A binary pulsar with a planet?
Detailed analysis of Fermi observations hints at pulsar exoplanet
The pulsar PSR J1555−2908 is in a binary system with a low-mass companion. The arrival times of its gamma-ray photons exhibit long-term variations, which are usually attributed to intrinsic effects and are called “timing noise”. For a rapidly rotating pulsar such as PSR J1555−2908, the observed variations are surprisingly large. Scientists from the “Pulsars” research group at AEI Hannover have now devised and tested an alternative explanation for the observations by the Fermi Gamma-ray Space Telescope: The arrival time variations could be due to the gravitational force of a Mercury-mass exoplanet orbiting the binary system on a 12-year eccentric orbit. While statistical evidence very slightly favors the pulsar exoplanet hypothesis, the timing noise hypothesis describes the data very well, too. At the moment it cannot be conclusively decided whether PSR J1555−2908 is in a hierarchical triple system, but Fermi will continue to collect data. Eventually, it will become clear whether the PSR J1555−2908 binary system actually is a triple with an exoplanet.
The 559 Hz black-widow pulsar PSR J1555−2908, originally discovered in radio, is also a bright gamma-ray pulsar. Timing its pulsations using 12 yr of Fermi-Large Area Telescope gamma-ray data reveals long-term variations in its spin frequency that are much larger than is observed from other millisecond pulsars. While this variability in the pulsar rotation rate could be intrinsic "timing noise," here we consider an alternative explanation: the variations arise from the presence of a very-low-mass third object in a wide multiyear orbit around the neutron star and its low-mass companion. With current data, this hierarchical-triple-system model describes the pulsar's rotation slightly more accurately than the best-fitting timing noise model. Future observations will show if this alternative explanation is correct.