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Hotspot Found on Geminga.
Astronomers have detected a small bright spot on a familiar Neutron star, Geminga, located 500 light-years away. The hot spot was discovered using the XMM-Newton x-ray observatory, and it's the size of a football field. Geminga itself is only 20 km (12.4 miles) across, and spins 4 times a second, but it's got 1.5 times the mass of our own Sun. The hot spot is created because material ejected from the rapidly spinning object is being recaptured by its gravity, and then funneled by its magnetic field to strike a specific point, heating it up millions of degrees.
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.
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