Hotter than expected neutron star surfaces help explain superburst frequency

April 13, 2007

EAST LANSING, Mich. -- A new theoretical thermometer built from heavy-duty mathematics and computer code suggests that the surfaces of certain neutron stars run significantly hotter than previously expected. Hot enough, in fact, to at least partially answer an open question in astrophysics -- how to explain the observed frequency of ultra-violent explosions known as superbursts that sometimes ignite on such stars' surfaces?

"This is the first model that goes into some reasonable detail about the nuclear physics that occur in the crusts of accreting neutron stars," said Hendrik Schatz, NSCL professor and co-author of a paper that will be published in The Astrophysical Journal in June. One of Schatz's co-authors, NSCL assistant professor Ed Brown, will present the results April 17 at a meeting of the American Physical Society in Jacksonville, Fla.

Superbursts emanate from binary systems in which a neutron star orbits a companion star. When the two stars get close enough together, a steady rain of material is sucked away from the companion star onto the surface of the neutron star.

Because a neutron star is so dense -- on Earth, one teaspoonful would weigh a billion tons -- the companion star material that reaches the neutron star surface is strongly compressed and heated. Eventually nuclear reactions trigger an explosion that burns through the surface layer of accumulated material, resulting in a burst of X-rays clearly detectable by ground- and space-based instruments.

X-ray bursts repeat every few hours to days, along the way fusing hydrogen and helium into a mixture of elements that is itself potentially reactive. In contrast, superbursts occur when, after many months, the accumulated "ashes" produced in the X-ray bursts ignite in a different, even more dramatic nuclear explosion.

The result is an outpouring of X-rays some 1,000 times as energetic as a standard X-ray burst. One superburst, which lasts only on the order of a few hours, releases as much energy as the sun will radiate in a decade.

Though hardly subtle astrophysical phenomena, superbursts remain shrouded in some mystery, largely because only twelve of the extreme events have ever been observed. This mystery is what attracted the attention of researchers participating in the Joint Institute for Nuclear Astrophysics, or JINA, project.

Working with colleagues at Los Alamos National Laboratory and the University of Mainz in Germany, JINA-affiliated NSCL scientists set out to build the most accurate model to-date of the crusts of accreting neutron stars. The team calculated that reactions in the stars' crusts release 10 times more heat than indicated by earlier models.

At least in part, this newly discovered heat helps to reconcile the work of theorists and experimentalists who study neutron stars. Prior to Schatz and Brown's research, theoretical astrophysicists predicted that superbursts should occur every ten years or so. Now, according to the new calculation, theorists can explain why the gigantic explosions should occur every three or four years.

But more work remains to be done. According to observational data, superbursts occur roughly annually -- and scientists still aren't altogether sure why.

"So this doesn't quite solve the problem," Brown said. "It's still an open question as to how nature ignites superbursts."
-end-
JINA is supported by the National Science Foundation.

NSCL is a world-leading laboratory for rare isotope research and nuclear science education.

Additional resources

Preprint of forthcoming Astrophysical Journal paper, "Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition": http://arxiv.org/abs/astro-ph/0609828; the paper's lead author is former NSCL-JINA postdoc Sanjib Gupta, who now works at Los Alamos

JINA science nugget: www.jinaweb.org/docs/nuggets_07/schatz_sanjib.pdf

Michigan State University

Related Neutron Star Articles from Brightsurf:

Black hole or no black hole: On the outcome of neutron star collisions
A new study lead by GSI scientists and international colleagues investigates black-hole formation in neutron star mergers.

UMD astronomers find x-rays lingering years after landmark neutron star collision
It's been three years since the landmark detection of a neutron star merger from gravitational waves.

Microscopic deformation of a neutron star inferred from a distance of 4500 light-years
Gravitational waves, which are ripples in spacetime, have recently provided a new window to the universe.

Method proposed for more accurate determinations of neutron star radii
Neutron stars are the smallest and densest astrophysical objects with visible surfaces in the Universe.

Unequal neutron-star mergers create unique "bang" in simulations
In a series of simulations, an international team of researchers determined that some neutron star collisions not only produce gravitational waves, but also electromagnetic radiation that should be detectable on Earth.

ALMA finds possible sign of neutron star in supernova 1987A
Based on ALMA observations and a theoretical follow-up study, scientists suggest that a neutron star might be hiding deep inside the remains of Supernova 1987A.

Scientists discover pulsating remains of a star in an eclipsing double star system
Scientists from the University of Sheffield have discovered a pulsating ancient star in a double star system, which will allow them to access important information on the history of how stars like our Sun evolve and eventually die.

How big is the neutron?
The size of neutrons cannot be measured directly: it can only be determined from experiments involving other particles.

The force is strong in neutron stars
Physicists at MIT and elsewhere have for the first time characterized the strong nuclear force, and the interactions between protons and neutrons, at extremely short distances.

New neutron detector can fit in your pocket
Researchers at Northwestern University and Argonne National Laboratory have developed a new material that opens doors for a new class of neutron detectors.

Read More: Neutron Star News and Neutron Star Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.