little green men (little green men) is the nickname with which Jocelyn Bell, whom I have the pleasure of knowing personally from my time working at Oxford, and Antony Hewish, who was her thesis director, baptized the peculiar signal they discovered with their new radio telescope in 1967. They detected constant pulses, which were repeated every 1.33 seconds like an electrocardiogram, and with the nickname they winked at the possibility that they were signals sent by an extraterrestrial civilization. It was a joke, but the reality is that they had no idea what was the nature of the pulses. They spent frantic days making sure they weren’t missing something, that the pulses weren’t actually radio interference from Earth or the product of instrument failure. Just a month later, Bell discovered a second signal from another part of the sky, this time with pulses that repeated every 1.2 seconds. This confirmed, without a doubt, that a new type of astronomical object had been discovered. “Something totally unexpected, something totally unknown and, therefore, something very funny” in the words of Jocelyn herself. She had discovered pulsars.
55 years and 3,000 pulsars later, we now know that these pulsating signals are produced by light beams emitted through the magnetic poles of fast-spinning neutron stars. They are something like cosmic lighthouses that we are capable of observing if the flash of light passes pointing towards our eyes, or towards the Earth, in this case. The time between flash and flash tells us the speed at which the neutron star rotates more precisely than our atomic clocks. And they rotate fast. Wow, they rotate fast.
Neutron stars are the result of the collapse of the core of massive stars as they die in a supernova explosion. The contracting core speeds up its rotation by conserving the angular momentum of the star it once was, in the same way that ice skaters spin faster and faster as they retract their arms. In this way, when neutron stars are born, they spin several times per second and with magnetic fields a billion times more intense than those of the Earth.
The “focuses” of the pulsars in our history are activated by the combination of these two features: the high energy generated by their rotation and their powerful magnetic fields. Specifically, as the neutron star rotates, and with it its large magnetic field, the particles on its surface accelerate. This results in a current of energy being given off by the poles of the magnetic field, which are misaligned from the axes of rotation. Like almost everything in this life, nothing lasts forever, and as energy is emitted, the neutron star loses rotational energy and slowly slows down. Thus, after a few million years, the neutron star will not spin fast enough to produce the light beams, and the stellar beacon will go out.
The discovery of pulsars was a revolution for the field, beginning with the very detection of neutron stars. Speculation about the existence of these stellar corpses, neutron nuclei created when massive stars die in supernova explosions, has been speculated since the 1930s. But it wasn’t until 1968, when a pulsar emitting signals every 33 milliseconds was detected in the center of the Crab Supernova Remnant, that the theories were confirmed. It was also the observations of a pulsar in a binary system that provided the first evidence for the existence of gravitational waves, thus confirming the predictions of Einstein’s Theory of Relativity. And, as if that were not enough, the first planet discovered outside our Solar System, that is, the first known exoplanet, orbits around a pulsar.
Today we continue to make new discoveries thanks to the study of pulsars and we find more and more of these exciting sources. For example, last month two new pulsars were discovered. MAXI J1957+032, which rotates 314 times per second, and MAXI J1816-195, which rotates 528 times per second. These galactic lighthouses are somewhat different from the ones we have talked about today. They are much faster, they make a turn in just a few milliseconds and, in addition, the light beams are emitted in X-rays. This is because they are activated by material that is accreted by the neutron star, which steals it from a neighboring star. Sure enough, they’re X-ray binaries, our old friends. Of these millisecond pulsar we will talk “more and better” at our next appointment.
Montserrat Armas Padilla is an astrophysicist from La Gomera who currently works at the Institute of Astrophysics of the Canary Islands. After graduating in Physics at the University of La Laguna and working in its Department of Applied Physics, she went to the Netherlands to carry out her doctoral thesis at the University of Amsterdam. She has enjoyed research stays at different institutions such as the University of Kyoto (Japan) and the University of Oxford (United Kingdom), always dedicated to the study of neutron stars using X-ray data.
* Section coordinated by Adriana de Lorenzo-Caceres Rodriguez