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The Race to Explain a Star's Mysterious Cooling
by Peter Edmonds
February 23, 2011 ::
Two new reports have uncovered remarkable details about the interior of a
tiny star located in our Galaxy about 11,000 light years from Earth. These
results, announced in this press release, reveal a superfluid forming in the neutron star in the famous Cassiopeia A (Cas A) supernova remnant. This unprecedented result supports the idea that
neutron stars contain the vast majority of all superconducting and
superfluid matter in the Universe.
The story behind this announcement is also interesting, and involves some
healthy competition and a tight race to explain the mysterious cooling of
this neutron star. There are striking similarities between the two
different papers, one led by Dany Page from the National Autonomous
University in Mexico, and the other by Peter Shternin from the Ioffe
Institute in St Petersburg, Russia. The studies were conducted by
independent research teams, yet they obtained almost identical results and
were submitted for publication only one day apart.
The detection of a faint X-ray point source at the center of Cas A in 1999
was one of the first scientific results from Chandra, but it wasn't until
2009 before a good theoretical model for X-ray emission from this neutron star was published in
Nature  by Wynn Ho from Southampton University in the UK, and Craig
Heinke, from the University of Alberta in Canada. They realized that their
estimate of the neutron star's surface temperature would be important for
understanding how such objects cool and evolve, and began collaborating
with a team led by Dmitry Yakovlev and Shternin, from the Ioffe Institute
in St Petersburg, Russia. However, they had just a single temperature
measurement and did not realize that much more information was waiting to
be discovered about the star's temperature.
In March 2010, a collaborator asked Ho whether the Cas A neutron star was
cooling. Heinke looked into this, and using multiple Chandra observations
of Cas A, he found a clear and unexpectedly large temperature decline of
about 4% over ten years, once some calibration problems were solved. The
result was quickly written up, submitted to The Astrophysical Journal
Letters  and accepted, and Heinke placed the paper on astro-ph on
Tuesday, July 27th, 2010. Only a few days later he received this excited
email from Page:
Dear Craig and Wynn,
I just found your paper on cooling of Cas A:
Just AMAZING !
In Heinke's words, "that's the kind of email one loves to get!" Page's
email to Heinke did not exaggerate his excitement, because there are 31
emails in his "Cas A" file during the weekend of July 31st, and he wrote to
his collaborators - Madappa Prakash (Ohio University), James Lattimer
(State Universty of New York at Stony Brook), and Andrew Steiner (Michigan
State University) - recommending that they interpret this cooling and get
a paper out by the next Monday, before Yakovlev and his colleagues did!
Page was right about his competitor's plans, but the star's cooling seemed
too rapid to be easily explained by either team. But then, in the middle of
September, Shternin enjoyed a "eureka" moment after hiking in a rugged part
of Russia and encountering some very steep slopes. He woke early the next
morning and went back to the Cas A cooling data. He knew that the onset of
superfluidity would help explain the rapid cooling, but realized that this
wasn't enough to explain the large drop in temperature. But, then he
remembered his mountain climbing and thought, "if you want to have a
steeper slope, just raise the top level." Adding in a proton superconductor
that formed much earlier in the lifetime of the star played this role,
because it slowed down the early cooling of their model neutron star. This
kept the temperature high before the cooling
from the neutron superfluid kicked in a few hundred years later, giving a
steeper slope for the temperature decline.
Shternin started writing a paper with Yakovlev, Heinke, Ho and Dan Patnaude
(Harvard-Smithsonian Center for Astrophysics) about these new results.
However, they were waiting for a new Chandra observation of Cas A to check
that the cooling was continuing as expected. The data were available by
November 5th, but Heinke had a busy teaching and research schedule and
didn't get to the data right away.
This would work in Page's favor. After his initial excitement at seeing
Heinke and Ho's paper at the end of July, Page also had difficulty
explaining the rapid cooling of the star, so he postponed working on it "to
tomorrow and the tomorrow became, as usual self generating!" But then a
conference approached where he was scheduled to give his usual talk about
neutron star cooling, a prospect that he did not find very stimulating. So,
he decided to turn "the tomorrow into a today and just do it, to have
something new and exciting to talk about."
Madappa Prakash recalled how frustrated Page was, in the numerous emails
they exchanged, before a light bulb went off in Page's head about the
effect of proton superconductivity to accelerate the cooling. Page's
jubilant e-mail said, "I got it," and left his colleagues waiting to figure
out what it was that he got. He explained the details in his conference
talk on November 18th and was approached afterwards by Sharon Morsink, a
colleague of Heinke from the University of Alberta. Morsink told Page,
"Craig will be glad to know you're working on his data." Page knew that the
clock had started ticking, since Morsink would report back to Heinke about
Page's progress. It was now very clear that the race between the two teams
was on and Page worked even harder than before to write his paper.
Heinke's reaction to Morsink's news was, "oh no, I've delayed the paper and
now we'll get scooped!" Heinke immediately analyzed the last observation
that weekend and on November 27th he emailed his collaborators both his
results and a report about Page's talk. Yakovlev was sure Page would submit
soon, but thought they should finish their paper quickly and "hope to
compete." But, it was too late. On November 29th, Page emailed Ho and
Heinke the paper that he had just submitted to Physical Review Letters  and was posting to astro-ph that day. Heinke emailed back,
congratulated him and explained they were working on a similar paper.
Shternin and his co-authors considered submitting and posting to astro-ph
the same day, but decided not to because such a paper wouldn't include the
new observation. They waited another day for Heinke to "frantically
scribble down a couple of paragraphs on the observations, some of which I
did standing outside, holding my laptop during an ill-timed fire alarm."
They finished the paper and submitted to the Monthly Notices of the Royal
Astronomical Society (MNRAS)  and astro-ph the next day, November
So, Page's team won the race, but with Shternin and co-authors so close
behind that the submissions were effectively simultaneous. The papers both
received good reports from referees and were soon accepted.
This story explains the similarity in timing between the two papers, but
not the similarity in the results. The obvious explanation is that they
have both explained, independently, what is really happening in the core of
the Cas A neutron star. This is an important advance in understanding these
remarkable stars and especially noteworthy because no other neutron star is
likely to provide a similar opportunity in the near future.
Competition plays an important role in motivating scientists. The prospect
of "going where no-one has gone before," at least in an intellectual sense,
is a key part of the allure of research. Also, astronomers, like other
scientists, are smart, ambitious people who are used to doing well
academically. So, competition has been a natural part of astronomical
research for a long time. Recent examples include the discovery of
observational evidence for dark energy and the estimation of the Hubble
constant. There is also intense competition for observing time on
telescopes like Chandra and Hubble, which are heavily oversubscribed. All
of this competition has sometimes intensified into rivalry or even
animosity between different groups, presumably because the stakes have been
perceived to be very high, or because minor problems and personality
clashes have escalated.
These problems do not appear to have been an issue at all for the Page and
Shternin teams, who clearly respect and admire each other, and behave
accordingly. For example, both teams have generously argued that the other
team should be mentioned first in early press articles that appeared. So,
while competition motivated them to work more quickly, their working
relationship remained very cordial and professional. They deserve a
significant amount of credit for their scientific work and for their
- Ho, W. & Heinke, C., 2009, Nature, 462, 71
- Heinke, C. & Ho, W., 2010, ApJ, 719, L167
- Page, D., et al, 2011, PRL (http://lanl.arxiv.org/abs/1011.6142)
- Shternin, P. et al., 2011, MNRAS (http://lanl.arxiv.org/abs/1012.0045)