The record-breaking pulsar system in numbers
The new search method / blind search
To unambiguously identify a gamma-ray pulsar, its properties must be known to a very high degree of precision. Only then can astronomers determine the rotational phase at which each of the gamma-ray photons was emitted by the pulsar. And only then can the gamma-ray pulsations be detected without doubt. None of the relevant pulsar properties, such as its position in the sky, its rotational frequency and how this changes, nor the orbital parameters of the binary system, are known a priori.
Researchers must check many combinations of these properties in a blind search. If the scientists were to immediately search through four years of Fermi data, the number of possible combinations would be so large that the computing effort required would make a practical implementation impossible.
The new analysis method splits the full data set into shorter overlapping sections. Each of the sections can now be searched separately; the individual results are then combined in an optimal way. Overall, this search method is almost as sensitive as a search through the full four-year data set in one run. If a promising signal is found at a particular parameter combination, the full set of data can be checked with this combination very quickly.
The key is to distribute the parameter combinations as cleverly as possible so that any signal is found with the highest possible probability and unnecessary computations are avoided. The new analysis method employs an algorithm which adaptively improves the parameter combinations – also called grid points – to cover the total parameter space while keeping computing costs as low as possible. “Initially, our method distributes the grid points purely randomly. After that, the algorithm checks whether the points are too close to each other and shifts them if required,” says Henning Fehrmann from the AEI.
Pulsars and the Fermi space observatory
Pulsars are compact neutron stars born in supernova explosions which rotate rapidly and steadily about their axis. Their intense magnetic field causes them to emit radio waves or gamma-ray photons in the form of a diverging beam. Their rotation sweeps the beam through space like the beam from a lighthouse. When the neutron star is pointing towards Earth, it is visible as a pulsar.
Not all pulsars show up in multiple spectral ranges simultaneously. In some cases, the scientists measure only the flashes as a radio pulsar; in other cases, only the periodic arrival times of gamma-ray photons can be registered. The most likely causes for the different pulsar types are the different orientations of the emission regions in the extremely strong magnetic field of the neutron star and its orientation towards Earth.
If the pulsar is in a binary system after its formation, it can accrete matter from its companion as the latter evolves. It transfers angular momentum to the pulsar and speeds up its spin. This process is called “recycling” of pulsars and explains the origin of millisecond pulsars, which spin several hundred times per second about their axis.
Until now, astronomers have found most pulsars in the radio wave range. However, thanks to NASA’s Fermi satellite, they are discovering more and more of these celestial bodies via their high-energy gamma radiation. Fermi has been observing the universe with its Large Area Telescope (LAT) in the gamma range since 2008. It has discovered hundreds of new sources, many of which are probably as yet undiscovered pulsars.
KNI / ME / HOR