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Discovery of Pulsars
Pulsars were discovered in mid-1967 by Cambridge University astronomers Jocelyn Bell Burnell and Antony Hewish. The first pulsar was named LGM-1, for "little green men," because they thought that they might be picking up signals from an extraterrestrial civilization.
Pulsars have been found with periods from just fractions of a second to a few seconds. More than 1,100 pulsars are known today.
Official Classes of Pulsars
There are three different classes of pulsars. The first class is powered by its own rotational energy and the conservation of angular momentum. The second is called an X-ray pulsar. These pulsars emit X-rays that we can observe; their energy is taken from the potential gravitational energy of attracted matter.
The last class was theorized about in 1992 and is called a Magnetar pulsar. These neutron stars have very powerful magnetic fields. However, these magnetic fields are decaying and it is this decay that emits electromagnetic radiation in the form of X-rays and gamma rays.
The Lighthouse Model of Pulsars
The initial explanation for pulsar activity was that they were highly dense white dwarf stars that were "pulsing," that is, dimming and brightening or contracting and expanding. There was initial speculation over whether a pulsar was some sort of signal of extraterrestrial origin.
In the lighthouse model of pulsars, the rotational axis and the magnetic axis of the neutron star are not aligned, causing the magnetic axis to form a cone-like shape. This neutron star emits two narrow opposed beams of radiation, sometimes called "synchrotron radiation," which we happen to see as pulses as they pass by Earth's viewing plane.
This model not only explained pulsar signals but also confirmed the existence of neutron stars.
Challenges to the Model
In January of 2009, the online magazine "Universe Today" released the article " 'Lighthouse' Analogy No Longer Works for Pulsars." Twelve unknown gamma-ray-only pulsars have been discovered using NASA's Fermi gamma-ray space telescope. The article states that "a new class of gamma-ray-only pulsars shows that the gamma rays must form in a broader region than the lighthouse-like radio beam." These new pulsars are showing that the gamma rays arise way above the neutron star itself.
Pulsars as Communication Beacons?
The idea of pulsars as a communications device for some far off alien civilization has never truly left the thoughts of some in the science community. Due to the uncertainty of the lighthouse model of pulsars, there are some in the community who think, however improbable, that the possibility exists of some alien race modifying the pulsars to reach out to other civilizations.
While it's not necessarily a mainstream idea in the science world, the idea still raises questions. Even Carl Sagan said, "Has anyone examined systematically the sequencing of pulsar amplitude and polarization nulls? One would need only a very small movable shield above a pulsar surface to modulate emission to Earth. This seems much easier than generating an entire pulsar for communications. For signaling at night it is easier to wave a blanket in front of an existing fire than to start and douse a set of fires in a pattern which communicates a desired message."