Thursday, November 21, 2013

Revealing galaxies beyond our sight

The secrets of the universe can be revealed in many different ways. Serendipity often plays a role, where astronomers use unexpected events to make discoveries. Here's a striking example. In a few cases, astronomers have been able to map the positions and distances of galaxies that are so remote and faint they cannot be seen using our most powerful telescopes. However, the spectacular explosion of a single star can give away the galaxy's presence.

For context, I’ll explain what the limits of our telescopic vision are for distant galaxies. A result published in Nature (arXiv paper) a few weeks ago by Steven Finkelstein and collaborators claimed the title of the most distant galaxy with a securely known distance (other possibilities for more distant galaxies have been claimed, but with less secure distance estimates). The galaxy is called “z8_GND_5296”, which is a candidate for the most unappealing galaxy name. The light seen from z8_GND_5296 was emitted when the universe was only about 700 million years old, or about 5% of its current age. The galaxy was also found to be forming stars at a prodigious rate.

A Hubble Space Telescope image of z8_GND_5296, which is claimed to be the most distant galaxy with a securely known distance. Credit: V. Tilvi, S.L. Finkelstein, C. Papovich, A. Koekemoer, CANDELS, and STScI/NASA
This is an impressive result, but it doesn't represent the record for the most distant object with a securely known distance. That record is likely held by an exploding star that was seen when the universe was only about 630 million years old, as published in Nature papers in 2009 by Nial Tanvir et al (arXiv paper) and Ruben Salvaterra et al (arXiv paper). Here’s a press release describing the discovery.

The center of this image shows the afterglow of an exploded star which is claimed to be the most distant known object in the universe. The image was obtained from the Gemini-South and the Very Large Telescope. Credit: A. J. Levan.
A deep Hubble Space Telescope (HST) search for the galaxy containing this exploded star came up empty, a testimony to the incredible light show that massive stars can perform when they explode. The burst of gamma rays and other radiation from these "gamma-ray bursts" (GRBs) can provide our only clue about the very distant galaxies they occurred in. Once this radiation fades, any information about the galaxy fades with it. To turn things around, we can imagine that an alien civilization billions of light years away just spotted the explosion of a massive star in our galaxy when it was an infant. Without this luminous event they may have had no idea our distant galaxy existed.

The field containing the explosion seen by Tanvir and Salvaterra is one of six that was observed with HST by Tanvir et al, as described in a 2012 paper in The Astrophysical Journal (arXiv paper). The explosions were traced back to times ranging from about 520 million (*) to about 1.2 billion years after the Big Bang. Only a hint of a galaxy was seen in one of the targets – the nearest one – and nothing at all in the others. Just empty backgrounds. The authors assume that the exploded stars are located in galaxies, because lone stars would be very difficult to understand. Conversely, exotic galaxies are not required. The team estimated that ordinary rates of star formation, at most, are occurring in these unseen galaxies, rates that are about a hundred or more times lower than in z8_GND_5296.

These results reveal an observational bias. The galaxy z8_GND_5296 was only detected at its great distance because it's unusually bright. To use a human analogy, a galaxy like this is over seven feet tall and isn’t representative of the larger population. The indirect detection of galaxies by observing exploded stars also involves limitations: there is an observational bias because the galaxies have to contain regions with young stars, and these destructive events are extremely rare, so huge numbers of galaxies are missed. However, for locating the most typical galaxies in the very distant universe, the exploded star technique still does a better job than direct detection. Tanvir et al. make this point with the title of their paper: “Star formation in the early universe: beyond the tip of the iceberg”.
There are two conclusions I’ll take from these results. One is that results with superlatives are exciting, but they can have limited significance. Another is that non-detections are sometimes interesting, and deserve attention.

(*) Technically this object is more distant but the technique used was not a secure one.
Update (Nov 24th): A few days ago a different stellar explosion was discussed in a NASA press conference and press release. This object, named GRB 130427A, was discovered earlier this year using NASA's Swift Space Telescope and the Fermi Gamma-ray Space Telescope. It was not nearly as distant as the GRBs discussed above, since it occurred at a time when the Universe was a much more mature age of 9.9 billion years, that is about 70% of its current age. However, it has other exceptional properties. The NASA press release, written by Francis Reddy, states that it is one of the brightest GRBs ever seen. Also, a new paper (arXiv paper) led by Alessandro Maselli (one of four papers published in Science) explains that its properties are similar to those of the most luminous GRBs seen in the very distant Universe. In the words of Maselli et al. a "common central engine is responsible for producing GRBs in both the contemporary and the early Universe". The common central engine, in academically understated jargon, is a newly formed black hole (see the illustration below).
In this artist's illustration, the most common type of GRB is shown. The collapse of a massive star forms a black hole (left) and a powerful jet is launched producing radiation across the electromagnetic spectrum. Credit: NASA's Goddard Space Flight Center
Another interesting detail, not mentioned in the press release, is that a supernova explosion was associated with GRB 130427A. Previously supernovas had only been associated with rather weak GRBs observed at relatively close distances. It's now clear that supernova explosions can be associated with very powerful GRBs, like the distant one seen by Tanvir and Salvaterra. This is important because arguments had been made that powerful GRBs didn't have enough power left over for a strong supernova. This result "definitively proves" this isn't the case, as the Maselli et al paper explains.

By coincidence I attended a short talk on GRBs at the Center for Astrophysics on the same day that the Science embargoes went down. The speaker, Tanmoy Laskar, pointed out that there had already been a burst of publicity for GRB 130427A soon after it was discovered, as explained in this slide.

A slide from Tanmoy Laskar's talk. Credit: Tanmoy Laskar
Following Laskar's slide, here's a NASA press release about this "shockingly bright" burst and here is an article by Miriam Kramer from space.com about this explosion being the "most powerful ever seen". This is a good demonstration of the speed that astronomy research can sometimes move at. GRB 130427A was discovered on April 27th, 2013, triggering a bunch of follow-up observations including the all-important estimate of the explosion's distance. The NASA release then followed on May 3rd and press articles soon after that. It wasn't until November 21st that the peer-reviewed science papers were published, confirming the quick analysis performed earlier. For some cosmic perspective on that delay, it took 3.8 billion years for the light from this event to reach us, so six months doesn't seem like much of an extra wait.


Note: In a future, more PR-oriented post I'll use some of these results to look at a few challenges involved with publicizing superlatives. 


Wednesday, October 9, 2013

The Nobel Prize for Physics: 2013 and Looking Ahead

It’s Nobel Prize week and early yesterday the winners of the physics prize were announced. Congratulations go to Peter Higgs and Francois Englert for "the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider” according to an official statement. Another scientist, Robert Brout was co-author of a key paper with Englert in 1964, but he died in 2011 so he couldn’t be given the prize, according to the Nobel rules.

Francois Englert (left) and Peter Higgs at CERN on July 4th, 2012, when the discovery of the Higgs boson was announced. Credit: Maximilien Brice/CERN 
Many media pundits predicted that scientists associated with the Higgs would win, but there was uncertainty about who would be given the award, since the Nobel can be given to a maximum of three recipients. One problem was that Gerald Guralnik, Carl Hagen and Tom Kibble performed very similar work and published it only a few weeks after the others. Here’s a reaction to the Nobel decision from Tom Kibble and one from Gerald Guralnik. I can certainly understand this stinging a little.

The Nobel Prize in physics has always been awarded to individuals, but some suggested that a share of the prize might be awarded to scientists at CERN’s Large Hadron Collider. That didn’t happen, but maybe it will in the future, since they certainly deserve it. For now they have a consolation prize with the official mention of their crucial confirmation.

One interesting feature of this year’s prize was the slight delay in the announcement yesterday morning. This article from the Associated Press gives some fascinating insight into the secretive Nobel process, while failing to actually explain the delay. The deliberations of the Nobel Committee are supposed to be kept secret for 50 years, which doesn’t say a lot for their openness and transparency. However, some details of the process are made available and I was surprised to learn that the decision on the Nobel isn’t made until the day of the announcement, using a majority vote by the full academy. With all of the media speculation that occurs in the build-up to the announcement, especially this year for the Higgs, I wonder if the academy imposes any sort of media blackout in an attempt to avoid bias. Unconscious bias is especially hard to avoid.

Many articles about the physics prize have already been written and more will be published over the next few days. They can easily be found with Google News. For detailed background on the science that does not mention the unfortunate nickname for the Higgs boson, I recommend this blog post by physicist Matt Strassler. For well-argued critiques of the Nobel Prize's rules and tradition, I recommend these articles by Sean Carroll at his blog and in the New York Times (I have some thoughts on this topic, but I'll save them for another blog post).

Looking ahead

I’ll now discuss who might be having a sleepless night one year from now, when the physics field will presumably be more open. My discussion will be limited to astrophysics (*), where my background lies.

Thomson Reuters suggested that the astronomers who did key work on exoplanets had a chance for the 2013 prize, namely Geoff Marcy, Michel Mayor and Didier Queloz. Another strong candidate is William Borucki, the Principal Investigator for the very successful Kepler mission. (Added note: I no longer think Marcy should be considered for a Nobel Prize, given his tarnished record.) When I saw this prediction I wondered if the relative lack of new physics that has come from exoplanet work might be a handicap. For example, in this article by Kate Becker, 2011 Laureate Adam Riess discusses the type of work that tends to win the Nobel: "I think the key is its importance must be fundamental, generally involving new physics."

A screenshot from paperscape.org. Credit: paperscape.org
Here’s a graphical way to look at this issue, using a great new visualization of the ArXiv called paperscape. The dots in this figure represent papers, where astrophysics papers are colored dull pink and papers in other fields of physics have different colors, such as green and blue. The points representing papers are located near other papers that they reference, using an N-body algorithm. As an example, in this close-up view you can see that papers involving dark energy and the cosmic microwave background show close links between astrophysics and other areas of physics. There’s a lot of color mixing going on. It’s notable that three different Nobels have been awarded for work in these areas.

A close-up of a screenshot from paperscape.org. Go to the web-site for a much clearer picture. Credit: paperscape.org
The field of “extrasolar planets” – now commonly called exoplanets – is located in the lower right, in a region that is clearly separate from other areas of physics, implying that there is a lack of contact between exoplanet work and other areas of physics. Despite this apparent weakness I still think that exoplanet workers should win the physics prize sometime in the future, as the field is remarkable in so many other ways. Astronomers have found evidence for a class of object that is more common than stars and that has incredibly diverse properties. They are beginning to answer one of the longest-standing questions in science: how common are earth-sized and earth-like planets? They have taken a crucial step towards understanding how common life might be in our galaxy. Currently I’m reading a terrific book by Lee Billings about this great adventure, called “Five Billion Years of Solitude”.

I can give a personal reason for supporting an exoplanet-related Nobel Prize: if Marcy or Mayor are given the award, I can boast that I was a co-author with a Nobel prize winner (Added note: I retract this "boast" in the case of Marcy.) One problem with this boast is that I've never met either of them and many other scientists can make a similar claim, in this era of author-inflation.

I'll note, at risk of revealing my earlier concern to be a strawman, that there is a precedent for awarding the Nobel for astrophysics work that is more applied physics than fundamental physics. For example, Nobel prizes were awarded to Sir Martin Hyle for developing aperture synthesis in radio astronomy and to Riccardo Giacconi for the discovery of X-ray sources

The map from paperscape can be used to discuss hints about other potential future Nobels involving astrophysics. The area of the circle for each paper is proportional to the number of citations a paper has received, and it’s easy to spot the two key papers by Adam Riess and Saul Perlmutter that led to the 2011 prize (go to paperscape.org, zoom in and click on the two largest circles near “dark energy”, using my figure as a guide). So, assuming that citations are a good reflection of scientific importance, the large circles near “cosmic microwave background” may hint that Charles Bennett, David Spergel and others involved with NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) will attract interest from the Nobel Committee in the future.

Glancing at this area of the map is a good reminder than no Nobel has yet been awarded for the discovery of dark matter. Here, Vera Rubin is an outstanding candidate for her work on galaxy rotation, as others have noted. It’s interesting to me that evidence of dark matter was found well before evidence for dark energy, and yet a Nobel has been awarded only for the latter. Another subject area that might receive attention is inflation, where Alan Guth, Andrei Linde and others have done critical work. However, extra observational evidence, from projects like Planck, the Atacama Cosmology Telescope and the South Pole Telescope, might be needed to convince the Nobel committee. 

Vera Rubin at the Kitt Peak 84-inch telescope in 1970. Credit: Carnegie Institution, Department of Terrestrial Magnetism and Vera Rubin.
Since the arXiv is only about 20 years old, the crucial papers by the new Nobel winners, and by others like Vera Rubin and Alan Guth do not appear in paperscape. A similar comment applies to the highly influential papers by Rashid Sunyaev on the Sunyaev Zeldovich effect and the Shakura-Sunyaev model for material in a disk around a compact object, such as a black hole. For some extra background, here's an interview I did with Sunyaev last year. I think he would be an excellent choice as a Nobel Laureate.

Rashid Sunyaev. Credit: Rashid Sunyaev.
I've already mentioned dark matter and dark energy. Another crucial part of observational cosmology is the Hubble constant, giving the expansion rate of the universe. Here, Wendy Freedman and others have done very important work, including this highly-cited paper. It should be noted that Freedman has already won the prestigious Gruber prize in cosmology, along with Charles Bennett and the WMAP team, the winners of the 2011 Nobel prize (Perlmutter, Riess and Schmidt), John Mather (co-winner of the 2006 Nobel prize) and the COBE team, along with Guth, Linde, Rubin and Sunyaev. No wonder it’s sometimes referred to as the Nobel Prize of astronomy.

This list of possibilities is not meant to be complete, and I'm sure there are better judges than me. What important and potentially Nobel-worthy work in astrophysics have I missed?



(*) Here are some quick Nobel statistics to give perspective. Of the 107 Nobel prizes in physics I count twelve that clearly involve astrophysics (I won’t count Einstein because the award didn’t explicitly mention general relativity). Ten out of twelve of these have been awarded since the 1960s and four since 2002, showing a possible rise in prominence of astrophysics compared to other areas of physics. Has astrophysics been pulling its weight compared to other areas? I didn’t find historical statistics on numbers of paper in different areas of physics, but I see that about 24% of physics papers on the arXiv in 2012 are in astrophysics. This isn’t too different from the recent fraction of astrophysics-related Nobels, although we're obviously dealing with small number statistics.

Other statistics: only one Nobel has been awarded for work involving optical astronomy: the 2011 prize. One notable omission, besides the discovery of dark matter, is the discovery that the universe is expanding. Edwin Hubble, Georges Lemaitre and Vesto Slipher, among others, did important work here, but died years ago.

I'll end with a disappointing statistic. Over the 196 Nobel Laureates in all areas of physics, only 2 have been women, none of them involved in astrophysics. It's time that changed.


Friday, October 4, 2013

The Race for Gold*

Gold ring. Credit: P.Edmonds
Astronomers know a lot, but they’re not certain how the gold in this ring was made. There are two leading ideas, both containing at least one destructive event. One idea is that it was created in the collapse of a massive star followed by a powerful explosion. These supernova explosions form neutron stars – dense objects with extraordinary properties – or black holes. In the extreme conditions produced by the explosion, gold and other heavy elements may form. This has been the default story for many people, including astronomers like Neil Tyson**.

The other idea is even more spectacular. Here, we follow the story of particles that were squashed down into a neutron star following a supernova. Normally they would stay locked away for countless billions of years, but in a few special cases they can be liberated. If the neutron star has a companion that also explodes to form a neutron star or black hole, and if these two collapsed objects are close to each other, they can form a kind of deadly dance. As they spin around each other, energy leaks away via ripples in space-time and the orbit gets tighter until the two objects merge and explode, forming a black hole. In the aftermath of the explosion, neutrons slam into the stellar remains to form gold and other heavy elements like platinum.

video
Animation showing a merger of two neutron stars resulting in a gamma-ray burst. CreditDana Berry, SkyWorks Digital, Inc.

That's quite a story, involving three powerful explosions and the warping of space-time. In this post I'll explain some details about the closely contested academic race that uncovered evidence for this exotic process, and two different publicity approaches that followed. I’ll focus on the aggressive one, because it’s interesting to see how boundaries were pushed in several ways.

I didn't know the merger story was a serious contender for making heavy elements like gold until this paper appeared on the arXiv: "Smoking Gun Or Smoldering Embers? A Possible R-Process Kilonova Associated with the Short-Hard GRB 130603B". A short-hard gamma ray burst (GRB) is thought to occur when the merger described above occurs, producing a powerful explosion and a burst of gamma rays. A chain of nuclear reactions called the "r-process" should then occur, producing heavy elements like gold and a second source of radiation called a "kilonova", a few days after the explosion.

In the paper mentioned above, Edo Berger and colleagues from Harvard-Smithsonian Center for Astrophysics (CfA; in full disclosure, where I work) used observations from the Hubble Space Telescope (HST) to argue that an infrared source appearing at the position of a GRB might be a kilonova. They also pointed out an alternative explanation was feasible, when particles slam into the surrounding environment giving an afterglow.

Their paper made the kilonova claim carefully, using "possible" in the headline and other cautious statements, like: "If true, the kilonova interpretation provides the strongest evidence to date that short GRBs are produced by compact object mergers, and places initial constraints on the ejected mass."

What followed really caught my attention, as someone who works in astronomy publicity. This paper was the basis for a CfA press release and press conference on July 17th. The press release framed the result as possibly explaining the origin of the gold found on Earth and in the universe, in the merger story described earlier. Bling was mentioned, and the result received a lot of press coverage. This coverage was heavily driven by the CfA publicity, as there was a clear gap between the content of the paper and that of the press release and press conference. The paper did not mention the debate about the source of heavy elements. Also, the paper did not explicitly mention gold, so no description of the expected yield from this possible kilonova was given, and no details of the extrapolation to calculate the total amount of gold in the universe were given, including the uncertainties.

Edo Berger at the CfA press conference announcing possible detection of a kilonova. Credit: CfA/Edo Berger
The paper was not accepted for publication at the time of the publicity. The authors did their analysis extremely quickly, as the HST data they used was available on June 13th and their paper was put onto the arXiv on June 17th. The authors could have delayed submission of the paper to allow more details to be included in it or publicized a follow-up paper with these details. An extra benefit of this approach is that more data could have been included, to check for expected fading of the source.

A competition emerges

Why the rush? I can see only one answer: competition. Another team, led by Nial Tanvir of the University of Leicester, originally proposed for the data that Berger et al. used, which was Director's Discretionary Time (DDT) and was publicly available straight away. Tanvir et al. submitted their own paper, titled "A search for kilonova emission associated with GRB 130603B: the smoking gun signature of a compact binary merger event" only 3 days after Berger et al submitted their paper. An Astrobites blog post by Elisa Chisari gives more details about the astrophysics.

The competition aspect is obvious because Tanvir et al. finished their abstract by saying
"We note that we felt compelled to submit this provisional report of our work, despite our HST DDT program being incomplete, due to other authors having already posted an analysis of the publicly available first epoch data." 
That’s not a statement you see every day on the arXiv, and I look at it regularly. The good news is that they also argued a kilonova may have been detected, but they cautioned that "we cannot yet say whether the light is transient in nature".

They soon cleared up this uncertainty when a second set of HST observations from the DDT program came in. This time the data was not made public straight away, as it usually is. Observers can ask for a short proprietary period if they have a good reason. Tanvir told me: 
"We didn't ask in the first instance, but after our experience with the first epoch, we argued that it was important to give sufficient time to do a careful analysis of the data without being rushed into publication." 
So, Tanvir and his team were given one month to work on the data exclusively, before it went public and they came out ahead in the next stage of the race with Berger et al. On Saturday August 3rd, the day that the second set of data went public, they had a paper published in Nature with Advance Online Publication: "A ‘kilonova’ associated with the short-duration γ-ray burst GRB 130603B" and posted it online at the arXiv. This paper incorporated the second epoch of HST data and new ground-based data and reported clear dimming of the source, as shown in the press release image put out by Space Telescope Science Institute (STScI) that accompanied the Nature paper. The publicity emphasized the strong evidence that the kilonova detection provides for the origin of short GRBs.
 
Publicity images from the Hubble Space Telescope for the Nature paper by Nial Tanvir et al. Credit: NASA, ESA, N. Tanvir (University of Leicester), A. Fruchter (STScI), and A. Levan (University of Warwick)
The Nature paper was received on July 16th and accepted on July 26th. I asked Tanvir what argument they made to Nature for such rapid publication and he said: 
"Nature were aware that there was a likelihood that the result of the second epoch would be published/publicised first elsewhere if their publication was not sufficiently quick. Of course, the normal refereeing process had to be adhered to, which could have delayed things, but thankfully didn't."
Their Nature paper is clearly very different from the "provisional report" they submitted to the arXiv earlier, not surprisingly because of the extra HST data. Ironically, their paper addresses the production of elements more directly than the Berger et al. paper by saying "it is speculated that this mechanism may be the predominant source of stable r-process elements in the Universe" and "If this simplest interpretation of the data is correct, it provides […] (ii) confirmation that such mergers are likely sites of significant r-process production."

However, Berger et al. weren't finished. They put a revised version of their paper on the arXiv on the same day that the Tanvir et al. paper was published in Nature. Along with the extra HST data they added data from Magellan, a ground-based telescope. Now the title of their paper makes a clear statement: "An r-Process Kilonova Associated with the Short-Hard GRB 130603B". Berger told me that they obtained the HST data from the archive as soon as it was available (August 3rd) and immediately worked on the data and then submitted their revised paper to the arXiv about 3 hours later. This is a remarkably fast turnaround, though I note that the PDF of the paper is dated August 6th, so they may have revised this quick submission.

Both groups ran a strong race, as they worked very quickly on their papers and got what seems to be the right interpretation when they only had some of the data. In a sense this was Olympic-level research in the race for gold's origin. The authors knew something about the other team's work because they both submitted reports to the Gamma-ray Coordinates Network (GCN) describing early progress. Tanvir et al's report came first on June 13th followed by Berger et al's more detailed report only 7 hours later.

Aggressive publicity

There were several striking features about the Berger team's publicity, as noted earlier: the heavily framed story about gold, the short schedule for the analysis of the data, the unfinished peer review and the incomplete data for the initial submission to the arXiv. It's the combination of these features that makes this publicity aggressive. Contrast this with the Tanvir et al. team who took the classical approach by not pursuing publicity until the full set of data came in and the paper was accepted in a peer-reviewed journal.
 
An artist's impression of two neutron stars merging. Credit: Dana Berry, SkyWorks Digital, Inc.
I have some experience with heavily framed stories, and my opinion has evolved with time. We've dug stories out of papers before, but I'm less interested in adopting this approach now. It's gratifying to find a hidden feature in a story, but issues with transparency and accessibility can result, both for science writers and for others who look at the paper after being directed from the release or news articles. Here, several key details of the gold calculation are missing, for example. I explain why this might be important below.

Research on transient phenomena is clearly a high-pressure field, where analysis and paper writing can occur at a very fast rate. One example is an interesting object called SN 2009ip. This object was originally thought to be a supernova, but when another outburst was observed it was reclassified as a supernova impostor. A subsequent outburst from SN 2009ip was claimed to be a real supernova, as I wrote in a blog post last year. The team who made this claim submitted a paper on the same day that some of the data were obtained (they provided some details later about how this was done). They clearly wanted to beat their competition in submitting a paper on this event (though note that they did not pursue publicity until their paper was accepted several months later). Subsequently, at least 4 papers have appeared disputing the claim that a supernova occurred. As time passes it will become clear if the star was destroyed and a supernova really occurred. If it didn’t, I’ll correct my blog posts and consider writing a new one. Although I’m impressed with the skill and drive shown by astronomers in these fields, it's not clear that extremely rapid analysis and submission consistently delivers the most reliable work.

What about the importance of peer review? In my work at the Chandra X-ray Center we have been advised that NASA only does press releases on papers that have been accepted after peer review (although their Communication Policy makes no mention of this requirement). Peer review isn’t infallible, but it is a useful filter and I think NASA’s policy is sound. (Note that CfA is not part of NASA and weren’t bound by their policy on peer review, though I believe they usually follow it.) It’s not common for major, publicity-killing revisions to occur in the papers that we track as they undergo peer review, but it's not unheard of either. It is very common to see comments along these lines: “we’ll recommend this paper be accepted if certain changes are made”.

In the case of Berger et al. the press conference was held one month after the paper was submitted and about two weeks before the second epoch of HST data became available. Because of the latter, I’ll speculate that the most reasonable reaction from a referee – who very likely would have known about the second epoch – would be to say “I’ll review the paper after you add the extra data in”. The potential was to turn those guarded statements in the first version of the paper into much stronger ones, by waiting just a few weeks and doing some extra analysis. At that stage only Tanvir et al. knew what the follow-up HST data were showing.

To summarize, I think the most aggressive aspect of the publicity was the timing – the paper that led to the CfA press conference was based on incomplete data. It was provisional work. In the revised paper they were soon able to make a much stronger case for a kilonova having been detected.

How much gold was made?

Both teams concluded that a kilonova was observed, a cosmic first. However, after reviewing the numbers for the Berger paper and publicity, I noticed a couple of potential discrepancies. The calculated ejecta mass was ~0.01 solar masses in the original Berger et al. paper – as quoted in the press release – and it was 0.03–0.08 solar masses in their revised paper (using the same ejecta velocity as the original paper it was 0.03). A factor of 3 isn't enormous for astronomy, especially for groundbreaking work, but I am curious about whether re-analysis with the new data was the reason for the difference (I asked Berger and a co-author but they did not answer).

The ejecta mass is used to estimate the gold yield, where there's potentially a bigger problem with the numbers. The amount of gold wasn't quoted in either version of the Berger et al. papers, but in the CfA press release a value "as large as 10 moon masses" was given. However, a Washington Post article by Joel Achenbach, quotes an estimate of 20 Earth masses of gold given by astrophysicist Daniel Kasen, an expert on kilonovas. This value is 200 times larger than the CfA release's estimate. I suspected a typo was made in the Washington Post article, but Kasen promptly confirmed this number and provided some details of his calculation when I contacted him (I asked Berger and a co-author about the discrepancy, but they did not answer). Taken at face value, these numbers suggest gold might be much more common than the authors thought, assuming the same neutron star merger rate. This is where some details about the gold calculation might have been valuable.
Daniel Kasen from Berkeley Astronomy Department. Credit: Berkeley and Daniel Kasen
This uncertainty detracts from the confident claims made in the press conference, including "This material also contains several moons worth of gold, suggesting that neutron star collisions are the primary producers of gold in the universe" and "we can in principle account for the amount of gold that we see in the Milky Way and the universe, so there's no real need to invoke another mechanism" [where this other mechanism is the supernova idea]. This statement was qualified somewhat with: "This doesn't mean that supernovae themselves do not contribute some fraction for the gold in the universe but I would argue that this is the dominant formation mechanism."

This confidence carried over into much of the press coverage. For example a USA Today story began with: "The gold glinting on your wedding band was likely born in a cataclysmic merger of two exceedingly exotic stars, astronomers report Wednesday". "A strange glow in space has provided fresh evidence that all the gold on Earth was forged from ancient collisions of dead stars, researchers reported Wednesday,” said the Associated Press.

A more cautious viewpoint came in an excellent article in New Scientist by Lisa Grossman, who concluded: 
"Daniel Kasen of the University of California, Berkeley, thinks the idea is intriguing, but he points out that we don't know for sure how common neutron star mergers are. 
"For reasonable rates, if they're ejecting that much mass, it's plausible or likely that they're ejecting a big chunk of the heavy elements of the universe," he says. "But it would be helpful if they get more data points.""
That sounds like excellent advice, and I'm sure both the Berger and Tanvir teams – and others – will be searching hard for more cases. In the revised version of their paper Berger et al. admit that the rate of compact binary mergers is "poorly known", and I don’t know how quickly it will improve.

In the meantime, it will be interesting to follow reactions to the work of Berger and Tanvir. I suspect that the supernova idea won't be easily tossed aside, because there are some strong supporters of it. For example, both Berger et al. and Tanvir et al. cite a paper by Metzger et al. (2010) that discusses the r-process and kilonovas in detail. Metzger et al. (2010) mentions the astrophysical origin of the r-process remaining one of the "great mysteries in nuclear astrophysics” and quotes a paper by Qian and Wasserburg (2007) for a recent review. One might guess that such a paper would heavily support neutron star mergers as a way to make heavy elements like gold. However, in a 52-page paper Qian and Wasserburg dismiss the idea of neutron star mergers in a 15-line footnote on page four, and don't otherwise discuss it! I didn’t ask their opinion about this study, but it would be interesting to hear their reaction. (Side comment: Wasserburg lists his address as: "The Lunatic Asylum, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA." I haven't heard of this department at Caltech.)

Who won?

Having described this academic competition as a race, it’s fair to ask who won? Overall, I don’t think there was a clear winner. The Berger et al. team certainly obtained more publicity than Tanvir et al. but the latter team had their paper published first. They also adopted a more conservative – and I think more robust – approach to the publicity.

To their credit, the Berger et al. team showed courage and confidence in publicizing their initial submission. They took a gamble by saying a kilonova was likely to have been observed and they were vindicated when the extra data came in. I know astronomers who won’t submit papers to the arXiv until they’ve been accepted, so posting a paper and then publicizing it before acceptance would definitely be considered too brave.

I am interested in hearing people’s reactions to this story, including the competition that took place and the aggressive publicity tactics that were employed. What do scientists think? What about science communicators and public information officers?


(*) This post was originally meant to be a follow-up to my earlier post It's been done before, or it's wrong. Part I. It was going to be Part II, corresponding to the "it's wrong" reference, but then the story developed.
(**) A hat-tip to Joe Hanson for posting about the Neil Tyson clip.

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