NASA's RXTE captures thermonuclear behavior of unique neutron star

Mar 09, 2012 by Francis Reddy
This graph based on RXTE data provides an overview of the changing character of T5X2's X-ray emission during outbursts from Oct. 13 to Nov. 19, 2010. As the persistent X-ray emission rises (upward steps in the plot), the burst rate increases while the burst brightness decreases. The abrupt dropout on Oct. 13 occurred when the moon briefly covered the source. Credit: Credit: NASA/Goddard Space Flight Center

(PhysOrg.com) -- A neutron star is the closest thing to a black hole that astronomers can observe directly, crushing half a million times more mass than Earth into a sphere no larger than a city. In October 2010, a neutron star near the center of our galaxy erupted with hundreds of X-ray bursts that were powered by a barrage of thermonuclear explosions on the star's surface. NASA's Rossi X-ray Timing Explorer (RXTE) captured the month-long fusillade in extreme detail. Using this data, an international team of astronomers has been able to bridge a long-standing gap between theory and observation.

"In a single month from this unique system, we have identified behavior not seen in observations of nearly 100 bursting during the past 30 years," said Manuel Linares, a postdoctoral researcher at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology in Cambridge. He led a study of the RXTE data that will be published in the March 20 issue of The .

On Oct. 10, 2010, the European Space Agency's INTEGRAL satellite detected a transient X-ray source in the direction of Terzan 5, a globular star cluster about 25,000 light-years away toward the constellation Sagittarius. The object, dubbed IGR J17480, is classed as a low-mass X-ray binary system, in which the neutron star orbits a star much like the sun and draws a stream of matter from it. As only the second bright X-ray source to be found in the cluster, Linares and his colleagues shortened its moniker to T5X2.

Three days after the source's discovery, RXTE targeted T5X2 and detected regular pulses in its emission, indicating that the object was a pulsar -- a type of neutron star that emits electromagnetic energy at periodic intervals. The object's powerful magnetic field directs infalling gas onto the star's magnetic poles, producing hot spots that rotate with the neutron star and give rise to X-ray pulses. At NASA's Goddard Space Flight Center in Greenbelt, Md., RXTE scientists Tod Strohmayer and Craig Markwardt showed that T5X2 spins at a sedate -- for neutron stars -- rate of 11 times a second. And because the pulsar's orbital motion imparts small but regular changes in the pulse frequency, they showed that the pulsar and its sun-like companion revolve around each other every 21 hours.

This illustration compares the size of a neutron star to Manhattan. The crushed core of a star that has exploded as a supernova, a neutron star packs more mass than the sun into a sphere just 10 to 15 miles wide. Credit: NASA/Goddard Space Flight Center

That same day, RXTE observed its first burst from the system: an intense spike in X-rays lasting nearly 3 minutes and caused by a thermonuclear explosion on the neutron star's surface. Ultimately, RXTE cataloged some 400 events like this between Oct. 13 and Nov. 19, with additional bursts observed by INTEGRAL and NASA's Swift and Chandra observatories. NASA decommissioned RXTE on Jan. 5, 2012.

In the T5X2 system, matter streams from the sun-like star to the neutron star, a process called accretion. Because a neutron star packs more than the sun's mass into a sphere between 10 and 15 miles across -- about the size of Manhattan or the District of Columbia -- its surface gravity is extremely high. The gas rains onto the pulsar's surface with incredible force and ultimately coats the neutron star in a layer of hydrogen and helium fuel. When the layer builds to a certain depth, the fuel undergoes a runaway thermonuclear reaction and explodes, creating intense X-ray spikes detected by RXTE and other spacecraft. The bigger the blast, the more intense its X-ray emission.

Models designed to explain these processes made one prediction that had never been confirmed by observation. At the highest rates of accretion, they said, the flow of fuel onto the neutron star can support continuous and stable thermonuclear reactions without building up and triggering episodic explosions.

At low rates of accretion, T5X2 displays the familiar X-ray pattern of fuel build-up and explosion: a strong spike of emission followed by a long lull as the fuel layer reforms. At higher accretion rates, where a greater volume of gas is falling onto the star, the character of the pattern changes: the emission spikes are smaller and occur more often.

But at the highest rates, the strong spikes disappeared and the pattern transformed into gentle waves of emission. Linares and his colleagues interpret this as a sign of marginally stable nuclear fusion, where the reactions take place evenly throughout the fuel layer, just as theory predicted.

"We see T5X2 as a 'model burster,' the one that's doing everything expected of it," said Diego Altamirano, an astrophysicist at the University of Amsterdam in The Netherlands and a co-author on the paper describing the findings.

The question now before the team is why this system is so different from all others studied in previous decades. Linares suspects that T5X2's slow rotation may hold the key. Faster rotation would introduce friction between the neutron star's surface and its fuel layers, and this frictional heat may be sufficient to alter the rate of nuclear burning in all other bursting neutron stars previously studied.

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Going
5 / 5 (3) Mar 10, 2012
Its incredible that we are able to build models which explain what is happening in such an alien environment as a neutron star surface.
Callippo
1 / 5 (15) Mar 10, 2012
It's not so amazing (if not unethical) if we realize, this research has no practical applications, whereas it's quite expensive and whole the world is facing energetic and economical crisis. We have a much more useful things to develop which could decrease the risk of wars and which are waiting for years. We cannot spend all money in research of far side of Pluto just because it's possible to spend it. So I'd propose to pay the scientists who are developing useless stuffs like the artists for balancing the apparent discrepancy between support of useful research (like the cold fusion) and abundance of useless research (like the astrophysics). If someone likes it, he should pay for it at private basis.
Pkunk_
4.6 / 5 (9) Mar 10, 2012
We cannot spend all money in research of far side of Pluto just because it's possible to spend it. So I'd propose to pay the scientists who are developing useless stuffs like the artists for balancing the apparent discrepancy between support of useful research (like the cold fusion) and abundance of useless research (like the astrophysics). If someone likes it, he should pay for it at private basis.


Considering the money spent on astrophysical research , nothing else gives the same bang for buck.
You are able to observe and learn far more about the Universe we live in by observing these phenomena occuring thousands to millions of light years away.

Also astrophysical research yields actual and measurable results , unlike "cold" fusion which yields a lot of hot air (due to the energy put in to the system to allegedly achieve "fusion" , a word used very loosely when it comes to the cold variety).
Callippo
1 / 5 (12) Mar 10, 2012
I'm quite glad, you're posting it, because it will serve as an evidence of the fact, how deeply the proponents of mainstream physics are actually separated from reality and incompetent to judge, what is real in nature and what not. The fact, you can derive and integrate mechanically doesn't mean, you're understanding the physics - actually the quite opposite effect may appear, because the learning of formal math is difficult and it consumes the time and the intellectual capacity, which is missing somewhere else after then.

This incompetence is a problem of educational system and whole scientific generation, which should be essentially replaced.
Shootist
4 / 5 (8) Mar 10, 2012
This incompetence is a problem of educational system and whole scientific generation, which should be essentially replaced.


Speaking strictly for yourself, I'm sure?
Ironhorse
2.8 / 5 (4) Mar 10, 2012
@Callippo The only way to decrease the risk of war is to eliminate the human race, which considering that's us, isn't practical.

Also, the education system, though not perfect, would be much improved by requiring stringent standards with respect to home schooling, so that it can't be used to hideout from inconvenient facts.
neiorah
not rated yet Mar 12, 2012
With all this money we could develop a drug that can be widely sprayed in the unruly Arab countries so it lowers their testosterone count and everyone settles down. Make it last 6 months and maybe their brain will kick in instead of them acting only on emotion like they are now.
Kinedryl
1 / 5 (1) Mar 12, 2012
The only way to decrease the risk of war is to eliminate the human race
Nope, the people made wars even when the civilization was a way smaller. The civilization goes into the war like massive star, which collapses, because it becomes too heavy. But it's the ratio of energy production and gravitation, which determines, whether the star will remain stable or it will collapse. Even the massive star may remain stable, if its regulating its energy consumption and production well. For example, when we will implement cold fusion, we could decrease the price of food and the social tension between Arabians and western world due the lowered consumption of oil and whole civilization will remain stable and growing at least for some time.
Pkunk_
1 / 5 (1) Mar 20, 2012
Kinedryl said-
For example, when we will implement cold fusion, we could decrease the price of food and the social tension between Arabians and western world due the lowered consumption of oil and whole civilization will remain stable and growing at least for some time.


While the goal is admirable , the means are pretty much vaporware. "Cold" fusion is pretty much a scientific fraud which requires more energy going in than coming out. I think it's ridiculous to expect an unproven tech like this to

The only proven technology which can meet the enormous energy requirements of the near future is Nuclear fission , until we crack the big mutah - HOT Fusion (which is actually documented to work , and can release thousands to millions of times the energy put into it ).
Manhar
1 / 5 (1) Apr 30, 2012
Compact mass of neutron star or black hole is an accepted fact. Nobody has raised doubt about it. I wonder if expert in this area can explain the existing forces in the universe and the process to achieve such compacted mass. The universe is assumed to be clear of such forces to allow dark matters, gypsy stars and planets and galaxies to float freely. How mass is compacted without apparent compacting forces?