Universe Today

Astronomers using ESA?s X-ray observatory XMM-Newton have detected a small, bright ?hot spot? on the surface of the neutron star called Geminga, 500 light-years away. The hot spot is the size of a football field and is caused by the same mechanism producing Geminga?s X-ray tails. This discovery identifies the missing link between the X-ray and gamma-ray emission from Geminga.

Neutron stars are the smallest kind of stars known. They are the super-dense remnants of massive stars that died in cataclysmic explosions called supernovae. They have been thrown through space like cannonballs and set spinning at a furious rate, with magnetic fields hundreds of billions of times stronger than Earth?s.

In the case of Geminga, this cannonball contains one and a half times the mass of the Sun, squeezed into a sphere just 20 kilometres across and spinning four times every second.

A cloud bustling with electrically charged particles surrounds Geminga. These particles are shepherded by its magnetic and electric fields. ESA?s XMM-Newton observatory had already discovered that some of these particles are ejected into space, forming tails that stream behind the neutron star as it hurtles along.

Scientists did not know whether Geminga?s tails are formed by electrons or by their twin particles with an opposite electrical charge, called positrons. Nevertheless, they expected that, if for instance electrons are kicked into space, then the positrons should be funnelled down towards the neutron star itself, like in an ?own goal?. Where these particles strike the surface of the star, they ought to create a hot spot, a region considerably hotter than the surroundings.

An international team of astronomers, lead by Patrizia Caraveo, IASF-CNR, Italy, has now reported the detection of such a hot spot on Geminga using ESA?s XMM-Newton observatory.

With a temperature of about two million degrees, this hot spot is considerably hotter than the one half million degrees of the surrounding surface. According to this new work, Geminga?s hot spot is just 60 metres in radius.

“This hot spot is the size of a football field,” said Caraveo, “and is the smallest object ever detected outside of our Solar System.” Details of this size can presently be measured only on the Moon and Mars and, even then, only from a spacecraft in orbit around them.

The presence of a hot spot was suspected in the late 1990s but only now can we see it ?live?, emitting X-rays as Geminga rotates, thanks to the superior sensitivity of ESA?s X-ray observatory, XMM-Newton.

The team used the European Photon Imaging Cameras (EPIC) to conduct a study of Geminga, lasting about 28 consecutive hours and recording the arrival time and energy of every X-ray photon that Geminga emitted within XMM-Newton?s grasp.

“In total, this amounted to 76 850 X-ray counts ? twice as many as have been collected by all previous observations of Geminga, since the time of the Roman Empire,” said Caraveo.

Knowing the rotation rate of Geminga and the time of each photon?s arrival meant that astronomers could identify which photons were coming from each region of the neutron star as it rotates.

When they compared photons coming from different regions of the star, they found that the ?colour? of the X-rays, which corresponds to their energy, changed as Geminga rotated. In particular, they could clearly see a distinct colour change when the hot spot came into view and then disappeared behind the star.

This research closes the gap between the X-ray and gamma-ray emission from neutron stars. XMM-Newton has shown that they both can originate through the same physical mechanism, namely the acceleration of charged particles in the magnetosphere of these degenerate stars.

“XMM-Newton?s Geminga observation has been particularly fruitful,” said Norbert Schartel, ESA?s Project Scientist for XMM-Newton. “Last year, it yielded the discovery of the source tails and now it has found its rotating hot spot.”

Caraveo is already applying this new technique to other pulsating neutron stars observed by XMM-Newton looking for hot spots. This research represents an important new tool for understanding the physics of neutron stars.

Original Source: ESA News Release

Nearly five billion years ago, the giant gaseous planets Jupiter and Saturn formed, apparently in radically different ways.

So says a scientist at the University of California’s Los Alamos National Laboratory who created exhaustive computer models based on experiments in which the element hydrogen was shocked to pressures nearly as great as those found inside the two planets.

Working with a French colleague, Didier Saumon of Los Alamos’ Applied Physics Division created models establishing that heavy elements are concentrated in Saturn’s massive core, while those same elements are mixed throughout Jupiter, with very little or no central core at all. The study, published in this week’s Astrophysical Journal, showed that refractory elements such as iron, silicon, carbon, nitrogen and oxygen are concentrated in Saturn’s core, but are diffused in Jupiter, leading to a hypothesis that they were formed through different processes.

Saumon collected data from several recent shock compression experiments that have showed how hydrogen behaves at pressures a million times greater than atmospheric pressure, approaching those present in the gas giants. These experiments – performed over the past several years at U.S. national labs and in Russia – have for the first time permitted accurate measurements of the so-called equation of state of simple fluids, such as hydrogen, within the high-pressure and high-density realm where ionization occurs for deuterium, the isotope made of a hydrogen atom with an additional neutron.

Working with T. Guillot of the Observatoire de la Cote d’Azur, France, Saumon developed about 50,000 different models of the internal structures of the two giant gaseous planets that included every possible variation permitted by astrophysical observations and laboratory experiments.

“Some data from earlier planetary probes gave us indirect information about what takes place inside Saturn and Jupiter, and now we’re hoping to learn more from the Cassini mission that just arrived in Saturn’s orbit,” Saumon said. “We selected only the computer models that fit the planetary observations.”

Jupiter, Saturn and the other giant planets are made up of gases, like the sun: They are about 70 percent hydrogen by mass, with the rest mostly helium and small amounts of heavier elements. Therefore, their interior structures were hard to calculate because hydrogen’s equation of state at high pressures wasn’t well understood.

Saumon and Guillot constrained their computer models with data from the deuterium experiments, thereby reducing previous uncertainties for the equation of state of hydrogen, which is the central ingredient needed to improve models of the structures of the planets and how they formed.

“We tried to include every possible variation that might be allowed by the experimental data on shock compression of deuterium,” Saumon explained.

By estimating the total amount of the heavy elements and their distribution inside Jupiter and Saturn, the models provide a better picture of how the planets formed through the accretion of hydrogen, helium and solid elements from the nebula that swirled around the sun billions of years ago.

“There’s been general agreement that the cores of Saturn and Jupiter are different,” Saumon said. “What’s new here is how exhaustive these models are. We’ve managed to eliminate or quantify many of the uncertainties, so we have much better confidence in the range within which the actual data will fall for hydrogen, and therefore for the refractory metals and other elements.

“Although we can’t say our models are precise, we know quite well how imprecise they are,” he added.

These results from the models will help guide measurements to be taken by Cassini and future proposed interplanetary space probes to Jupiter.

Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA’s Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.

Los Alamos develops and applies science and technology to ensure the safety and reliability of the U.S. nuclear deterrent; reduce the threat of weapons of mass destruction, proliferation and terrorism; and solve national problems in defense, energy, environment and infrastructure.

Original Source: Los Alamos News Release