Binary star system precisely timed with pulsar's gamma rays

Max Planck Scientists Find Evidence for Stellar Companion's Activity Cycles

July 29, 2015

Pulsars are rapidly rotating compact remnants born in the explosions of massive stars. They can be observed through their lighthouse-like beams of radio waves and gamma rays. Scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Hannover, Germany, now have precisely measured the properties of a binary star system with a gamma-ray millisecond pulsar. Using new methods, the researchers analyzed archival data from the Fermi Gamma-ray Space Telescope more precisely than possible before. They discovered variations in the orbital period of the interacting binary system that can be explained by magnetic activity cycles of the companion star.


Neutron stars are exotic objects. They are made up of matter much more densely packed than normal, giving the entire star a density comparable to an atomic nucleus. The diameter of our Sun would shrink to less than 30 kilometers if it was that dense.

Neutron stars also have extremely strong magnetic fields. Charged particles accelerated along the field lines emit electromagnetic radiation in different wavelengths. This radiation is bundled into a cone along the magnetic field axis. As the neutron star turns about its rotational axis, the cones of radiation sweep through the sky like a lighthouse beam because the rotational axis is usually inclined relative to the magnetic field axis. The neutron star becomes visible as a pulsar, if the beams sweep over Earth. Pulsars rotate with cycles of a few seconds up to only milliseconds. Their rotational periods can be highly stable with a precision that places them among the most accurate clocks in the Universe.

These cosmic lighthouses were first discovered in 1967 by Jocelyn Bell Burnell and identified as radio pulsars. X-ray and gamma-ray pulsars are also known to exist today. Even though not all pulsars are observable in all wavelengths, scientists assume that they still emit radiation in the entire electromagnetic spectrum. However the mechanisms which govern radiation emission in different frequency ranges are not yet completely understood.

Gamma-ray pulsars and radio pulsars

A plausible 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). Another model has gamma radiation originating not in the polar regions of the magnetic field but rather the equatorial plane where the field lines are disrupted. It is therefore very important to observe as many pulsars as possible in all wavelengths to better understand these mechanisms.

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