Fermi-LAT sky map with the celestial neighborhood of the newly discovered pulsar PSR J1906+0722 featuring several other gamma-ray pulsars (not labeled). The color scale shows the gamma-ray intensity. The dashed square at the centre encloses the position of the pulsar and the part of the sky shown in more detail in the figure below. Credit: Knispel/AEI/NASA/DOE/Fermi LAT Collaboration Read more at: http://phys.org/news/2015-08-scientists-elusive-gamma-ray-pulsar.html#jCp
Gamma-ray pulsars are remnants of explosions that end the lives of massive stars. They are highly-magnetized and rapidly rotating compact neutron stars. Like a cosmic lighthouse they emit gamma-ray photons in a characteristic pattern that repeats with every rotation. However, since only very few gamma-ray photons are detected, finding this hidden rhythm in the arrival times of the photons is computationally challenging. Now, an international team has discovered a new gamma-ray pulsar hidden in plain sight in data from the Fermi Gamma-ray Space Telescope. The improved, adaptive data analysis methods and the computing power from the distributed volunteer computing project Einstein@Home were key to their success.
“Everyone believed that the source now known as PSR J1906+0722 was a pulsar. The tricky part was to show that the gamma-ray photons carry the imprint of the pulsar’s rotation and arrive according to that hidden rhythm. Many had tried that before, but to no avail,” says Pletsch, AEI. The Fermi-LAT observations cover a total time of >6 yrs now. For every single gamma-ray photon received from the pulsar, the AEI scientists had to identify during which of the up to billions of pulsar rotations it was emitted.
The pulsar (artist’s impression) in the center of the image and its celestial neighborhood. The supernova remnant (magenta) nearby is shown towards the bottom left in Chandra space telescope images. The molecular clouds (green and red) are shown in WISE images. For the Fermi-LAT the supernova remnant and the pulsar merged into a single source which complicated the pulsar discovery. Credit: Knispel/AEI/DSS/WISE/Chandra Read more at: http://phys.org/news/2015-08-scientists-elusive-gamma-ray-pulsar.html#jCp
It turned out that the new adaptive search method was exactly what was needed to get to the bottom of the riddle of PSR J1906+0722. Its actual sky position is outside the conservatively large region covered by the sky grid, which is why earlier (non-adaptive) searches did not find it. Still, the computing costs required for the search are enormous: Blind searches like this one would take several dozens of years on a regular laptop. Thus they used distributed computing project Einstein@Home. Each week, 10s of 1000s of volunteers from all around the word donate idle compute cycles on their laptops and desktop computers to the project.
By analyzing only those gamma-ray photons received while no pulsed radiation from the pulsar was detected, they were able to identify a 2nd gamma-ray source nearby. In the Fermi-LAT catalogue it had been lumped together with the pulsar into a single source offset from the true pulsar position. “…the nearby gamma-ray source could be the shock front of a different supernova remnant slamming into a neighboring molecular cloud and generating gamma-rays in the process,” explains Clark.
A pulsar is a compact neutron star that accelerates charged particles to relativistic speeds in its extremely strong magnetic field. This process produces gamma radiation (violet) far above the surface of the compact remains of the star, for example, while radio waves (green) are emitted over the magnetic poles in the form of a cone. The rotation moves the emission regions across the terrestrial line of sight, making the pulsar light up periodically in the sky. Credit: NASA/Fermi/Cruz de Wilde Read more at: http://phys.org/news/2015-08-scientists-elusive-gamma-ray-pulsar.html#jCp
An explanation why some pulsars are visible as as gamma-ray pulsars and not as radio pulsars could be that lower-energy radio waves are bundled in a tighter cone at the magnetic poles than high-energy gamma-radiation. Since radiation is mainly emitted along the surface of the cone and different wavelengths are emitted in cones with a different spread, radio waves and gamma waves would leave the neutron star in different directions. A pulsar might thus become visible as a gamma-ray or radio pulsar to a distant observer (depending on which cone sweeps across the observers position).
http://phys.org/news/2015-08-scientists-elusive-gamma-ray-pulsar.htmljCp
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