domingo, 24 de marzo de 2013

The Discovery of a Colossal Stellar Corpse


There are believed to be millions of neutron stars in the Milky Way. The most massive known to date -a stellar corpse twice as massive as our Sun, was discovered in 2010 by astronomers using the National Science Foundation's Green Bank Telescope (GBT). With all their mass packed into a sphere the size of a small city, neutron stars protons and electrons are crushed together into neutrons. A neutron star can be several times more dense than an atomic nucleus, and a thimbleful of neutron-star material would weigh more than 500 million tons. This tremendous density makes neutron stars an ideal natural "laboratory" for studying the most dense and exotic states of matter known to physics. 

A pulsar is a rotating neutron star that can produce radiation by spinning its powerful magnetic field through space. A neutron star has used up most of its rotational energy moving its magnetic field, and and so it gradually slows down. When it slows down enough, it no longer radiates much energy, and so it is no longer considered a pulsar. This usually happens within a few million years. If a neutron star had only a weak magnetic field, it would also not be a pulsar. The first pulsar, dubbed LGM-1 for "Little Green Men," was observed on November 28, 1967 by Jocelyn Bell and Antony Hewish (who was awarded the Nobel Prize for the discovery creating a hugely justified controvery), who were  initially baffled as to the seemingly unnatural regularity of its emissions.   
At the time people had no idea what these radio signals were so the unknownsource was dubbed " little green men" (LGM) as one possibility could have been that the sounds came from some extraterrestrial form of life.  This link takes you to an article originally presented as an after-dinner speech by Bell - it's a very personal and entertaining account of the discovery: "Little Green Men, White Dwarfs or Pulsars? during which Bell asked "Where these signals made by men, but men from another civilization." 


The more recent discovery of the monster object was surprising because that much mass means that several theoretical models for the internal composition of neutron stars are ruled out," said Paul Demorest, of the National Radio Astronomy Observatory (NRAO). "This mass measurement also has implications for our understanding of all matter at extremely high densities and many details of nuclear physics," he added.

The scientists used an effect of Albert Einstein's theory of General Relativity to measure the mass of the neutron star and its orbiting companion, a white dwarf star. The neutron star, a pulsar, called PSR J1614-2230, spins 317 times per second, and the companion completes an orbit in just under nine days. The pair, some 3,000 light-years distant, are in an orbit seen almost exactly edge-on from Earth. That orientation was the key to making the mass measurement.


As the orbit carries the white dwarf directly in front of the pulsar, the radio waves from the pulsar that reach Earth must travel very close to the white dwarf. This close passage causes them to be delayed in their arrival by the distortion of spacetime produced by the white dwarf's gravitation. This effect, called the Shapiro Delay, allowed the scientists to precisely measure the masses of both stars. 

"We got very lucky with this system. The rapidly-rotating pulsar gives us a signal to follow throughout the orbit, and the orbit is almost perfectly edge-on. In addition, the white dwarf is particularly massive for a star of that type. This unique combination made the Shapiro Delay much stronger and thus easier to measure," said Scott Ransom, also of NRAO.

The astronomers used a newly-built digital instrument called the Green Bank Ultimate Pulsar Processing Instrument (GUPPI), attached to the GBT, to follow the binary stars through one complete orbit earlier this year. Using GUPPI improved the astronomers' ability to time signals from the pulsar severalfold.The researchers expected the neutron star to have roughly one and a half times the mass of the Sun. Instead, their observations revealed it to be twice as massive as the Sun. That much mass, they say, changes their understanding of a neutron star's composition.

Some theoretical models postulated that, in addition to neutrons, such stars also would contain certain other exotic subatomic particles called hyperons or condensates of kaons."Our results rule out those ideas," Ransom said.

"This measurement tells us that if any quarks are present in a neutron star core, they cannot be 'free,' but rather must be strongly interacting with each other as they do in normal atomic nuclei," said Feryal Ozel of the University of Arizona, lead author of the second paper.

There remain several viable hypotheses for the internal composition of neutron stars, but the new results put limits on those, as well as on the maximum possible density of cold matter.The scientific impact of the new GBT observations also extends to other fields beyond characterizing matter at extreme densities.

A leading explanation for the cause of one type of gamma-ray burst -- the "short-duration" bursts -- is that they are caused by colliding neutron stars. The fact that neutron stars can be as massive as PSR J1614-2230 makes this a viable mechanism for these gamma-ray bursts.

Such neutron-star collisions also are expected to produce gravitational waves that are the targets of a number of observatories operating in the United States and Europe. These waves, the scientists say, will carry additional valuable information about the composition of neutron stars.

"Pulsars in general give us a great opportunity to study exotic physics, and this system is a fantastic laboratory sitting out there, giving us valuable information with wide-ranging implications," Ransom explained. "It is amazing to me that one simple number -- the mass of this neutron star -- can tell us so much about so many different aspects of physics and astronomy," he added.

Source: The Daily Galaxy via the National Radio Astronomy Observatory 

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