: "It's been a goal right from the beginning of the project to reach interstellar space" as though leaving the solar system and reaching interstellar space are the same thing. He doesn't make an attempt to correct the question. Later Wall asks: "You'd been saying for a long time that you were looking for three signs that Voyager 1 had left the solar system…” Stone answers by discussing the solar bubble and again does not correct the question.

This agnostic approach might have been fine if it wasn’t for all of the other messages out there. They had to counter previous reports that Voyager had left the solar system, as described in several of the press articles mentioned above. Perhaps even more problematic was a News and Analysis article from Science titled: "It's Official - Voyager Has Left the Solar System". This article was part of Science's embargo package and was made available to reporters on Wednesday, September 11th, a day before the embargo came down and two days before the paper was published (update: an earlier version of my post said "several days before the paper was published, which was too vague). It may have influenced much of the press coverage, since a large number of articles used similar wording. Some stories were prepared during the embargo period and published as soon as the embargo went down, or not long after. Because the NASA release and press conference occurred after the embargo went down, they had less influence on the first wave of stories. I expect that NASA did not know about the News and Analysis article’s headline in advance.

Given all of these challenges, it's hardly surprising that the story got muddled. To be clear I'm not sure there was any completely satisfactory way to explain this milestone. This isn't unusual in science communication. I know astronomy is full of fuzzy terms and challenging concepts. Examples include the definition of a planet, defining the edge of a galaxy and explaining cosmological distances. This is a field where planetary nebulas have nothing to do with planets and higher magnitudes mean that a star is fainter. Other fields may be even more difficult.

Another problem is that a catchy name or impressive line can be very seductive. “Voyager enters interstellar space for the first time” is cool, but “Voyager leaves the solar system for the first time” is even cooler. It’s hard to remove a cool concept or name once it has taken root - think of the popular alternative name for the Higgs Boson, which I won’t repeat here - but, I think it’s worth trying.

There are many challenges in publicizing new science findings. It's obviously important to be accurate and interesting. There are also times when describing what you didn’t do is almost as important as describing what you did. 

Wednesday, September 11, 2013

On the wrong side of 3 sigma

The claim was made just over ten years ago, that we could see matter in the grip of the supermassive black hole at the center of our galaxy. This black hole is called Sagittarius A*, or Sgr A* for short. A 2003 paper in The Astrophysical Journal (preprint on arXiv) by Fred Baganoff and colleagues analyzed images from NASA's Chandra X-ray Observatory and showed that there was a slightly extended X-ray source coinciding with the position of the black hole. They argued that the best explanation for this extended source was matter caught in the gravitational hold of the black hole and falling inwards. Some of this matter would likely make the ultimate one-way trip and fall over the black hole’s event horizon, never to be seen again.

The central region of the Milky Way galaxy. The large image contains X-ray from the Chandra X-ray Observatory (blue) and infrared data from the Hubble Space Telecope (red and yellow). The inset shows a close-up in X-rays only, covering a region only half a light year wide centered on Sgr A*, the supermassive black hole at the center of our galaxy. Credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI.

Jump forward 8 years and - based partly on their 2003 paper - Baganoff and team are awarded a 3 million second long observation with Chandra, to make a much more detailed study of Sgr A*. In their proposal they argued that this large dataset would tell them how matter falls towards the black hole and how much and where some of it flows back. This is one of the largest Chandra programs ever, so the Time Assignment Committee obviously thought the science was compelling.

However, before even a single photon was obtained in this program, a paper appeared with a different spin on the X-ray source at the center of the galaxy. Sergey Sazonov and co-authors argued that a large population of volatile stars with masses less than the Sun may be orbiting around the black hole. If they were spun up by interactions with other stars in their crowded neighborhood, these young stars could be very active, producing flares and copious amounts of X-rays. Sazonov et al. suggested these X-rays could produce much of the extended emission seen near Sgr A*. Baganoff et al. had briefly mentioned a similar idea in their paper but pointed out there is no evidence for such a population at other wavelengths, including infrared and radio data.

Sazonov et al. explained that X-ray data by itself could help decide between the two competing arguments. The emission of X-rays at a specific energy would support the volatile star idea, because this signal is seen in young stars located nearby, but shouldn't be seen when matter falls onto black holes. Also, giant flares should regularly be seen in the extended X-ray source, because similar events are also observed in nearby stars.

Their paper even provided support for the first piece of evidence based on Chandra data. A hint of X-ray emission was seen at exactly the predicted energy - 6.4 keV - with a significance of “~3 sigma” (actually 2.75 sigma). They mentioned that the long observation, coming in 2012, could help confirm this possible detection.
Figure 7 from Sazonov et al. (2012) showing a Chandra spectrum of the extended source (within 1.5 arcseconds) of Sgr A*. Note the small amount of X-ray emission possibly detected at 6.4 keV, a hint that the X-ray source is dominated by emission from low-mass stars rather than hot gas captured by the black hole. The figure is taken from the arXiv version of the paper.

This was an intriguing result, and it was tempting to do a press release on the paper. It provided a different take on a familiar object, with possible evidence for a cocoon of interacting, overactive stars around our black hole that hadn't been detected by any other observatory. It would also imply that the X-ray emission generated by material falling towards the black hole was even fainter than previously thought, deepening a well-known mystery. I didn't know much about the first author, but the second author, Rashid Sunyaev, is one of the most outstanding astrophysicists in the world (here’s an interview I did with him in 2012) and I was familiar with some of the fine work done by Mikhail Revnivtsev, the third author.

However, the strength of the possible signal was on the wrong side of 3 sigma, meaning that it was slightly less significant than a commonly used threshold for evidence (3 sigma) and much less significant than the threshold for a discovery (5 sigma). Even significance levels this high are not an absolute guarantee. For example, the result claiming faster than light speed for neutrinos involved an apparent detection at 6 sigma. But that claim famously proved to be wrong.

There was more to the Sazonov paper than this hint of a signal at ~3 sigma, so a release would have been reasonable, but I thought it would have to be carefully worded with some speculation. Also, we knew that a much deeper observation was already scheduled and that doesn't happen too often. So, it seemed best to wait for more data. It's good that we did, because a definitive answer did come from the 3 million second observation. The apparent signal seen before at ~3 sigma did not survive and flaring from the extended source was not seen, as described in this Science paper (arXiv) by Q. Daniel Wang and collaborators, including Fred Baganoff. We did a press release on the paper and Wang wrote a blog post giving more details.
Figure S.3 from Wang et al. (2013) showing the X-ray spectrum of the point source corresponding to Sgr A* (black) and the extended X-ray source around it (red; 2-5 arcseconds annulus). Neither of these shows significant evidence for X-ray emission at 6.4 keV, ruling out the hints reported in Sazonov et al. (2012). The figure is taken from the arXiv version of the paper

The basic claim made in 2003 by Baganoff et al. was confirmed. The picture of a disk of hot gas surrounding the black hole is described by the artist's impression shown below. Despite the reputation black holes have for engulfing everything that's nearby, less than 1% of the material that is captured by the black hole ends up being pulled across the event horizon, and the rest is expelled in an outflow.

By holding back we avoided having to do a correction. Sometimes it's good to be patient.

This artist's illustration shows the environment around Sgr A*. The red disk shows hot gas that has been captured by the black hole and is being pulled inwards. The source of the hot gas is young, massive stars, shown in blue, orbiting around Sgr A*. The illustration also shows a large amount of material being thrown outwards, a key factor in explaining why there is so little radiation from material near Sgr A*. Credit: NASA/CXC/M.Weiss