Shedding New Light on a Pulsar Mystery
Astronomers at the fledgling Low Frequency Array (LOFAR) are teaming up with established radio observatories to study the weird and wonderful properties of pulsars. These dense, spinning, highly magnetized neutron stars emit radiation in focused beams which produce a “lighthouse” effect as these beams sweep across the Earth. Despite over four decades of study, the mechanism for generating these beams is not fully understood.
Neutron stars are those incredibly dense end products of supernovae where the mass of our sun is crammed into the space of a city. Pulsars are a particular subclass of neutron stars that made their presence known to radio astronomers Jocelyn Bell Burnell and Anthony Hewish in 1967 with a *blip blip blip* that one would expect to see from extraterrestrials. However, neutron stars had already been hypothesized in 1934, and the lighthouse model was soon formulated.
In this model, pulsars have two bright hotspots, or beams, of emission from the magnetic poles. If the magnetic axis and rotation axis are not aligned, and if Earth is in just the right spot, we see the hotspot spinning past us as a light turning on and off, or “pulsing.”
One hypothesis for the emission mechanism has light of different radio frequencies coming from different heights above the pulsar’s surface.
By observing a pulsar with the Lovell Telescope in the UK, the Effelsberg Telescope in Germany, and both the high and low LOFAR bands, astronomers span eight octaves in frequency, thus different heights in the pulsar magnetosphere, as shown in the graphic (right). The pulses, or “blips,” have a different shape in each frequency.
Pulsars are not only themselves interesting objects of study, but excellent probes of the gas in our galaxy along the line of sight, as well as laboratories for studying general relativity and searching for gravity waves. They may not be the beacons of “little green men,” but they are some of the coolest things in astronomy, and the new insight from low frequency observatories is sure to make them even cooler.
Images: ESA (top), Aris Karastergiou, University of Oxford (bottom).